The mine’s firing mechanism or target detecting device (TDD) may incorporate one or more of the following influence sensors depending on its type: magnetic, pressure, acoustic, or seismic.  A firing mechanism or TDD analyzes stimuli external to the mine to determine whether or not a valid “target” (surface vessel or submarine) generated them.  If this analysis shows the target to be valid and is operating within the mine’s damage zone, the firing mechanism or TDD initiates a sequence that detonates the mine.  Hence, under the proper conditions, the proximity of a target to a mine is all that is necessary to detonate it. 

It is important to note that a firing mechanism or TDD may not use a single type of influence sensor, but rather a number of them functioning in a complementary fashion.  This provides a diversified mix that increases the mine’s detection capability and accuracy.  It is quite possible that natural influences such as earth tremors, marine life, or severe sea states could trigger one of the sensors, but the absence of an output signal from the other sensor(s) under these conditions would inhibit a detonation. 

The firing mechanism or TDD is programmed to understand that a valid target produces multiple stimuli as it passes within range of the submerged mine and it requires the presence of more than one sensor output to command a detonation.  When used, multiple influence sensors also make defeating the mine much more difficult for the enemy if it chooses to do so.  The four classes of influence sensors utilized by firing mechanisms or TDDs are briefly explained in the following paragraphs.  The incorporation of a firing mechanism or TDD, and the types of influence sensors used by our current Naval mines, are included in the descriptive data for each mine in Section 8 of this publication.

Firing mechanisms or TDDs employ relatively small, versatile lightweight magnetometers as their magnetic influence sensors and use computerized solid-state circuitry to process received magnetic target data.  Magnetometers are three-axis (total field) detectors, as opposed to the earlier mine search coils that sensed magnetic fields along only a single axis.

As the firing mechanism or TDD functions within the mine, sensed magnetic data are converted into small electrical pulses that are amplified and analyzed by pre-programmed circuitry.  When this analysis determines that the magnetic inputs are from a valid target and that this target is operating close enough to inflict damage, the firing mechanism or TDD initiates detonation of the mine.  In this manner, the target is either damaged or sunk.

Pressure sensors are electro-hydraulic devices that convert negative pressures into electrical signals that are then analyzed and processed by the TDD.  In performing this function, the pressure influence sensor identifies a decrease in pressure caused by a moving ship as it displaces a given volume of water beneath its hull throughout the course of its passage over the submerged mine.  This phenomenon is an example of Bernoulli’s Principle.  It should be noted that ocean swells and tidal activity may also be sensed and processed by the pressure sensor.  However, since the magnitude of the signals produced by these natural disturbances is small, the TDD is programmed to ignore them.  If analysis determines that the strength and duration of a given negative pressure signal meets the programmed requirements for a valid target, the TDD initiates detonation of the mine

  Utilizing a hydrophone, acoustic sensors convert underwater sounds from targets, such as propeller beat and cavitation, propulsion machinery noises, and hull vibrations into electrical signals for processing and analysis by the mine’s firing mechanism.  If the acoustic inputs meet the pre-programmed requirements for a valid target, the firing mechanism initiates detonation of the mine.  In performing its function, the firing mechanism incorporates the ability to reject underwater sounds not produced by valid targets such as marine life or countermining explosions.  In either of these instances, the mine will not respond since these sounds either build up too slowly or develop too quickly.  In short, firing mechanisms respond only to sounds that are within a given frequency band as well as those that increase in intensity at a prescribed rate of change.

Seismic sensors are closely related to acoustic sensors in that they utilize the same acoustic stimuli in order to perform their function.  Sound waves are actually slight changes in pressure that travel quite readily over great distances in water.  The hydrophone in an acoustic sensor functions much the same as a human ear drum, vibrating a diaphragm in response to the “acoustic” pressure pulses and creating electrical outputs for the TDD to process.  A seismic sensor is designed to detect case movements as the mine firmly rests on the sea bottom.  The very same acoustic stimuli that vibrate the hydrophone’s diaphragm in the acoustic sensor also actually vibrate the mine’s outer case.  The extremely sensitive seismic sensor within the mine is designed to detect these small case movements.  If these seismic inputs meet the pre‑programmed requirements for a valid target, the TDD initiates detonation of the mine.  It’s important to note that the seismic sensor does not have to be presented to the water outside the mine as does the acoustic hydrophone.  This allows for a much greater freedom during the design of the mine.