Manticore
RETIRED MOD
- Joined
- Jan 18, 2009
- Messages
- 10,115
- Reaction score
- 114
- Country
- Location
Mine Warfare
by Stuart Slade
Mine Warfare
The most important point to start with is that the destruction of ships, whether civilian or military, is not the primary purpose of a minefield. Mines are intended to prevent the use of, or passage through an area of sea. Mine countermeasures are intended to permit the exploitation of an area of sea or allow safe passage through such an area. Following from these concepts, it is most effective to define offensive mine warfare as the laying of minefields and the maintenance of same while defensive mine warfare is definable as the destruction (or neutralization) of existing minefields and preventing their replenishment or creation. Both offensive and defensive mine warfare can form part of both offensive and defensive operations undertaken by the armed forces as a whole. Under such circumstances, the offensive mine warfare forces always have the initiative and the mine countermeasures groups will always be forced into a position where they are responding to those initiatives.
Asset protection
Historically, the use of minefields to impede and defeat an enemy attack is the oldest application of the technique. This was first attempted, on a systematic basis, by Confederate forces during the American Civil War. In the absence of significant naval forces and with severe limitations on their ability to construct conventional coastal defenses, mines offered the Confederacy the only possible way of defending critical ports and coastal cities. They were aided in these endeavors by the absence of any effective mine countermeasures technology on the Union side.
Minefields are extensively used to protect ports and other installations from intrusion by sealing off all but a few, very secret, passages. The same technique is used to defend coastal shipping lanes by laying comprehensive minefields to seaward of the route. With the development of mine countermeasures, such defenses are only effective when used in combination with other coastal defenses. Thus, to defeat such defenses, a balanced naval task force has to be committed. The mine warfare ships themselves must be extremely capable and ships other than mine countermeasures vessels have to be risked in order to protect them
In order for such defenses to be effective, the minefields need to be dense and extensive. Any attack is likely to be determined and its threat axis cannot be reliably predicted by the defenses. As a result, the numbers of mines required is immense. The sheer effort required to lay mines in this magnitude cannot possibly go un-noticed by the enemy so the areas mined in this manner are relatively well known and defined. It would be possible to use guile and deception to blur this perception but the results would not be worth the effort. The minefields also need to be continually maintained and supported. This suggests that attacking and destroying the minelaying vessels may be as valuable a counter as attempting to clear the mines.
Interdiction
In mine warfare terms this involves the obstruction of shipping lanes and transit points by well-placed minefields intended to restrict the ability of hostile forces to mount attacks from the sea, to prevent the infiltration of submarines into critical areas or to inhibit the free flow of commerce. Interdiction differs from asset protection in that the mines are not laid in close proximity to own installations and may be far removed from support by friendly forces. Indeed, in many cases of interdiction mining, the minefields will be laid off enemy ports or assets. In effect, the use of interdiction rather asset protection strategy turns many of the mine warfare problems on their heads.
The problem distills to getting the mines to their target. Since the area to be infested is usually covered by enemy defenses, the effort involved must be covert. This prevents the use of the huge numbers of mines used for asset protection. Yet the density of the minefield must be maintained in order to be effective; the implication is that the minefields resulting are very small and must be placed with the greatest of care. Asset protection minefields can be used with great effect here. If the minefield can be mapped, then the interdiction fields can be placed in the safe channels. This raises the delicious prospect of the presence of the hostile minefield not being suspected and the ship losses attributed to mines drifting from the defensive fields or being incompetently laid.
From the mine countermeasures point of view, the problems posed by interdiction minefields are also turned on their head. The threat now is small, dense fields that crop up in unexpected places without prior warning. Once such a minefield is discovered, the port or other installation in question is undeniably closed until the mines have been swept. The mine warfare ships are not at serious risk, except, of course, from the mines themselves. The problem distills to the massive effort required for routine surveillance at every port, on all shipping routes, all of the time. The Second World War solution of using converted fishing trawlers is no longer viable due to the efficiency of modern mines. Yet constructing dedicated mine warfare ships in sufficient numbers is quite impossible. Combining numbers, availability, crew considerations and financial constraints is a serious challenge to naval ingenuity.
Attrition
Interdiction is effectively mining somebody or something out of a given area; attrition can be defined as the art of mining high value assets into a selected area. Usually the effort is aimed at preventing a particularly crucial naval unit (typically an aircraft carrier or a ballistic missile submarine) from getting to sea and deploying. In a major conflict, attrition mining can be a crucial part of the naval balance of power - the consequences of destroying even a single SSBN or aircraft carrier (in both military and political terms) are incalculable. Usually attrition is achieved by selecting a suitable choke point through which the asset in question must pass and then mining it into oblivion.
In essence, countering attrition mining is a simple task. It is necessary to carve a single path, once, through the suspected choke point. The problem is that since the target is, by definition, of the highest value, the mines laid will be of the most sophisticated and dangerous types. The enemy will also make every effort to lay them in the largest possible quantities. The situation is complicated by the fact that the mine countermeasures ships cannot afford to make a single mistake (the commander of a minehunter responsible for mine clearance when, for example, the USS Abraham Lincoln hits one is unlikely to have much in the way of career prospects afterwards). To make matters worse, getting such assets to sea is often time-critical. At its most extreme, this could be to get the ship or submarine out before its base is incinerated. In such cases, low value ships are likely to be used as mine bumpers, accepting their loss to save the crucial unit.
The fact that the primary examples of attrition mining countermeasures have quoted aircraft carriers and SSBNs as the primary assets to be protected does not mean that this task is restricted to superpower navies. Once the overall size of the fleet decreases, smaller assets become equally important. In a war between two Newly Emerging Maritime Powers (NEMPs), a single frigate or amphibious warfare ship can be a crucial element in the naval balance of power. For example, many nations have a submarine fleet of two or three boats. If they cannot get out of port, all the investment placed in acquiring, supporting and maintaining that force have been wasted.
Terrorism
At first sight, the terrorist use of mines can be considered a sub-set of interdiction, yet the reality is that the problems and demands of countering terrorist mining are in a class by themselves. A terrorist "minefield" can consist of as little as a single covertly-placed mine. The objective is not so much the destruction of the ship that touches off the mine but the economic damage caused by disruption to trade, the financial penalties resulting from increased shipping insurance rates and extended journey times and the political impact of "demonstrating" the inability of a government to defend its citizens from outrage. The intended result is to cause pressure on the Government from the public and the business community to change policy in the direction desired by the terrorists.
Countering terrorist mining is very difficult. Essentially it involves proving that no mines are present to a business community that has nothing to gain and everything to loose by accepting the validity of any proof offered. They are quite right to adopt such a position; the fact that a given area is mine-free at 8 pm one evening does not mean it will be mine-free at 8 am the next morning. This situation boils down to a command and control problem; it is necessary to be able to demonstrate, quickly and unobtrusively that nothing has appeared on the seabed or, if something has appeared, it can be investigated and, if necessary, neutralized.
Force multiplication
A further use of minefields is to accentuate the effectiveness of other weapons and to provide a suitable environment for their use rather than as a primary weapon. This may be achieved by using minefields to channel enemy shipping into selected killing grounds or to restrict their maneuverability and thus enhance their vulnerability. A good example was the Silkworm attack on the USS Missouri during the Second Gulf War. Here, the Iraqis had placed minefields offshore of a coastal defense missile battery with the intention that either the Coalition warships would hit mines while maneuvering to avoid the attack or remain on their restricted course and get hit. This plan was defeated by the combination of mine avoidance sonars and integrated command systems on the escorting British destroyer HMS Gloucester. Other potential uses of force multiplier minefields include decoying submarines into attacks on "high value targets" (actually dummies surrounded by mines) or herding surface ships into positions suitable for submarine ambushes. Another, and very important, role is to draw scarce and irreplaceable mine warfare ships into positions where they (and their highly trained crews) can be destroyed. The only real counter to these strategies is the use of mine detection equipment in combination with fully integrated command systems to create a multi-dimensional tactical picture.
Riverine Mine Warfare
The subject of mine warfare carried out within the environment of inland waterways (including canals as well as rivers) is often neglected. Yet such waterways often represent a major means of commerce movement, and in many Asian countries, are the primary means of communication in rural areas. Such mining activities may also affect food production in, for example rice growing areas.
The environment of rivers is not an easy one for either offensive or defensive mine warfare operations. The combination of fast-flowing waters and silt-covered beds makes conventional mining activities very difficult. Usually resort is made to floating mines which are dropped in upstream of the target and allowed to run down into the target. This is actually an offense against international law but that has never appeared to stop anybody. Other techniques include off-route mines, rocket launchers, fougasses or claymore mines which are installed on the river or canal banks and detonated when the target passes. These are all, basically, terrorist techniques and are not usually part of operations carried out by regular military forces (more for want of opportunity than any reticence over the tactics).
Mine countermeasures in such environments is eased by the absence of acoustic or pressure mines; such would be detonated by the water and flow conditions. Since most craft will be built of wood and many will be oar- or sail-powered, magnetic mines are not that great a threat. Floating mines can usually be spotted visually. However, contact and off-route mines are very serious problems indeed and the latter are almost as insoluble a problem as pressure mines are on the high seas. The major difficulty is that the riverine environment forces the adoption of very small craft which have correspondingly limited capability. They are vulnerable, subject to intense threat, not necessarily mine related, and liable to substantial operational attrition.
The most attractive solution for this environment is the use of remote-controlled craft which can be used to sweep ahead of manned vessels. The Chinese have designed the Type 312 minesweeper drone for this role and have sold eight to Thailand for the same purpose. The problem with them is that the command link is vulnerable and this can result in serious operational unpredictability.
[The Thailand Army no longer uses the Type 312 as drones. Instead, they now stretch a oil-drum supported net across the river above the bridge to be protected. This then traps any mine floating down the river.]
Mine Categories
There are two generalized categories of mines, moored mines and ground mines. Moored mines float at a given depth and are held in place by an anchor. These are by far the least sophisticated and expensive form of naval mines. The problem with these weapons is that they cause less-destructive contact explosions rather than devastating under-the-keel hits. Ground mines detect the target by the acoustic, magnetic, or pressure signatures of a vessel, or a combination of these signatures (this detection method has caused these weapons to be called influence mines). After the target has been detected and is within range, the mine explodes at a set distance from the target. This ensures that the explosion is of the under-the-keel variety and maximizes damage.
Moored Mines
Contact mines
Contact mines are detonated when the ship strikes the mine. This bends horns on the outside of the mine, causing glass cylinders of acid inside the horns to break. This acid then ignites the detonator, either directly or by acting as an electrolyte for a battery. Other types of contact mines have used inertia switched but these proved to be very vulnerable to premature explosions. Contact mines have been fitted with many ingenious anti-sweep devices including explosive charges to cut sweeping wires and ratchet devices that enable a sweep wire to pass through the mooring cable without cutting it. Contact mines are practically restricted to the anti-ship role. Since the hull of the target actually has to touch the mine, using contact mines for ASW means that mines have to be set at all the depths a submarine is likely to adopt - requiring huge numbers
Antenna mines
The desirability of exploding a charge under a ship rather than in contact it has long been recognized. The first practical attempt at this was by the US in 1919. The antenna mine is moored on a short cable so it is a set distance under the surface. A long copper wire stretched upwards, terminating in a float. If any steel object touches the wire, anywhere along its length, an electrical potential is generated which detonates the mine. Antenna mines are particularly valuable in anti-submarine work since they can be set to sit deep in the water with their detonation antennas terminating a set depth under the surface. Thus, surface ships can sail over the wires in relative safety but submarines are in mortal danger.
String mines
The problem with antenna mines is that the explosive charge has to be within about 100 feet of the hull of the submarine if significant damage is to be achieved. The chance of achieving this was greatly increased by the introduction of string mines. These feature tiers of charges which may be either contact or antenna fused. The explosion of one mine necessarily means the discharge of all due to sympathetic detonation. The whole assembly is incredibly clumsy, looks rather like a perverted Christmas tree and can effectively block water up to 800 feet deep. It is, therefore, exclusively an asset protection system but one which is very effective. A slightly cleaned up version, in which the tiers of charges are replaced by much smaller single ring charges at 4 meter intervals has been marketed by the Italian Whitehead Motofides Company.
Overall
Moored mines are by far the most common in the world's mine warstocks and any minefield encountered will probably contain mostly contact mines of varying types. The manufacture of such mines or their equivalents, is a worldwide industry. The going cost for a new contact mine is around US$5,000. It is therefore impossible to ignore these weapons or to dismiss them as obsolescent. The use of moored mines is nowhere near as easy as it appears. The option of air-dropping moored mines is also now also virtually extinct since few aircraft have the weight-lifting capability and capacious, airborne-accessible bomb bays required for the role. Those nations that have such aircraft have better things to do with them. All these considerations mean that laying moored minefields is likely to be undertaken only by converted surface ships. If these can be identified and terminated with extreme violence, the threat from contact mines will be greatly alleviated. This is basically a C3I function and one which needs to be integrated with other mine warfare operations. Inexperienced construction of moored mines can also be dangerous. The Iraqis decided to "improve" the Pattern 1908 mine by increasing the explosive charge. In order to preserve buoyancy, a lighter cable was substituted for the heavier chain used in the Russian original. As a result, the inertia of the heavier mine in sea swell caused the less robust cable to break and resulted in the mines drifting out of control.
A very effective technique is visual observation; lookouts in the bows saved a British destroyer and two frigates from hitting floating mines in the Second Gulf War. The mines were then destroyed by 20 mm fire. A much more sophisticated variant of the same idea is the mine avoidance sonar. Effectively, this is a high-frequency imaging sonar scanning ahead and below the ship. It does not need the very high resolution of the mine warfare classification sonars since the identity of the mine is not required; just the data necessary to keep well away from it. Another essential requirement is the provision of parvanes. These prevent a contact mine being drawn against the hull of a ship by the suction generated as a result of its motion through the water. It is this suction that gives moored mines their lethal radius.
Bottom Mines
Magnetic Mines
Magnetic mines were developed by the British who wanted a mine which sat on the bottom and was, therefore, limited to relatively shallow water. The original magnetic mines built by the British used the vertical component of the ship's magnetic field to trigger the mine when a given field density was reached. Mines of this type were first used in 1918 off the Belgian coast as anti-submarine devices and were used in the anti-ship role of Russia in 1919. Examples were captured by the Russians at that time, found their way to Germany which copied them. The small number of magnetic mines laid by Germany in 1939 virtually brought the infested ports to a standstill until sweeping techniques could be developed. By this time, the British had developed a superior derivative of their 1919 mines which worked off the horizontal component of the ship's magnetic field. This apparently insignificant change made it possible to design a mine fuze which responded to the rate of change of field strength rather than absolute field strength. This made defense against magnetic mines by degaussing and magnetic sweeping procedures far less effective. Later, the mines were further improved by introducing double-tap fuzing. In this case the mine would be activated by increasing magnetic field strength but only detonated by decreasing field strength. Thus the mine would explode as the ship passed, under the screws rather than under the bows. The combination of horizontal-component and double-tap fuzing was an order of magnitude more damaging and was also much more difficult to simulate when sweeping. It made the whole generation of Second World War built minesweepers obsolete. Magnetic bottom mines were probably the last naval mines which could be built simply, inexpensively and in very large quantities by unskilled labor. The going cost for a magnetic mine is around US$10,000.
Defenses against the magnetic mine revolve around reducing the magnetic signature of the targets as much as possible. This predicates avoiding the use of steel components wherever practical. Hulls have to be made of non-magnetic material and fittings, such as anchor chains and windlasses fabricated from bronze. All low magnetic materials are extremely expensive, yet the use of steel in some areas remains unavoidable. Thus degaussing is still essential to correct the resulting magnetic field. An often-neglected aspect of this problem is that drastically reduced magnetic signatures have a disturbing tendency to increase unpredictably. This is partly the result of natural processes and partly the effect of apparently minor modifications to the ship..
Acoustic Mines
Acoustic mines were first introduced by Germany in 1940 and exploit the principle that all ships and submarines have a specific acoustic signature. This is generated by the vessel's machinery, the design of the hull, the propellers, and many other factors. Delay clocks can be included to leave the mine inert (and thus unsweepable) for up to twelve days after it has been laid while the incorporation of counters means that the mine will only be detonated after a certain number of impulses (usually up to 16) have been received. This means a sweeper would have to make a large number of passes over a suspected field before it could be sure that all the mines within the field had been exploded.
Acoustic mines can be set to work in either broadband or narrow band modes. The original broadband mines were those introduced by Germany and work on the integrated volume of noise emissions across all frequencies. As such, the fuzing system is relatively simple (all that could be achieved by the electronics technology available at the time) and mines of this type can be swept using relatively simple noise generators. Mines of this type are very widespread and are produced extensively. They do, however, require a degree of electronic sophistication in the fuzing making them more expensive and time-consuming to produce. Such mines are available for about US$25,000.
During the 1970s and 1980s, advancing electronics technology made it possible to increase the degree of electronic sophistication that could be packaged into small units. This had a major influence on all aspects of military technology and made the development of relatively intelligent weapons possible. In mine warfare, the result was the development of narrow-band signals processing software for mine fuzes. These revolutionized the prospects for acoustic mines and have had a major effect on mine countermeasures technology.
Narrow band noise processing exploits distinctive frequencies centered around specific shipboard activities. These may be the thumping of a diesel, the cavitation caused by screws in water or the characteristic whine of a gas turbine. Flow noise is generally not used since it is insufficiently precise for the sort of fuzing here in use. The implications of narrow-band processing are that the sound profiles used are so precise that the mine can be programmed to listen for specific types of ships or powerplants and ignore others. One Russian narrow band acoustic mine, for example, can be set to listen specifically for the LM-2500 gas turbine and ignore other powerplants.
This makes sweeping exceptionally difficult. In effect, the sweep devices have to emulate the sound profile of a target in order to detonate the mine. The processing capability of modern mines means that the match has to be fairly exact since errors will be detected and used to filter out the sweep. Such acoustic sweeps are in service but their use faces many problems. One is that the sweep may not be simulating the intended target; or may be simulating the wrong aspects (or combination or aspects) for the mine in question. Thus many sweeps will be necessary to cover the available aspect combinations (and then repeated up to 16 time search to allow for counting devices in the fuze). The resulting time consumption is so high that the development of narrow band acoustic fuzes for mines is widely believed to have rendered sweeping techniques ineffective when faced with this threat.
The good news is that these mines are very expensive, are very difficult to make and require great hydro-acoustic expertise if the fuzing system is to work. Such mines are usually found only in the inventories of major powers such as the USA, Russia, France etc. Even where mines of this type are exported, their fuzes are much simplified. For example, the Italians produce the MR-80 narrow-band acoustic mine for their own and NATO use. An export customer can buy an MR-80 (as did the Iraqis) but the version delivered will be the export-only MRP (which the Iraqis got but still labeled MR-80!) which has a broad band acoustic fuze. Few export customers have the sophistication to know the difference. The Russians have the same policy, openly calling the simplified mines "monkey models". Non-traditional mine producers (for example, Iraq, Iran, North Korea and Chile) have all tried to produce narrow band acoustic mines and failed. Chile tried to duplicate a British Stonefish mine in the mid-1980s and failed. They got around the problem by duplicating the outside casing (including markings) and general appearance of Stonefish while releasing data sheets that duplicated GEC-Marconi's corporate style. They also hinted that they had received a secret license to produce Stonefish. In fact, the Chilean mine had a simple broad band fuze.
The cost of acoustic mines differs dramatically depending on the capability of the fuzing system. Since these mines are produced in much smaller quantities than their simpler cousins, the operations of economy of scale are less pronounced, again increasing unit cost. As a result, acoustic mines are priced between US$50,000 and US$150,000.
Magnetic-Acoustic Mines
A vulnerability noted with acoustic mines is that the fuzing mechanism must be powered-up all the time the mine is active. Running the fuze powered-up requires electrical power, albeit in small quantities. The provision of such electrical power is restricted to banks of batteries which must be limited in volume. This limits the life of the mine to that of the battery power provided, typically as little as twelve days. Since the mines cannot be recovered and replenished, it is obvious that minefields consisting entirely of acoustic mines will be short-lived. Another potential problem is that any electrical system that runs constantly emits electrical energy that can be detected. There have been frequent claims that mine hunting systems have been devised which can use this emitted energy to locate and classify mines, the methodology does seem theoretically feasible; this does not mean it is practically realistic.
These problems are overcome by a magnetic-acoustic fuzing system. In this arrangement a very simple magnetic switch (comparable to those used in the original magnetic mines) is used as an initiator. This needs not be sophisticated or complex since it does not directly detonate the mine. What it does do is activate the acoustic portion of the fuze. If this determines the target is worth engaging, it will do so, otherwise it automatically turns off after a few minutes. These systems have many advantages. The very simple magnetic switch does not require energy so has almost infinite life. The acoustic fuze is switched off most of the time, (thus conserving battery power and limiting any suggestion of detectability). Even more useful is the fact that any sweep must not only combine magnetic and acoustic generators but must do so in carefully integrated and carefully phased proportions and sequences. In effect the combination of magnetic and acoustic fuzing not only integrates the benefits of both, it does so with a synergistic effect that adds an order of magnitude to the lethality of the system and the problems involved in countering it.
Pressure Mines
Pressure mines are invariably bottom mines since they measure the absolute drop in pressure associated with the difference between the known pressure due to water depth and the depth of water under the hull of a passing ship. This differential is directly related to the depth at which the mine is situated, the ratio involved being roughly proportional to the square root of the pressure drop. In other words, if the depth at which the mine is located is increased by a factor of 1.414, the baseline pressure drop required to detonate that mine must be doubled. In addition there is a necessary minimum below which a pressure drop will not explode the mine; this is an absolute requirement since wave motion would otherwise cause sufficient pressure drops to explode the mines. For these reasons, pressure mines are inevitably shallow-water inshore weapons. As with acoustic mines, pressure mines have selective time delay fuzes, letting the mine become active after a certain day and, if no targets have passed over it after a certain period, making it inoperative again. They also have similar multiple target ship count devices allows a predetermined number of ships to pass before detonation.
Pressure mines are completely unsweepeable by any known means. A variety of experimental methods have been tried including the use of mine bumpers. These were adopted in 1945 by the US Navy to clear pressure mines dropped off the Japanese coast by the US Air Force. These mine bumpers consisted of old freighters with their holds filled with sealed drums (to preserve buoyancy) and with all ship functions automated so that the crew could concentrate above decks. In fact, the ships were run entirely from the bridge where the crew sat of 12 layers of conventional bedding mattresses. These precautions proved inadequate since the mine explosions disabled the bumpers and the crew were seriously hindered (though not actually injured) by the shock waves. Pressure mines do have serious operational limitations. Since the pressure drop required to detonate them is directly related to the draft of the ship in question, shallow draft vessels can traverse most pressure minefields with impunity. Shallow draft implies restrained displacement and this consideration is the reason why the maximum size of NATO coastal mine warfare ships is restricted to 800 tons (and means that the US Avenger class minehunters should never go near a suspected pressure minefield). Air cushion craft should be able to traverse a pressure minefield in relative safety. On the other hand, many types of amphibious warfare craft (for example the AAV-7) have high pressure signatures and are very vulnerable indeed to such mines.
These characteristics mean that pressure mines are usually regarded as inshore defense and, specifically, anti-amphibious operation, weapons. In these roles, they would be located in such shallow water (even surf zone) that even minute pressure signatures would detonate them. In such operations, even the minimum pressure levels set by wave action are low since deep rollers will be attenuated as they come inshore. Thus, pressure fuzing has been adopted for an entire generation of small anti-invasion mines, specifically intended to destroy amphibious craft. Much of the current US mine warfare effort is being diverted to counter this problem. Pressure mines are available at costs between US$25,000 and US$50,000.
Rising Mines
This type of mine is particularly useful in either very deep water, which could not otherwise be mined, or where the seabed is soft and glutinous. The rising mine lies on or under the sea floor. It is normally equipped with a passive acoustic sensor to listen for a ship or submarine to pass within range. When contact is made, it switches to an active mode and jettisons ballast to change its buoyancy from negative to positive. This causes it to float up and explode at the appropriate moment. These weapons are particularly dangerous since no reliable means has yet been developed for detecting buried mines. A much more dangerous version of the rising mine fires a projectile rather than just floating upwards A rising mine's case depth is probably fixed by the crush depths of the mechanism and the projectile. In really deep water, then, the mine must be moored, and its case must be buoyant enough to support a considerable weight of cable. Consequently, the mine must be large, and the laying rate will be limited. In shallower water, down to a thousand feet or more, the mine case might well lie on the bottom, unsweepable by mechanical means (i.e., by any device that would cut the mooring cable).
The effect of the existence of deep-water rising mines is to extend the minable area of the world significantly. Countermeasures are very expensive because the mine may cover a substantial lethal area (the projectile may maneuver to one side or the other) and because any attempt to destroy mines or mine mooring requires very deep operations. Although not technically rising mines (the projectile shoots sideways rather than up), the same basic philosophy has been adopted for a range of surf-zone anti-invasion mines. These use similar acoustic sensors to detect landing craft and amphibious armored vehicles approaching and then fire their projectiles at them. The Italians released a mine of this type in the late 1980s while the Russians have offered the KPM anti-invasion mine which uses the same principle. These are particularly serious deterrents to an invasion since their large target acquisition area means that the conventional anti-mine tactics of clearing narrow paths to the beach are no longer adequate - the anti-invasion mines would cover such paths from the flanks.
Dummies
Not every mine-like object is a mine. An object on the seabed could be domestic rubbish, sunken buoys, oddly-shaped rocks, debris for sunken ships, or even a mine. The problem in clearing them is the very large numbers of mine-like objects that have to be checked and, if dangerous, eliminated. A skilled mine warfare team (which the Iraqis were not) will lay thousands of cheap dummies for every genuine mine and bring the mine clearance process to a congested halt. It is very easy indeed to make a cheap plastic imitation of a mine casing, paint it with the correct markings and fill it with concrete. The estimated cost is around US$10.00. Each has to be treated as a live mine until proven otherwise. The reverse is also true, of course. The installation of modern and very dangerous fuzing packages in apparently simple, old contact mines comes under this heading.
The ultimate in dummy mines are the mines that are not there. For example, in a future Second Gulf War-like intervention, the leader of the country about to be intervened on would simply have to solemnly declare that his country was not going to lay any mines in a given area and if any were found, they would have been laid by his enemies trying to discredit him. At the same time his two Type 209 submarines are seen surreptitiously leaving port at night with their external minelaying cradles in place. Any statements that no mines have been found are met with a smug grin and "No comment". The result would have to be a massive on-going mine clearance operation that could not be stopped (in spite of its negative results) and would represent a significant diversion of very scarce assets away from mine clearance operations in other sectors. Current efforts to counter dummies and decoys work on the thesis that improving the discrimination of the classification sonar used to identify the mine-like object is the best route. This means that a higher frequency sonar is required. A related approach uses a broad-band sonar to determine whether the mine is a dummy (the exact technique is classified). However, higher frequencies have shorter ranges in water. This means the ROV carrying the sonar will have to approach closer to the suspected mine, making them more vulnerable to booby traps.
Booby trapping
Another profitable line of attack is to attempt the destruction of the unmanned underwater vehicles (UUVs) or divers used by the minehunters to inspect and destroy the targets. Each minehunter usually carries two remote observation vehicles (ROVs) or UUVs with the total number held in inventory being small. It is very unlikely that more than a 100% excess of stock over deployed units is held. Therefore, if the skilled divers or their mechanical substitutes can be killed, the minehunting effort will be very severely impeded.
Booby trapping can be carried out in many ways; the installation of anti-handling devices on the mines to kill divers that attempt to close them, the location of small anti-diver charges around mines and rigged with various fuzing options or the development of mines specifically intended to detect and kill the remote-controlled submersibles. These developments could make the exchange rate for mines swept as opposed to ROVs destroyed unacceptable. A research program, launched by the British Defense Research Agency, has attacked this problem two ways. One is to produce a low-observables ROV in which all metallic structures are removed and all machinery is sound rafted. The other is to play with the emissions of the classification sonar so the ROV can stand off at a safe distance and simulate an approach. The exact methodology is classified but a calculated guess would suggest that the volume is increased at the appropriate rate while a doppler component is added.
Overall
The really unpleasant thing about bottom mines is that they are easy and safe to lay in comparison with moored mines. Their provision of delay fuzes mean they can be safely handled on board unspecialized platforms by unskilled crews. Effectively, the minelaying procedure is simply sailing to a specified point and dropping the packages over the side. Once in the water, they will create chaos out of all proportion to their numbers and actual effect. This means that bottom mines are ideal for covert laying and thus for the offensive use of minefields. They can be delivered by submarines (in place of torpedoes), by aircraft (especially if the destructor concept has been adopted) or by converted ship. The technique could be as simple as shipping mines to supporters in a hostile country (by Federal Express or DHL for example) who would take them to the nearest bridge over the river to be mined (or out on a ferry) and heave them over the side.
.
by Stuart Slade
Mine Warfare
The most important point to start with is that the destruction of ships, whether civilian or military, is not the primary purpose of a minefield. Mines are intended to prevent the use of, or passage through an area of sea. Mine countermeasures are intended to permit the exploitation of an area of sea or allow safe passage through such an area. Following from these concepts, it is most effective to define offensive mine warfare as the laying of minefields and the maintenance of same while defensive mine warfare is definable as the destruction (or neutralization) of existing minefields and preventing their replenishment or creation. Both offensive and defensive mine warfare can form part of both offensive and defensive operations undertaken by the armed forces as a whole. Under such circumstances, the offensive mine warfare forces always have the initiative and the mine countermeasures groups will always be forced into a position where they are responding to those initiatives.
Asset protection
Historically, the use of minefields to impede and defeat an enemy attack is the oldest application of the technique. This was first attempted, on a systematic basis, by Confederate forces during the American Civil War. In the absence of significant naval forces and with severe limitations on their ability to construct conventional coastal defenses, mines offered the Confederacy the only possible way of defending critical ports and coastal cities. They were aided in these endeavors by the absence of any effective mine countermeasures technology on the Union side.
Minefields are extensively used to protect ports and other installations from intrusion by sealing off all but a few, very secret, passages. The same technique is used to defend coastal shipping lanes by laying comprehensive minefields to seaward of the route. With the development of mine countermeasures, such defenses are only effective when used in combination with other coastal defenses. Thus, to defeat such defenses, a balanced naval task force has to be committed. The mine warfare ships themselves must be extremely capable and ships other than mine countermeasures vessels have to be risked in order to protect them
In order for such defenses to be effective, the minefields need to be dense and extensive. Any attack is likely to be determined and its threat axis cannot be reliably predicted by the defenses. As a result, the numbers of mines required is immense. The sheer effort required to lay mines in this magnitude cannot possibly go un-noticed by the enemy so the areas mined in this manner are relatively well known and defined. It would be possible to use guile and deception to blur this perception but the results would not be worth the effort. The minefields also need to be continually maintained and supported. This suggests that attacking and destroying the minelaying vessels may be as valuable a counter as attempting to clear the mines.
Interdiction
In mine warfare terms this involves the obstruction of shipping lanes and transit points by well-placed minefields intended to restrict the ability of hostile forces to mount attacks from the sea, to prevent the infiltration of submarines into critical areas or to inhibit the free flow of commerce. Interdiction differs from asset protection in that the mines are not laid in close proximity to own installations and may be far removed from support by friendly forces. Indeed, in many cases of interdiction mining, the minefields will be laid off enemy ports or assets. In effect, the use of interdiction rather asset protection strategy turns many of the mine warfare problems on their heads.
The problem distills to getting the mines to their target. Since the area to be infested is usually covered by enemy defenses, the effort involved must be covert. This prevents the use of the huge numbers of mines used for asset protection. Yet the density of the minefield must be maintained in order to be effective; the implication is that the minefields resulting are very small and must be placed with the greatest of care. Asset protection minefields can be used with great effect here. If the minefield can be mapped, then the interdiction fields can be placed in the safe channels. This raises the delicious prospect of the presence of the hostile minefield not being suspected and the ship losses attributed to mines drifting from the defensive fields or being incompetently laid.
From the mine countermeasures point of view, the problems posed by interdiction minefields are also turned on their head. The threat now is small, dense fields that crop up in unexpected places without prior warning. Once such a minefield is discovered, the port or other installation in question is undeniably closed until the mines have been swept. The mine warfare ships are not at serious risk, except, of course, from the mines themselves. The problem distills to the massive effort required for routine surveillance at every port, on all shipping routes, all of the time. The Second World War solution of using converted fishing trawlers is no longer viable due to the efficiency of modern mines. Yet constructing dedicated mine warfare ships in sufficient numbers is quite impossible. Combining numbers, availability, crew considerations and financial constraints is a serious challenge to naval ingenuity.
Attrition
Interdiction is effectively mining somebody or something out of a given area; attrition can be defined as the art of mining high value assets into a selected area. Usually the effort is aimed at preventing a particularly crucial naval unit (typically an aircraft carrier or a ballistic missile submarine) from getting to sea and deploying. In a major conflict, attrition mining can be a crucial part of the naval balance of power - the consequences of destroying even a single SSBN or aircraft carrier (in both military and political terms) are incalculable. Usually attrition is achieved by selecting a suitable choke point through which the asset in question must pass and then mining it into oblivion.
In essence, countering attrition mining is a simple task. It is necessary to carve a single path, once, through the suspected choke point. The problem is that since the target is, by definition, of the highest value, the mines laid will be of the most sophisticated and dangerous types. The enemy will also make every effort to lay them in the largest possible quantities. The situation is complicated by the fact that the mine countermeasures ships cannot afford to make a single mistake (the commander of a minehunter responsible for mine clearance when, for example, the USS Abraham Lincoln hits one is unlikely to have much in the way of career prospects afterwards). To make matters worse, getting such assets to sea is often time-critical. At its most extreme, this could be to get the ship or submarine out before its base is incinerated. In such cases, low value ships are likely to be used as mine bumpers, accepting their loss to save the crucial unit.
The fact that the primary examples of attrition mining countermeasures have quoted aircraft carriers and SSBNs as the primary assets to be protected does not mean that this task is restricted to superpower navies. Once the overall size of the fleet decreases, smaller assets become equally important. In a war between two Newly Emerging Maritime Powers (NEMPs), a single frigate or amphibious warfare ship can be a crucial element in the naval balance of power. For example, many nations have a submarine fleet of two or three boats. If they cannot get out of port, all the investment placed in acquiring, supporting and maintaining that force have been wasted.
Terrorism
At first sight, the terrorist use of mines can be considered a sub-set of interdiction, yet the reality is that the problems and demands of countering terrorist mining are in a class by themselves. A terrorist "minefield" can consist of as little as a single covertly-placed mine. The objective is not so much the destruction of the ship that touches off the mine but the economic damage caused by disruption to trade, the financial penalties resulting from increased shipping insurance rates and extended journey times and the political impact of "demonstrating" the inability of a government to defend its citizens from outrage. The intended result is to cause pressure on the Government from the public and the business community to change policy in the direction desired by the terrorists.
Countering terrorist mining is very difficult. Essentially it involves proving that no mines are present to a business community that has nothing to gain and everything to loose by accepting the validity of any proof offered. They are quite right to adopt such a position; the fact that a given area is mine-free at 8 pm one evening does not mean it will be mine-free at 8 am the next morning. This situation boils down to a command and control problem; it is necessary to be able to demonstrate, quickly and unobtrusively that nothing has appeared on the seabed or, if something has appeared, it can be investigated and, if necessary, neutralized.
Force multiplication
A further use of minefields is to accentuate the effectiveness of other weapons and to provide a suitable environment for their use rather than as a primary weapon. This may be achieved by using minefields to channel enemy shipping into selected killing grounds or to restrict their maneuverability and thus enhance their vulnerability. A good example was the Silkworm attack on the USS Missouri during the Second Gulf War. Here, the Iraqis had placed minefields offshore of a coastal defense missile battery with the intention that either the Coalition warships would hit mines while maneuvering to avoid the attack or remain on their restricted course and get hit. This plan was defeated by the combination of mine avoidance sonars and integrated command systems on the escorting British destroyer HMS Gloucester. Other potential uses of force multiplier minefields include decoying submarines into attacks on "high value targets" (actually dummies surrounded by mines) or herding surface ships into positions suitable for submarine ambushes. Another, and very important, role is to draw scarce and irreplaceable mine warfare ships into positions where they (and their highly trained crews) can be destroyed. The only real counter to these strategies is the use of mine detection equipment in combination with fully integrated command systems to create a multi-dimensional tactical picture.
Riverine Mine Warfare
The subject of mine warfare carried out within the environment of inland waterways (including canals as well as rivers) is often neglected. Yet such waterways often represent a major means of commerce movement, and in many Asian countries, are the primary means of communication in rural areas. Such mining activities may also affect food production in, for example rice growing areas.
The environment of rivers is not an easy one for either offensive or defensive mine warfare operations. The combination of fast-flowing waters and silt-covered beds makes conventional mining activities very difficult. Usually resort is made to floating mines which are dropped in upstream of the target and allowed to run down into the target. This is actually an offense against international law but that has never appeared to stop anybody. Other techniques include off-route mines, rocket launchers, fougasses or claymore mines which are installed on the river or canal banks and detonated when the target passes. These are all, basically, terrorist techniques and are not usually part of operations carried out by regular military forces (more for want of opportunity than any reticence over the tactics).
Mine countermeasures in such environments is eased by the absence of acoustic or pressure mines; such would be detonated by the water and flow conditions. Since most craft will be built of wood and many will be oar- or sail-powered, magnetic mines are not that great a threat. Floating mines can usually be spotted visually. However, contact and off-route mines are very serious problems indeed and the latter are almost as insoluble a problem as pressure mines are on the high seas. The major difficulty is that the riverine environment forces the adoption of very small craft which have correspondingly limited capability. They are vulnerable, subject to intense threat, not necessarily mine related, and liable to substantial operational attrition.
The most attractive solution for this environment is the use of remote-controlled craft which can be used to sweep ahead of manned vessels. The Chinese have designed the Type 312 minesweeper drone for this role and have sold eight to Thailand for the same purpose. The problem with them is that the command link is vulnerable and this can result in serious operational unpredictability.
[The Thailand Army no longer uses the Type 312 as drones. Instead, they now stretch a oil-drum supported net across the river above the bridge to be protected. This then traps any mine floating down the river.]
Mine Categories
There are two generalized categories of mines, moored mines and ground mines. Moored mines float at a given depth and are held in place by an anchor. These are by far the least sophisticated and expensive form of naval mines. The problem with these weapons is that they cause less-destructive contact explosions rather than devastating under-the-keel hits. Ground mines detect the target by the acoustic, magnetic, or pressure signatures of a vessel, or a combination of these signatures (this detection method has caused these weapons to be called influence mines). After the target has been detected and is within range, the mine explodes at a set distance from the target. This ensures that the explosion is of the under-the-keel variety and maximizes damage.
Moored Mines
Contact mines
Contact mines are detonated when the ship strikes the mine. This bends horns on the outside of the mine, causing glass cylinders of acid inside the horns to break. This acid then ignites the detonator, either directly or by acting as an electrolyte for a battery. Other types of contact mines have used inertia switched but these proved to be very vulnerable to premature explosions. Contact mines have been fitted with many ingenious anti-sweep devices including explosive charges to cut sweeping wires and ratchet devices that enable a sweep wire to pass through the mooring cable without cutting it. Contact mines are practically restricted to the anti-ship role. Since the hull of the target actually has to touch the mine, using contact mines for ASW means that mines have to be set at all the depths a submarine is likely to adopt - requiring huge numbers
Antenna mines
The desirability of exploding a charge under a ship rather than in contact it has long been recognized. The first practical attempt at this was by the US in 1919. The antenna mine is moored on a short cable so it is a set distance under the surface. A long copper wire stretched upwards, terminating in a float. If any steel object touches the wire, anywhere along its length, an electrical potential is generated which detonates the mine. Antenna mines are particularly valuable in anti-submarine work since they can be set to sit deep in the water with their detonation antennas terminating a set depth under the surface. Thus, surface ships can sail over the wires in relative safety but submarines are in mortal danger.
String mines
The problem with antenna mines is that the explosive charge has to be within about 100 feet of the hull of the submarine if significant damage is to be achieved. The chance of achieving this was greatly increased by the introduction of string mines. These feature tiers of charges which may be either contact or antenna fused. The explosion of one mine necessarily means the discharge of all due to sympathetic detonation. The whole assembly is incredibly clumsy, looks rather like a perverted Christmas tree and can effectively block water up to 800 feet deep. It is, therefore, exclusively an asset protection system but one which is very effective. A slightly cleaned up version, in which the tiers of charges are replaced by much smaller single ring charges at 4 meter intervals has been marketed by the Italian Whitehead Motofides Company.
Overall
Moored mines are by far the most common in the world's mine warstocks and any minefield encountered will probably contain mostly contact mines of varying types. The manufacture of such mines or their equivalents, is a worldwide industry. The going cost for a new contact mine is around US$5,000. It is therefore impossible to ignore these weapons or to dismiss them as obsolescent. The use of moored mines is nowhere near as easy as it appears. The option of air-dropping moored mines is also now also virtually extinct since few aircraft have the weight-lifting capability and capacious, airborne-accessible bomb bays required for the role. Those nations that have such aircraft have better things to do with them. All these considerations mean that laying moored minefields is likely to be undertaken only by converted surface ships. If these can be identified and terminated with extreme violence, the threat from contact mines will be greatly alleviated. This is basically a C3I function and one which needs to be integrated with other mine warfare operations. Inexperienced construction of moored mines can also be dangerous. The Iraqis decided to "improve" the Pattern 1908 mine by increasing the explosive charge. In order to preserve buoyancy, a lighter cable was substituted for the heavier chain used in the Russian original. As a result, the inertia of the heavier mine in sea swell caused the less robust cable to break and resulted in the mines drifting out of control.
A very effective technique is visual observation; lookouts in the bows saved a British destroyer and two frigates from hitting floating mines in the Second Gulf War. The mines were then destroyed by 20 mm fire. A much more sophisticated variant of the same idea is the mine avoidance sonar. Effectively, this is a high-frequency imaging sonar scanning ahead and below the ship. It does not need the very high resolution of the mine warfare classification sonars since the identity of the mine is not required; just the data necessary to keep well away from it. Another essential requirement is the provision of parvanes. These prevent a contact mine being drawn against the hull of a ship by the suction generated as a result of its motion through the water. It is this suction that gives moored mines their lethal radius.
Bottom Mines
Magnetic Mines
Magnetic mines were developed by the British who wanted a mine which sat on the bottom and was, therefore, limited to relatively shallow water. The original magnetic mines built by the British used the vertical component of the ship's magnetic field to trigger the mine when a given field density was reached. Mines of this type were first used in 1918 off the Belgian coast as anti-submarine devices and were used in the anti-ship role of Russia in 1919. Examples were captured by the Russians at that time, found their way to Germany which copied them. The small number of magnetic mines laid by Germany in 1939 virtually brought the infested ports to a standstill until sweeping techniques could be developed. By this time, the British had developed a superior derivative of their 1919 mines which worked off the horizontal component of the ship's magnetic field. This apparently insignificant change made it possible to design a mine fuze which responded to the rate of change of field strength rather than absolute field strength. This made defense against magnetic mines by degaussing and magnetic sweeping procedures far less effective. Later, the mines were further improved by introducing double-tap fuzing. In this case the mine would be activated by increasing magnetic field strength but only detonated by decreasing field strength. Thus the mine would explode as the ship passed, under the screws rather than under the bows. The combination of horizontal-component and double-tap fuzing was an order of magnitude more damaging and was also much more difficult to simulate when sweeping. It made the whole generation of Second World War built minesweepers obsolete. Magnetic bottom mines were probably the last naval mines which could be built simply, inexpensively and in very large quantities by unskilled labor. The going cost for a magnetic mine is around US$10,000.
Defenses against the magnetic mine revolve around reducing the magnetic signature of the targets as much as possible. This predicates avoiding the use of steel components wherever practical. Hulls have to be made of non-magnetic material and fittings, such as anchor chains and windlasses fabricated from bronze. All low magnetic materials are extremely expensive, yet the use of steel in some areas remains unavoidable. Thus degaussing is still essential to correct the resulting magnetic field. An often-neglected aspect of this problem is that drastically reduced magnetic signatures have a disturbing tendency to increase unpredictably. This is partly the result of natural processes and partly the effect of apparently minor modifications to the ship..
Acoustic Mines
Acoustic mines were first introduced by Germany in 1940 and exploit the principle that all ships and submarines have a specific acoustic signature. This is generated by the vessel's machinery, the design of the hull, the propellers, and many other factors. Delay clocks can be included to leave the mine inert (and thus unsweepable) for up to twelve days after it has been laid while the incorporation of counters means that the mine will only be detonated after a certain number of impulses (usually up to 16) have been received. This means a sweeper would have to make a large number of passes over a suspected field before it could be sure that all the mines within the field had been exploded.
Acoustic mines can be set to work in either broadband or narrow band modes. The original broadband mines were those introduced by Germany and work on the integrated volume of noise emissions across all frequencies. As such, the fuzing system is relatively simple (all that could be achieved by the electronics technology available at the time) and mines of this type can be swept using relatively simple noise generators. Mines of this type are very widespread and are produced extensively. They do, however, require a degree of electronic sophistication in the fuzing making them more expensive and time-consuming to produce. Such mines are available for about US$25,000.
During the 1970s and 1980s, advancing electronics technology made it possible to increase the degree of electronic sophistication that could be packaged into small units. This had a major influence on all aspects of military technology and made the development of relatively intelligent weapons possible. In mine warfare, the result was the development of narrow-band signals processing software for mine fuzes. These revolutionized the prospects for acoustic mines and have had a major effect on mine countermeasures technology.
Narrow band noise processing exploits distinctive frequencies centered around specific shipboard activities. These may be the thumping of a diesel, the cavitation caused by screws in water or the characteristic whine of a gas turbine. Flow noise is generally not used since it is insufficiently precise for the sort of fuzing here in use. The implications of narrow-band processing are that the sound profiles used are so precise that the mine can be programmed to listen for specific types of ships or powerplants and ignore others. One Russian narrow band acoustic mine, for example, can be set to listen specifically for the LM-2500 gas turbine and ignore other powerplants.
This makes sweeping exceptionally difficult. In effect, the sweep devices have to emulate the sound profile of a target in order to detonate the mine. The processing capability of modern mines means that the match has to be fairly exact since errors will be detected and used to filter out the sweep. Such acoustic sweeps are in service but their use faces many problems. One is that the sweep may not be simulating the intended target; or may be simulating the wrong aspects (or combination or aspects) for the mine in question. Thus many sweeps will be necessary to cover the available aspect combinations (and then repeated up to 16 time search to allow for counting devices in the fuze). The resulting time consumption is so high that the development of narrow band acoustic fuzes for mines is widely believed to have rendered sweeping techniques ineffective when faced with this threat.
The good news is that these mines are very expensive, are very difficult to make and require great hydro-acoustic expertise if the fuzing system is to work. Such mines are usually found only in the inventories of major powers such as the USA, Russia, France etc. Even where mines of this type are exported, their fuzes are much simplified. For example, the Italians produce the MR-80 narrow-band acoustic mine for their own and NATO use. An export customer can buy an MR-80 (as did the Iraqis) but the version delivered will be the export-only MRP (which the Iraqis got but still labeled MR-80!) which has a broad band acoustic fuze. Few export customers have the sophistication to know the difference. The Russians have the same policy, openly calling the simplified mines "monkey models". Non-traditional mine producers (for example, Iraq, Iran, North Korea and Chile) have all tried to produce narrow band acoustic mines and failed. Chile tried to duplicate a British Stonefish mine in the mid-1980s and failed. They got around the problem by duplicating the outside casing (including markings) and general appearance of Stonefish while releasing data sheets that duplicated GEC-Marconi's corporate style. They also hinted that they had received a secret license to produce Stonefish. In fact, the Chilean mine had a simple broad band fuze.
The cost of acoustic mines differs dramatically depending on the capability of the fuzing system. Since these mines are produced in much smaller quantities than their simpler cousins, the operations of economy of scale are less pronounced, again increasing unit cost. As a result, acoustic mines are priced between US$50,000 and US$150,000.
Magnetic-Acoustic Mines
A vulnerability noted with acoustic mines is that the fuzing mechanism must be powered-up all the time the mine is active. Running the fuze powered-up requires electrical power, albeit in small quantities. The provision of such electrical power is restricted to banks of batteries which must be limited in volume. This limits the life of the mine to that of the battery power provided, typically as little as twelve days. Since the mines cannot be recovered and replenished, it is obvious that minefields consisting entirely of acoustic mines will be short-lived. Another potential problem is that any electrical system that runs constantly emits electrical energy that can be detected. There have been frequent claims that mine hunting systems have been devised which can use this emitted energy to locate and classify mines, the methodology does seem theoretically feasible; this does not mean it is practically realistic.
These problems are overcome by a magnetic-acoustic fuzing system. In this arrangement a very simple magnetic switch (comparable to those used in the original magnetic mines) is used as an initiator. This needs not be sophisticated or complex since it does not directly detonate the mine. What it does do is activate the acoustic portion of the fuze. If this determines the target is worth engaging, it will do so, otherwise it automatically turns off after a few minutes. These systems have many advantages. The very simple magnetic switch does not require energy so has almost infinite life. The acoustic fuze is switched off most of the time, (thus conserving battery power and limiting any suggestion of detectability). Even more useful is the fact that any sweep must not only combine magnetic and acoustic generators but must do so in carefully integrated and carefully phased proportions and sequences. In effect the combination of magnetic and acoustic fuzing not only integrates the benefits of both, it does so with a synergistic effect that adds an order of magnitude to the lethality of the system and the problems involved in countering it.
Pressure Mines
Pressure mines are invariably bottom mines since they measure the absolute drop in pressure associated with the difference between the known pressure due to water depth and the depth of water under the hull of a passing ship. This differential is directly related to the depth at which the mine is situated, the ratio involved being roughly proportional to the square root of the pressure drop. In other words, if the depth at which the mine is located is increased by a factor of 1.414, the baseline pressure drop required to detonate that mine must be doubled. In addition there is a necessary minimum below which a pressure drop will not explode the mine; this is an absolute requirement since wave motion would otherwise cause sufficient pressure drops to explode the mines. For these reasons, pressure mines are inevitably shallow-water inshore weapons. As with acoustic mines, pressure mines have selective time delay fuzes, letting the mine become active after a certain day and, if no targets have passed over it after a certain period, making it inoperative again. They also have similar multiple target ship count devices allows a predetermined number of ships to pass before detonation.
Pressure mines are completely unsweepeable by any known means. A variety of experimental methods have been tried including the use of mine bumpers. These were adopted in 1945 by the US Navy to clear pressure mines dropped off the Japanese coast by the US Air Force. These mine bumpers consisted of old freighters with their holds filled with sealed drums (to preserve buoyancy) and with all ship functions automated so that the crew could concentrate above decks. In fact, the ships were run entirely from the bridge where the crew sat of 12 layers of conventional bedding mattresses. These precautions proved inadequate since the mine explosions disabled the bumpers and the crew were seriously hindered (though not actually injured) by the shock waves. Pressure mines do have serious operational limitations. Since the pressure drop required to detonate them is directly related to the draft of the ship in question, shallow draft vessels can traverse most pressure minefields with impunity. Shallow draft implies restrained displacement and this consideration is the reason why the maximum size of NATO coastal mine warfare ships is restricted to 800 tons (and means that the US Avenger class minehunters should never go near a suspected pressure minefield). Air cushion craft should be able to traverse a pressure minefield in relative safety. On the other hand, many types of amphibious warfare craft (for example the AAV-7) have high pressure signatures and are very vulnerable indeed to such mines.
These characteristics mean that pressure mines are usually regarded as inshore defense and, specifically, anti-amphibious operation, weapons. In these roles, they would be located in such shallow water (even surf zone) that even minute pressure signatures would detonate them. In such operations, even the minimum pressure levels set by wave action are low since deep rollers will be attenuated as they come inshore. Thus, pressure fuzing has been adopted for an entire generation of small anti-invasion mines, specifically intended to destroy amphibious craft. Much of the current US mine warfare effort is being diverted to counter this problem. Pressure mines are available at costs between US$25,000 and US$50,000.
Rising Mines
This type of mine is particularly useful in either very deep water, which could not otherwise be mined, or where the seabed is soft and glutinous. The rising mine lies on or under the sea floor. It is normally equipped with a passive acoustic sensor to listen for a ship or submarine to pass within range. When contact is made, it switches to an active mode and jettisons ballast to change its buoyancy from negative to positive. This causes it to float up and explode at the appropriate moment. These weapons are particularly dangerous since no reliable means has yet been developed for detecting buried mines. A much more dangerous version of the rising mine fires a projectile rather than just floating upwards A rising mine's case depth is probably fixed by the crush depths of the mechanism and the projectile. In really deep water, then, the mine must be moored, and its case must be buoyant enough to support a considerable weight of cable. Consequently, the mine must be large, and the laying rate will be limited. In shallower water, down to a thousand feet or more, the mine case might well lie on the bottom, unsweepable by mechanical means (i.e., by any device that would cut the mooring cable).
The effect of the existence of deep-water rising mines is to extend the minable area of the world significantly. Countermeasures are very expensive because the mine may cover a substantial lethal area (the projectile may maneuver to one side or the other) and because any attempt to destroy mines or mine mooring requires very deep operations. Although not technically rising mines (the projectile shoots sideways rather than up), the same basic philosophy has been adopted for a range of surf-zone anti-invasion mines. These use similar acoustic sensors to detect landing craft and amphibious armored vehicles approaching and then fire their projectiles at them. The Italians released a mine of this type in the late 1980s while the Russians have offered the KPM anti-invasion mine which uses the same principle. These are particularly serious deterrents to an invasion since their large target acquisition area means that the conventional anti-mine tactics of clearing narrow paths to the beach are no longer adequate - the anti-invasion mines would cover such paths from the flanks.
Dummies
Not every mine-like object is a mine. An object on the seabed could be domestic rubbish, sunken buoys, oddly-shaped rocks, debris for sunken ships, or even a mine. The problem in clearing them is the very large numbers of mine-like objects that have to be checked and, if dangerous, eliminated. A skilled mine warfare team (which the Iraqis were not) will lay thousands of cheap dummies for every genuine mine and bring the mine clearance process to a congested halt. It is very easy indeed to make a cheap plastic imitation of a mine casing, paint it with the correct markings and fill it with concrete. The estimated cost is around US$10.00. Each has to be treated as a live mine until proven otherwise. The reverse is also true, of course. The installation of modern and very dangerous fuzing packages in apparently simple, old contact mines comes under this heading.
The ultimate in dummy mines are the mines that are not there. For example, in a future Second Gulf War-like intervention, the leader of the country about to be intervened on would simply have to solemnly declare that his country was not going to lay any mines in a given area and if any were found, they would have been laid by his enemies trying to discredit him. At the same time his two Type 209 submarines are seen surreptitiously leaving port at night with their external minelaying cradles in place. Any statements that no mines have been found are met with a smug grin and "No comment". The result would have to be a massive on-going mine clearance operation that could not be stopped (in spite of its negative results) and would represent a significant diversion of very scarce assets away from mine clearance operations in other sectors. Current efforts to counter dummies and decoys work on the thesis that improving the discrimination of the classification sonar used to identify the mine-like object is the best route. This means that a higher frequency sonar is required. A related approach uses a broad-band sonar to determine whether the mine is a dummy (the exact technique is classified). However, higher frequencies have shorter ranges in water. This means the ROV carrying the sonar will have to approach closer to the suspected mine, making them more vulnerable to booby traps.
Booby trapping
Another profitable line of attack is to attempt the destruction of the unmanned underwater vehicles (UUVs) or divers used by the minehunters to inspect and destroy the targets. Each minehunter usually carries two remote observation vehicles (ROVs) or UUVs with the total number held in inventory being small. It is very unlikely that more than a 100% excess of stock over deployed units is held. Therefore, if the skilled divers or their mechanical substitutes can be killed, the minehunting effort will be very severely impeded.
Booby trapping can be carried out in many ways; the installation of anti-handling devices on the mines to kill divers that attempt to close them, the location of small anti-diver charges around mines and rigged with various fuzing options or the development of mines specifically intended to detect and kill the remote-controlled submersibles. These developments could make the exchange rate for mines swept as opposed to ROVs destroyed unacceptable. A research program, launched by the British Defense Research Agency, has attacked this problem two ways. One is to produce a low-observables ROV in which all metallic structures are removed and all machinery is sound rafted. The other is to play with the emissions of the classification sonar so the ROV can stand off at a safe distance and simulate an approach. The exact methodology is classified but a calculated guess would suggest that the volume is increased at the appropriate rate while a doppler component is added.
Overall
The really unpleasant thing about bottom mines is that they are easy and safe to lay in comparison with moored mines. Their provision of delay fuzes mean they can be safely handled on board unspecialized platforms by unskilled crews. Effectively, the minelaying procedure is simply sailing to a specified point and dropping the packages over the side. Once in the water, they will create chaos out of all proportion to their numbers and actual effect. This means that bottom mines are ideal for covert laying and thus for the offensive use of minefields. They can be delivered by submarines (in place of torpedoes), by aircraft (especially if the destructor concept has been adopted) or by converted ship. The technique could be as simple as shipping mines to supporters in a hostile country (by Federal Express or DHL for example) who would take them to the nearest bridge over the river to be mined (or out on a ferry) and heave them over the side.
.
