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Geophysical intelligence is a branch of Measurement and Signature Intelligence (MASINT) that involves phenomena transmitted through the earth (ground, water, atmosphere) and manmade structures including emitted or reflected sounds, pressure waves, vibrations, and magnetic field or ionosphere disturbances. ."

As with many branches of MASINT, specific techniques may overlap with the six major conceptual disciplines of MASINT defined by the Center for MASINT Studies and Research . This taxonomy breaks MASINT into:
 * Electro-optical
 * Nuclear radiation
 * Geophysical
 * radar
 * radiofrequency ,
 * materials

Acoustic Intelligence
This includes the collection of passive or active emitted or reflected sounds, pressure waves or vibrations in the atmosphere (ACOUSTINT) or in the water (ACINT). Hydrophones and sonar, in antisubmarine warfare, are well known, but less well known is the often clandestine process of measuring the signatures of other countries' ships and submarines. It is even possible to use passive sonar to detect aircraft flying low over the sea.

Historically, acoustic sensors were first used in the air, as with artillery ranging.

For surface and subsurface MASINT, the US Navy operates the Integrated Undersea Surveillance System (IUSS) with multiple subsystems in the Fixed Surveillance System (FSS, also known as SOSUS), Fixed Distributed System (FDS), and the Advanced Deployable System (ADS or SURTASS).

Artillery Ranging
One of the first applications of acoustic and optical MASINT was locating enemy artillery by the sound and flash of their firing, a technique pioneered by Canadian Forces under Gen. Arthur Currie, with Andrew McNaughton in a key staff role. The combination of sound ranging (i.e., acoustic MASINT) and flash ranging (i.e., before modern optoelectronics) gave information unprecedented for the time, in both accuracy and timeliness. Enemy gun positions were located within 25 to 100 yards, with the information coming in three minutes or less. Today's latest counterbattery radars can give the position within 15-30 seconds.



In the "Sound Ranging" graphic, the Listening Post, which is well forward of the microphone stations, sends an electrical signal to the microphone stations (MS) when the LP operator hears the gun's sound at time T0. Either manually or electrically, each MSx sends a starting pulse to an oscillograph. When the MS operator hears the sound, he stops the signal sent to the oscillograph. The oscillograph operator can then compute a time of arrival Ax, which is the difference between the T0 and TMx. Without computer assistance, the range had to be computed manually.

The positions of the microphone stations and listening posts are precisely known. Each Ax can be graphed as a hyperbola. Where the asympotes of the hyperbola meet is the position at which the gun is assumed to be located.

Where sound ranging is a time-of-arrival technique not dissimilar to that of modern multistatic sensors, flash ranging used theodolites to take bearings on the flash from the presurveyed flash observation post. The location of the gun was determined goniometrically, where the bearings intersected. Flash ranging, today, would be called electro-optical MASINT.

Artillery sound and flash ranging remained in use through WWII and into the postwar years, until mobile counterbattery radar, itself a MASINT radar sensor, became available. These techniques anticipated, and then paralleled, radio direction finding in SIGINT, which first was goniometric and now, with the precision time synchronization from GPS, is often time-of-arrival.

If the observation was at night, the Canadian master gunner could compare the sound and flash, while only sound was available in daylight. The Canadian units still had to estimate wind, temperature, and barometric pressure on the trajectory to the German artillery. and manually -- and quickly -- compute firing orders. Optimal times were on the order of three minutes.

Artillery positions now are located primarily with specialized radar, such as the US AN/TPQ-37, as well as IMINT. SIGINT also may give clues to positions, both with COMINT for firing orders, and ELINT for such things as weather radar.

Hydrophones
Modern hydrophones convert sound to electrical energy, which then can undergo additional signal processing, or that can be transmitted immediately to a receiving station. They may be directional or omnidirectional.

Sea-Based
Navies a variety of acoustic systems, especially passive, in antisubmarine warfare, both tactical and strategic. For tactical use, passive hydrophones, both on ships and airdropped sonobuoys, are used extensively in antisubmarine warfare. They can detect targets far further away than with active sonar, but generally will not have the precision location of active sonar, approximating it with a technique called Target Motion Analysis (TMA). Passive sonar does have the advantage of not revealing the position of the sensor.

Passive sonobuoys, such as the AN/SSQ 53F, are directional and can be set to sink to a specific depth. These would be dropped from helicopters and maritime patrol aircraft such as the P-3.

The US installed massive SOSUS hydrophone arrays on the ocean floor, to track Soviet and other submarines. Later, with SOSUS put into standby, "tuna boat" sensing vessels called SURTASS used longer, more sensitive towed passive acoustic arrays than could be deployed from maneuvering vessels, such as submarines and destroyers. SURTASS is now being complemented by Low Frequency Active (LFA) sonar; see the sonar section.

US submarines made extensive clandestine patrols to measure the signatures of Soviet submarines and surface vessels. This acoustic MASINT mission included both routine patrols of attack submarines, and submarines sent to capture the signature of a specific vessel. US antisubmarine technicians on air, surface, and subsurface platforms had extensive libraries of vessel acoustic signatures.

Land-based
Vietnam-era acoustic MASINT sensors included " Acoubuoy (36 inches long, 26 pounds) floated down by camouflaged parachute and caught in the trees, where it hung to listen. The Spikebuoy (66 inches long, 40 pounds) planted itself in the ground like a lawn dart. Only the antenna, which looked like the stalks of weeds, was left showing above ground."

Part of the AN/GSQ-187 Improved Remote Battlefield Sensor System (I-REMBASS) is a passive acoustic sensor, which, with other MASINT sensors, detects vehichles and personnel on a battlefield. Passive acoustic sensors provide additional measurements that can be compared with signatures, and used to complement other sensors.

For example, a ground search radar may not be able to differentiate between a tank and a truck moving at the same speed. Adding acoustic information, however, may quickly distinguish between them.



Sonar
Combatant vessels, of course, made extensive use of active sonar, which is yet another acoustic MASINT sensor.

Surface Combatant
A modern Arleigh Burke class destroyer carries active and passive acoustic systems including


 * AN/SQQ-89(V)10 Surface ASW Combat System:* AN/SQS-53-C(V)4 sonar detecting and ranging set w/Kingfishers sonar dome rubber window.
 * AN/UQN-4A sonar sounding set.
 * AN/BQH-7A(V)2 bathythermograph set.
 * AN/UYQ-25B(V) Sonar in situ Mode Assessment System (SIMAS).

Air-Dropped Active Sonobuoys
Active sonobuoys, containing a sonar transmitter and receiver, can be dropped from fixed-wing maritime patrol aircraft (e.g., P-3, Nimrod, Chinese Y-8, Russian and Indian Bear ASW variants), antisubmarine helicopters, and carrier-based antisubmarine aircraft (e.g., S-3). While there have been some efforts to use other aircraft simply as carriers of sonobuoys, the general assumption is that the sonobuoy-carrying aircraft can issue commands to the sonobuoys and receive, and to some extent process, their signals.

The Directional Hydrophone Command Activated Sonobuoy system (DICASS) both generate sound and listen for it. A typical modern active sonobuoy, such as the AN/SSQ 963D, generates multiple acoustic frequencies. Other active sonobuoys, such as the AN/SSQ 110B, generate small explosions as acoustic energy sources.

Dipping Sonar
Antisubmarine helicopters can carry a "dipping" sonar head at the end of a cable, which the helicopter can raise from or lower into the water. The helicopter would typically dip the sonar when trying to localize a target submarine, usually in cooperation with other ASW platforms or with sonobuoys. Typically, the helicopter would raise the head after dropping an ASW weapon, to avoid damaging the sensitive receiver. Not all variants of the same basic helicopter, even assigned to ASW, carry dipping sonar; some may trade the weight of the sonar for more sonobuoy or weapon capacity.

The EH101 helicopter, used by a number of nations, has a variety of dipping sonars. The (British) Royal Navy version has Ferranti/Thomson-CSF sonar, while the Italian version uses the HELRAS.

Russian Ka-25 helicopters carry dipping sonar, as does the A current US system is on the LAMPS SH-60 helicopter, which carries a "dipping" AQS-13F sonar, plus AN/SQQ-28(V)10 sonar signal processing for active sonobuoys it drops.

Surveillance Vessel
Newer Low-Frequency Active (LFA) systems are controversial, as their very high sound pressures may be hazardous to whales and other marine life. A decision has been made to employ LFA on SURTASS vessels, after an environmental impact statement that indicated, if LFA is used with decreased power levels in certain high-risk areas for marine life, it would be safe when employed from a moving ship. The ship motion, and the variability of the LFA signal, would limit the exposure to individual sea animals. LFA operates in the low-frequency (LF) acoustic band of 100-500 Hz. It has an active component, the LFA proper, and the passive SURTASS hydrophone array. "The active component of the system, LFA, is a set of 18 LF acoustic transmitting source elements (called projectors) suspended by cable from underneath an oceanographic surveillance vessel, such as the Research Vessel (R/V) Cory Chouest, USNS Impeccable (T-AGOS 23), and the Victorious class (TAGOS 19 class).

"The source level of an individual projector is 215 dB. These projectors produce the active sonar signal or “ping.” A "ping," or transmission, can last between 6 and 100 seconds. The time between transmissions is typically 6 to 15 minutes with an average transmission of 60 seconds. Average duty cycle (ratio of sound “on” time to total time) is less than 20 percent. The typical duty cycle, based on historical LFA operational parameters (2003 to 2007), is normally 7.5 to 10 percent."

This signal "...is not a continuous tone, but rather a transmission of waveforms that vary in frequency and duration. The duration of each continuous frequency sound transmission is normally 10 seconds or less. The signals are loud at the source, but levels diminish rapidly over the first kilometer."

Submarine
sonar (bow, flank, active intercept, and towed array sonar)

Acoustic sensing of large explosions
An assortment of time-synchronized sensors can characterize conventional or nuclear explosions. One pilot study, the Active Radio Interferometer for Explosion Surveillance (ARIES). This technique implements an operational system for monitoring ionospheric pressure waves resulting from surface or atmospheric nuclear or chemical explosives. Explosions produce pressure waves that can be detected by measuring phase variations between signals generated by ground stations along two different paths to a satellite. This is a very modernized version, on a larger scale, of WWI sound ranging.

As can many sensors, ARIES can be used for additional purposes. Collaborations are being pursued with the Space Forecast Center to use ARIES data for total electron content measures on a global scale, and with the meteorology/global environment community to monitor global climate change (via tropospheric water vapor content measurements), and by the general ionospheric physics community to study travelling ionospheric disturbances.

Sensors relatively close to a nuclear event, or a high-explosive test simulating a nuclear event, can detect, using acoustic methods, the pressure produced by the blast. These include infrasound microbarographs (acoustic pressure sensors) that detect very low-frequency sound waves in the atmosphere produced by natural and man-made events.

Closely related to the microbarographs, but detecting pressure waves in water, are hydro-acoustic sensors, both underwater microphones and specialized seismic sensors that detect the motion of islands.

Temperature
Antisubmarine aircraft, ships, and submarines can release independent sensors that measure the water temperature at various depths. The water temperature is critically important in acoustic detections, as changes in water temperature at thermoclines can act as a "barrier" or "layer" to acoustic propagation. To hunt a submarine, which is aware of water temperature, the hunter must drop acoustic sensors below the thermocline.

Seismic Intelligence
defines seismic intelligence as "The passive collection and measurement of seismic waves or vibrations in the earth surface." One strategic application of seismic intelligence makes use of the science of seismology to locate and characterize nuclear testing, especially underground testing. Seismic sensors also can characterize large conventional explosions that are used in testing the high-explosive components of nuclear weapons. Seismic intelligence also can help locate such things as large underground construction projects.

For nuclear test detection, seismic intelligence is limited by the "threshold principle" coined in 1960 by George Kistiakowsky, which recognized that while detection technology would continue to improve, there would be a threshold below which small explosions could not be detected. .

The most common sensor in the Vietnam-era "McNamara Line" of remote sensors was the ADSID (Air-Delivered Seismic Intrusion Detector) sensed earth motion to detect people and vehicles. It resembled the Spikebuoy, except it was smaller and lighter (31 inches long, 25 pounds). The challenge for the seismic sensors (and for the analysts) was not so much in detecting the people and the trucks as it was in separating out the false alarms generated by wind, thunder, rain, earth tremors, and animals—especially frogs."

Magnetic MASINT
Magnetometers detect changes in the earth's magnetic field, which can be die o

A magmetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument.

Magnetometers are used in geophysical surveys to find deposits of iron because they can measure the magnetic field variations caused by the deposits. Magnetometers are also used to detect archaeological sites, shipwrecks and other buried or submerged objects. Magnetic anomaly detectors detect submarines for military purposes.

A magnetometer can also be used by satellites like GOES to measure both the magnitude and direction of the earth's magnetic field.

Magnetometers are very sensitive, and can give an indication of possible auroral activity before one can see the light from the aurora. A grid of magnetometers around the world constantly measures the effect of the solar wind on the earth's magnetic field.

Magnetometers can be divided into two basic types:
 * scalar magnetometers measure the total strength of the magnetic field to which they are subjected, and
 * vector magnetometers have the capability to measure the component of the magnetic field in a particular direction.

The use of three orthogonal vector magnetometers allows the magnetic field strength, inclination and declination to be uniquely defined. Examples of vector magnetometers are fluxgates and superconducting quantum interference devices, or SQUIDs. Some scalar magnetometers are discussed below.

A magnetograph is a special magnetometer that continuously records data.

Earth's magnetism varies from place to place and differences in the Earth's magnetic field (the magnetosphere) can be caused by two things:

1. The differing nature of rocks 2. The interaction between charged particles from the sun and the magnetosphere

Metal detectors use electromagnetic induction to detect metal. Uses include de-mining (the detection of land mines), the detection of weapons such as knives and guns, especially at airports, geophysical prospecting, archaeology and treasure hunting. Metal detectors are also used to detect foreign bodies in food, and in the construction industry to detect steel reinforcing bars in concrete and pipes and wires buried in walls and floors.

In its simplest form, a metal detector consists of an oscillator producing an alternating current that passes through a coil producing an alternating magnetic field. If a piece of electrically conductive metal is close to the coil, eddy currents will be induced in the metal, and this produces an alternating magnetic field of its own. If another coil is used to measure the magnetic field (acting as a magnetometer), the change in the magnetic field due to the metallic object can be detected.

Modern developments
The modern development of the metal detector began in the 1930s. Gerhard Fisher had developed a system of radio direction-finding, which was to be used for accurate navigation. The system worked extremely well, but Fisher noticed that there were anomalies in areas where the terrain contained ore-bearing rocks. He reasoned that if a radio beam could be distorted by metal, then it should be possible to design a machine which would detect metal, using a search coil resonating at a radio frequency.

Technological changes were taking place at a rapid rate too, and very few of the smaller companies managed to stay in competition with the big outfits. GOLDAK, METROTECH, IGWT, TEC, and, quite recently, ARADO ceased production of hobby machines. Some devotees of metal detecting still treasure their Arado machines, which had a reputation for being difficult to set up, but were reputed to be the deepest-seeking hobby detectors ever made. The biggest technical change in detectors was the development of the induction-balance system, where two coils are set up in an electrical equilibrium to produce a 'null' or zero balance. Introducing metal to the vicinity of the coils caused them to unbalance, producing a change of tone in the machine's speaker. Scientists had long known that every metal has a specific response to stimulation by alternating current. Each metal produces a time lag or 'phase angle' in its induced current, in relation to the drive current. This meant that detectors could now be set up to ignore unwanted phase angles, and respond positively only to desired metals. But there was also a downside to the development of the 'discriminator' detectors. Introducing discrimination always had the effect of reducing the sensitivity of the machine, so it was less able to find deep objects. In addition, there was the fact that some desirable metals were quite near the area of unwanted metals, such as iron. Gold, particularly in alloy form, was quite close to tinfoil in the overall spectrum, so the discrimination control had to be used carefully. The price to be paid for setting up a detector to ignore iron and tinfoil was the possibility that, sooner or later, the user would scan over, and ignore, a valuable find - perhaps a diamond engagement ring on a beach.

New coil designs
Coil designers also tried out innovative designs. The original Induction Balance coil system consisted of two identical coils placed on top of one another. Compass Electronics produced a new design; the two coils were made in a D shape, and were mounted back-to-back to form a circle. This system was widely used in the 1970s, and both concentric and D type (or Widescan as they became known) had their fans. Another development was the invention of detectors which could cancel out the effect of mineralization in the ground. This gave greater depth, but was a non-discriminate mode. It worked best at lower frequencies than those used before, and frequencies of 3 to 20 kHz were found to produce the best results. Many detectors in the 1970s had a switch which enabled the user to switch between the discriminate mode and the non-discriminate mode. Later developments switched electronically between both modes. The development of the Induction Balance detector would ultimately result in the Motion detector, which constantly checked and balanced the background mineralization.

Pulse induction
At the same time, developers were looking at using a completely different type of technology in metal detectors. This was the process known as Pulse Induction. Unlike the Beat Frequency Oscillator or the Induction Balance machines which both used a uniform alternating current at a low radio frequency, the pulse induction machine simply fired a high-voltage pulse of signal into the ground. In the absence of metal, the 'spike' decayed at a uniform rate, and the time it took to fall to zero volts could be accurately measured. However, if metal was present when the machine fired, a small current would flow in the metal, and the time for the voltage to drop to zero would be increased. These time differences were minute, but the improvement in electronics made it possible to measure them accurately and identify the presence of metal at a reasonable distance. These new machines had one major advantage: they were completely impervious to the effects of mineralization, and rings and other jewelery could now be located even under highly-mineralized 'black sand'. They had one major disadvantage too: there was no way to incorporate discrimination into a Pulse induction detector. At least, that was the perceived wisdom of scientists and engineers until Eric Foster, who had run Location Technology in Ireland for many years, started a new company in Britain and produced the Goldscan, the first Pulse Induction detector which had the apparent ability to differentiate between metals. This was a new type of 'junk eliminator' circuit, which relied on the size of the target as well as its metallic response to give a control that would show positive for a gold ring and negative for a copper coin. Its ability to differentiate between non-ferrous metals was not an exact science, but gave unparalleled depth on mineralized soil or sand. Pulse Induction detectors are now widely used in the construction industry; the Whites PI-150 is an industrial machine which can detect large objects to 10 feet, using a 12 or 15 inch coil.

--Future detectors
Modern top models are fully computerized, using microchip technology to allow the user to set sensitivity, discrimination, track speed, threshold volume, notch filters, etc., and hold these parameters in memory for future use. Compared to just a decade ago, detectors are lighter, deeper-seeking, use less battery power, and discriminate better.

New genres of metal detector have made their appearance. BB (Beat Balance) and CCO (Coil Coupled Operation) were unveiled by the electronics press in 2004. Both were invented by electronics writer and designer Thomas Scarborough and combine unprecedented simplicity with good sensitivity.*

MAD
A magnetic anomaly detector (MAD) is an instrument used to detect minute variations in the Earth's magnetic field. The term refers specifically to magnetometers used either by military forces to detect submarines (a mass of ferromagnetic material creates a detectable disturbance in the magnetic field)Magnetic anomaly detectors were first employed to detect submarines during World War II. MAD gear was used by both Japanese and U.S. anti-submarine forces, either towed by ship or mounted in aircraft to detect shallow submerged enemy submarines. After the war, the U.S. Navy continued to develop MAD gear as a parallel development with sonar detection technologies.

Operation
To reduce interference from electrical equipment or metal in the fuselage of the aircraft, the MAD sensor is placed at the end of a boom or a towed aerodynamic device. Even so, the submarine must be very near the aircraft's position and close to the sea surface for detection of the change or anomaly. The detection range is normally related to the distance between the sensor and the submarine. The size of the submarine and its hull composition determine the detection range. MAD devices are usually mounted on aircraft.

Function
There is some misunderstanding of the mechanism of detection of submarines in water using the MAD boom system. Magnetic moment displacement is ostensibly the main disturbance, yet submarines are detectable even when oriented parallel to the earth's magnetic field, despite construction with non-ferromagnetic hulls. For example, the Soviet-Russian Alfa class submarine, whose hull is constructed out of titanium to give dramatic submerged performance and protection from detection by MAD sensors, is still detectable.

The Alfa's detectability has led some analysts to deduce that its name is an intentional deception, so effective that the Soviet Union decided to construct the Alfa and even consider building the Typhoon class submarine SSBN out of titanium at one point. Since titanium structures are detectable, MAD sensors do not directly detect deviations in the earth's magnetic field. Instead, they may be described as long-range electric and electromagnetic field detector arrays of great sensitivity.

An electric field is set up in conductors experiencing a variation in physical environmental conditions, providing that they are contiguous and possess sufficient mass. Particularly in submarine hulls, there is a measurable temperature difference between the bottom and top of the hull producing a related salinity difference, as salinity is affected by temperature of water. The difference in salinity creates an electric potential across the hull. An electric current then flows through the hull, between the laminae of sea-water separated by depth and temperature.

The resulting dynamic electric field produces an electromagnetic field of its own, and thus even a titanium hull will be detectable on a MAD scope, as will a surface ship for the same reason.

Vehicle Detectors
The Remotely Emplaced Battlefield Surveillance System (REMBASS) is a US Army program for detecting the presence, speed, and direction of a ferrous object, such as a tank. Coupled with acoustic sensors that recognize the sound signature of a tank, it could offer high accuracy. It also collects weather information.

The Army's AN/GSQ-187 Improved Remote Battlefield Sensor System (I-REMBASS) includes both magnetic-only and combined passive infrared/magnetic intrusion detectors. The DT-561/GSQ hand emplaced MAG "sensor detects vehicles (tracked or wheeled) and personnel carrying ferrous metal. It also provides information on which to base a count of objects passing through its detection zone and reports their direction of travel relative to its location. The monitor uses two different (MAG and IR) sensors and their identification codes to determine direction of travel.

Demining
Landmines often contain enough ferrous metal to be detectable with appropriate magnetic sensors.

Manual detection with a metal detector


The first step in manual demining is to scan the area with metal detectors, which are sensitive enough to pick up most mines but which also yield about one thousand false positives for every mine, and cannot detect landmines with very low metal content. Areas where metal is detected are carefully probed to determine if a mine is present, and must continue until the object that set off the metal detector is found.

Technologies that improve safety include large, pillow-like pads strapped to the bottoms of shoes that distribute weight and dull the impact of footsteps, as very slight disturbances of the ground can tip off old, unstable, or intentionally sensitive mine triggers. Demining can be safer than construction work if procedures are followed rigorously.

For a variety of reasons, it may be advisable to have the mine detector on a remotely operated ground vehicle.