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The first thing to understand is that there is no such thing as a UXO detector - instead there are anomaly detectors. This is an important distinction - if you understand this you have an appreciation of the technical challenges and difficulties associated with UXO detection. Below are descriptions of several technologies that are often used to detect buried UXO. These technologies include: magnetometers, electromagnetic induction (EM), ground penetrating radar (GPR), infrared detectors, natural detectors, and trace gas analysis .
Magnetometers are instruments designed to measure the minute magnetic fields associated with ferrous materials. In general there are two types of magnetometers - vector and total field. Vector magnetometers include fluxgate and SQUIDs measure a single component of the field. Total field magnetometers include proton precision and cesium vapor sensors, which measure the magnitude of the magnetic field but not its direction. Magnetometers used in UXO detection usually have sensitivities of less than one nano Tesla (nT). A ferrous object in an ambient field has associated with it a magnetic dipole moment, which in general may be composed of a permanent moment and an induced moment. In the case of UXO the induced moment is proportional to the ambient earth's magnetic field assuming that the ambient field is spatially and temporally constant during the transverse of the magnetometers. A typical ambient field at northern latitudes is around 60,000 nT so even small fractional variations can strongly influence a survey and the ability to detect anomalies. During a magnetometer survey geophysics often use a base or reference station to correct for fluctuation in the ambient field over the survey time. The base station and survey data are usually related by time tags for comparison and correcting.
Magnetometers are common anomaly detectors used in UXO surveys because many of the UXO contain a considerable amount of ferrous material. Magnetometers can be used to determine the approximate location of a ferrous anomaly by the field of its dipole moment. The size of an object can also be determined from the size and orientation of the induced moment.
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Electromagnetic Induction (EM) works by exposing an object to a time-varying magnetic field and detecting the secondary magnetic field produced by the eddy currents induced in the object. The magnetic field is generated and detected using coils of wire. Existing EM systems are essentially of two types - continuous wave and transient. The continuous wave method uses a continuous wave form to generate the primary field and the secondary field, which is superimposed on the primary, is detected by electronically canceling out the primary field component at the receiver. In the transient design, a pulsed magnetic field is used as the primary and the secondary field is detected after the primary has sufficiently decayed. In principle by employing either system the same type of information can be obtained. The choice of one system over the other is based upon the operational considerations and ease of interpretation in a particular application. In the case of UXO detection the transient design is usually favored because it does not require the critical balancing of the primary and secondary fields.
EM sensors are commercially available and are commonly used in UXO detection work because they have the ability to detect both ferrous and non-ferrous targets. Commercial systems available on the market today come in wheeled or towed versions and smaller handheld designs.
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Ground penetrating radar (GPR) is used to locate anomalies or voids under the soil. GPR surveys are done by towing a transducer or antenna of an appropriate wavelength by hand, or vehicle over an area of interest. The GPR antennas work as transmitters and receivers with commercial antennas ranging in frequencies of about 50 to 1000 Mhz. These antennas emit pulses of electromagnetic (EM) energy through the soil which traverse the ground at sub-light velocities such that ample time is available for transmitted pulses to be reflected back and processed before another pulse is emitted. As the transmitted pulse moves from one medium into another the pulse slows, depending on the dielectric constant of the material, and the wavelength decreases. In addition the contrasting electrical properties of the two mediums also cause the EM energy waves it be reflected, scattered or diffracted which affects the return signal of the radar. The variations in return signals are feed into a computer and analyzed and are used to determine the presence of anomalies. Radar waves can generally resolve objects on the order of one-half a wavelength.
GPR is not ideal for all types of soils, in order to determine if GPR is a viable technology one must understand how the properties of the soil affect the performance of the radar waves. Radar works best passing through low conductivity materials such as sand, dry granites and limestones. Clays with high conductivity are hard on radar waves, and the longest wavelength antennas can only get down 0.5 meters or so in wet clays. Water has a dielectric constant of 81 and radically alters the velocity of the radar-wave traveling through materials and can cause serious errors in depth estimating. In comparison saturated quartz sands have a dielectric constant of up to 30.
A traditional GPR system consists of a source generator (pulse), transmitter antenna (dipole), matched receiver antenna, fast analogue to digital converter (ADC) and computer to record and display data. The antennas are mounted on a carrier or cart and separated by a small distance. The antennas are usually dragged across the ground instead of being elevated on a wheeled cart because air and soil have different dielectric properties that would effect the energy waves.
Note: Dielectric constant is a measure of the capacity of a material to store a charge when an electric field is applied. |
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IR sensor technologies can be used to identify objects by measuring their thermal energy signatures. UXO on or near the soil surface may possess a different heat capacity or heat transfer properties than the surrounding soil, and this temperature difference can theoretically be detected and used to identify UXO. For IR sensor technologies to produce results useful for detecting UXO, a sharp thermal contrast must exist between the UXO and its surroundings (usually the soil surface). IR sensor technology results also depend on the type and density of vegetation present, weather conditions, time of day (thermal loading and gradient), and specific size and properties of the UXO. In practice, IR sensor technologies can only detect UXO located on an unvegetated soil surface.
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Yes - Believe it or not Honey Bees have been evaluated by scientists to see if they could be trained to detect landmines or shallow buried UXO by detecting trace amounts of explosives. A team of scientists from the University of Montana at Missoula and Sandia National Laboratory in Albuquerque, N.M have studied the potential.
The theory is based on the ability of bees to pick up dust and airborne chemicals on their bodies. The theory is that trace levels of TNT, an explosive used in some landmines, can be found in the air around the buried weapons. TNT may also be absorbed from the soil into the pollen of flowering plants and picked up by the bees as they search for nectar. The bees can then be screened for explosive chemicals on their return to the hive. The scientists are also training bees to sniff out TNT chemicals by teaching them to link the smell of explosives with the smell of sugary substances.
One question that usually comes to mind is how would track the bees location to back track that information into a location of a landmine or UXO. Believe it or not researchers at the Natural Resources Institute at University of Greenwich in England have developed tiny antennas to attach to bees so that they can be tracked by radar. These antenna can track the range and direction of a bee every three seconds.
The use of bees has not been perfected for UXO detection but maybe we will see such radical technologies in the future - in the mean time don't throw that magnetometer out just yet.
CaninesCanines have been used in explosives detection work for years and have proven to be very effective. Canines have been studied over the years for use in landmine and UXO detection but are not deployed in the field during UXO remediation efforts.
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In trace gas analysis vapors emanating from munitions are sensed above ground and are separated into their various molecular and ionic constituents for identification. Since the materials used in manufacture vary dramatically across ordnance design most trace gas analysis sensors try to detect the explosive vapor emanating from the UXO as opposed to trying to detect the plastic casing in the case of mines. The problem facing trace gas analysis is that most explosives have low vapor pressures which makes it difficult to detect not to mention the soil that covers or buries the item. The most sensitive trace gas technologies include atmospheric pressure chemical ionization mass spectrometry (CIMS), plasma chromatography, gas chromatography with electron capture detectors, opto-acoustic absorption and multi-path laser absorption. A detailed discussion of these technologies is beyond the scope of this section. Trace gas analysis is usually left for research and development efforts in UXO detection and is not used in current UXO clearance efforts.
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