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By contrast, passive geoprospecting instruments operate on the basis of the principle that the continually varying density of the earth's crust alters the earth's magnetic field in measurable and predictable ways. The strength of the earth's gravitational field at any point on the surface of the earth depends on the density of the rock beneath the surface, which changes as one moves across the surface of the earth. This physical law enables the development of gravity sensing instruments, which are commonly used by geologists to measure the force of gravity over many small sections of land.23 The measurements that are obtained are compiled into a gravity map for an area, which are regularly used by the oil, gas, and mining industries to indicate the presence of hydrocarbon or mineral deposits below the surface of the earth.
In the same way, gravity mapping can be used to indicate the existence of cavities or deeply buried facilities. But, defining the exact location of an underground facility is not a simple matter.
Gravity Field Mapping
Given the different instruments that are available for seeing under the surface of the earth, the instruments most widely used for depths greater than 20 meters are those that are based on sensing the force of gravity. Gravity surveys, which are traditionally used for detecting salt domes and cavities in bedrock, are time consuming and overt activities. The gravimeter is moved meter by meter to presurveyed "benchmarks" to precisely measure the gravity vector (i.e., force and direction of gravity) As the force of gravity changes with changes in the density of subterranean features, one looks for the characteristic alterations in the gravity field that result from variations in the density (or absence) of material underneath the surface of the earth.24 Small intervals between measurements are necessary to accurately define the edges of cavities that may exist underground. The microgravity engineering and archaeological surveys that are traditionally used for detecting cavities in bedrock often involve taking measurements at intervals of one meter. But the presence of soft soil under the gravimeter or the effect of wind blowing on the instrument can adversely affect the gravity reading and therefore skew the results. Furthermore, precise measurements of altitude (within 10 cm) and latitude (within 30 meters) are required for accurate results.25 In fact, obtaining accurate altitude and latitude measurements is currently the most difficult and time-consuming aspect of conducting gravity field surveys.
While gravimeters can measure the gravitational force at discrete points, this technology requires highly accurate, three-dimensional prospecting at each point prior to measurement. Fortunately, GPS has vastly simplified this operation. Modern gravimeters can make highly accurate gravity measurements at each station in less than half-an-hour, but are impractical for detecting underground facilities under the pressure of time that would exist in military contingencies.
Another instrument that is used in virtually all commercial and military aircraft, as well as intercontinental ballistic missiles, is the inertial measurement unit, or IMU. In comparison with a gravimeter, inertial measurement units measure changes in acceleration due to movement and reduce the movements to a calculation of its three-dimensional location in space. What would be most useful to prospectors would be to integrate the gravimeter and the IMU into a single instrument that accurately measures changes in gravitational fields while dynamically moving over the surface of the earth. This technology, which is known as a gradiometer, is being improved and miniaturized for mining and prospecting applications, and has operational benefits for military contingencies.
Gradiometers have been used in the US Navy's submarine fleet to stealthily detect underwater obstacles without having to visually sight them, and without having to emit an audible sonar "ping" that reveals the location of a submarine. Gradiometers can dynamically measure extremely small changes in the gravity gradient as the instrument passes over the surface of the earth or near objects.
The concept of gravity gradients is central to understanding gradiometers. As described earlier, while conventional gravimeters measure the overall force of gravity at a given point on the surface of the earth, gradiometers are comprised of up to six pairs of identical sensors (called accelerometers) in an instrument that takes twelve separate measurements of gravity at any given time.26 Each paired set of accelerometers is separated by a small gap between the two sensors.
For an aircraft flying over land whose subsurface consists of both low density and high density rocks, the gravity gradient immediately indicates the presence of less dense rock (or cavities) that lie underneath the surface of the earth, or the increase in force of gravity due to the presence of a large landform, such as a mountain.27 The ability to use gradiometers as a part of airborne surveys has significant military implications for locating deeply buried facilities.28 In the past, airborne surveys lacked detail, principally because of the limitations associated with the sensing equipment on aircraft Airborne surveys, rather than detailed mapping, were used to determine the gross features of the gravity field over wide areas. The subtle gravity perturbations that are produced by buried facilities would have been missed by a quick overflight of gravity sensing instruments because the instrument is unable to produce sufficiently accurate data, process that information quickly, and sense the micro-perturbations in gravity over the ever-changing subterranean density of the earth. However, with the increasing miniaturization of electronics and sensors, gradiometers can be used for the explicit purpose of detecting deeply buried facilities.
A concept for locating underground facilities involves the integration of a gradiometer, GPS receiver, and the ability to transmit raw gradiometer data to an airborne platform, such as an uninhabited aerial vehicle (UAV). In this case, a UAV could be programmed to survey an area and transmit the results in real-time. The ability to program a UAV to autonomously accomplish such a mission, from take off to landing, is feasible and in fact serves as the fundamental concept for the Air Force's Global Hawk UAV. Depending on the location of a buried facility and the threat posed by it, one could fly small remotely piloted vehicles (RPVs) over the suspected area.
The various RPVs of differing sizes and payload capacities that exist on the commercial market for prospecting and surveying could be modified for military operations. For example, the Sensoar RPV, which is manufactured by Remote Sensing Research, is a slow flying, radio controlled, gas or electric powered aircraft.29 With a wingspan of 12 feet and weight of about 12 pounds, it is capable of taking low-altitude, high resolution photographs, or operating at altitudes greater than 10,000 feet. Its 4-pound payload capacity includes a GPS receiver and camera However, in place of the camera, a properly sized gradiometer could be integrated with GPS to perform airborne gravity surveys above areas that are suspected of containing deeply buried facilities.
Satellite Intelligence, Surveillance, and Reconnaissance
As the capabilities and flexibility of satellites for gathering accurate and highly detailed intelligence information continue to increase, their role is becoming more central to intelligence operations, including the ability to detect deeply buried facilities. For example, reconnaissance satellites use an array of high resolution imaging and sensors, such as the Landsat's multispectral scanner, to provide clues about the existence of Underground facilities and their activities. This relies on infrared, thermal, and multispectral imaging of the surrounding land and the facility.
Furthermore, reconnaissance satellites can be used to estimate what is being produced at a particular site based on the size of storage tanks, number of rail cars, size of the roads, and other external features. Landsat's thermal imagery can detect, in sections of land that are the size of a front lawn, vent duct arrays or the heat generated by underground facilities if they are close enough to the surface. Its blue-band filter can also detect the smoke and gases that are emitted from underground vents. While underground facilities are difficult to locate, roads or tracks leading into the side of a mountain or disappearing underground often help to reveal their location. Furthermore, commercial firms have developed the software that detects changes between images that are generated over time, which is known as change detection software. Acquiring images of the same terrain over a period of time is a common way for using satellites to monitor activities and changes in areas where deeply buried facilities are suspected to exist.
Human Intelligence, Surveillance, and Reconnaissance
Human sources of information will remain central to the ability to locate deeply buried facilities. The information that is obtained from defectors, covert agents, photographs, documents, and soil samples, among other types of knowledge about a facility, help military planners to deal with these targets.
Not surprisingly, it is quite difficult to find deeply buried facilities, and to complicate matters this is an area in which experience and technology are not fully developed.30 However, by using an integrated combination of geoprospecting instruments, satellites, and human intelligence reports, it is likely that one can determine where deeply buried facilities are located. There are numerous signs of the existence of underground facilities, including gravity perturbations, the presence of ventilation shafts, electrical power lines (above or below ground), water and sewage hookups, and emergency exits Satellite imaging with a variety of sensors can indicate the presence and location of underground facilities, and human intelligence can help to locate these facilities. If one uses a broad array of sources, these facilities can be found, but characterizing the shape, depth, and mission of an underground facility may be more difficult than locating it.31 While this discussion highlights the value of gravity sensing instruments, it is essential to develop a comprehensive approach for integrating all resources in order to produce reasonable estimates about the location of underground facilities.
IV. Neutralizing Deeply Buried Facilities
For the reasons outlined in this study, it is difficult to locate and neutralize deeply buried facilities, especially when one must consider the fact that these facilities may contain nuclear, chemical, or biological agents whose destruction might inadvertently release dangerous substances into the atmosphere. This condition would endanger friendly forces and noncombatants alike, and create the possibility of regional disasters. An underground facility also may contain important military and governmental assets that have value for subsequent exploitation, non-proliferation purposes, or intelligence analysis. A further complication is that an underground facility may be located in an urban area, perhaps under a school or hospital or surrounded by a neighborhood in which the danger of collateral damage precludes the use of conventional or nuclear weapons. For this reason, specialized personnel who are properly equipped to neutralize a facility may be the best option. In any case, there is no guarantee that neutralizing a deeply buried facility will be an antiseptic operation because it could easily be costly in terms of lives and equipment for both sides.
The concept of neutralization includes the full range of "kill" levels that are necessary to accomplish the objectives of the mission. Those objectives may be to recover weapons of mass destruction or hostages from an underground facility, disable biological weapon manufacturing equipment, or completely destroy a command and control center. There will be cases when the United States will want to destroy or disrupt deep underground facilities that are heavily guarded, largely invulnerable, and possibly located in urban areas.