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Transient electromagnetic exploration techniques: can they be applied to the landmine discrimination problem?
African Earth Sciences 33 (2001) 693–698 www.elsevier.com/locate/jafrearsci
Transient electromagnetic exploration techniques: can they be applied to the landmine discrimination problem? M. Combrinck Department of Earth Sciences, University of Pretoria, Pretoria 0002, South Africa Received 31 March 2001; accepted 9 July 2001
Abstract The use of metal detectors in the humanitarian demining industry closely resembles the use of electromagnetic techniques in the mineral exploration industry to find conductive orebodies. An overview is given for the landmine problem and some of the current technologies used in the demining process. The similarities of metal detection to geophysical transient electromagnetic data acquisition, processing and interpretation are used to suggest ways in which more information than just the ‘‘presence of metal’’ may be obtained when using pulse induction (PI) metal detectors. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Landmines; Mineral exploration industry; Metal detection
1. Introduction Despite efforts of nations around the world to reduce the total number of buried landmines, more landmines are deployed in armed conflict every year than are removed by mine clearance personnel. According to statistics laid before the US Congress by the Office of International Security and Peacekeeping Operations (OISPO, 1994), the world was littered with an estimated 80–110 million anti-personnel (AP) landmines in 64 countries, which maim or kill an estimated 500 people every week – mostly civilians. Angola and Mozambique are estimated to have more than one million landmines each. In addition to the lives that are lost due to explosions of abandoned landmines, the mere suspicion that landmines may be present in an area lays waste to large areas of land that could otherwise be used for mining, agriculture or social infrastructure. Removing landmines from a designated area to such an extent that civilian populations can continue their normal existence is known as humanitarian demining (OISPO, 1994). The most widely used standard for these operations is the United Nations standard of 99.6% clearance (Blagden, 1998). This is a very different problem compared to military countermine operations in which the object is mainly to
E-mail address: [email protected] (M. Combrinck).
detect and avoid mined areas during times of conflict (Trevelyan, 1998). The level of detection and discrimination in the humanitarian demining industry therefore has to be superior to that used in the military operations. When mines are used in proper military operations the positions of minefields and individual mines are mapped and the danger area is clearly marked. New developments also include self-deactivating and selfdestruct mines that can limit innocent lives to be claimed once conflict situations have been resolved. The greatest challenge for deminers occurs in countries which have been subject to years of guerilla warfare in which landmines have been planted indiscriminately (OISPO, 1994). This is the case in many African countries where independence and civil wars have been raging for decades.
2. Landmines and demining techniques There are many types of landmines developed in countries all over the world, consisting of different materials and activated in different ways. One can, however, distinguish between AP and anti-tank (AT) landmines based on their preferred application, destructive potential, approximate size, depth of burial and triggering mechanism. AP mines are approximately 7.5 cm in diameter and between 2.5 and 5 cm thick (Chignell, 1998), are buried at depths less than 7.6 cm
0899-5362/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 9 - 5 3 6 2 ( 0 1 ) 0 0 0 8 6 - 0
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(Amazeen and Locke, 1998), triggered by pressure, tripwire or command and are designed to kill or maim a person within a radius of up to 50 m (OISPO, 1994). AT mines are approximately 20–30 cm in diameter, buried at depths less than 20 cm (Amazeen and Locke, 1998) and are aimed at demobilizing armored vehicles. Landmines are traditionally constructed from or at least contain measurable quantities of metal, and metal detectors are used with great success (if not always speed) to find these. A more recent development is the minimal metal or plastic mines, e.g., the Chinese Type 72 AP, the PMA-2 and R1M1 mines (Joynt, 1998). In areas with high soil conductivity these mines cannot be detected reliably with metal detectors and have given rise to a number of new technological innovations that have shown different measures of success under different conditions. In considering humanitarian demining these plastic mines still constitute less than 5% of the total world occurrence (Joynt, 1998). In reality, the operational mine detection process has evolved very little in technological terms since World War II. Metal detection still forms the basis for clearing operations and works very well under most circumstances. There is, however, one very pronounced drawback and that is the number of false alarms generated by a metal detector sensitive enough to detect minimum metal mines. Every fuse-holder, spent case, nail, piece of shrapnel and any other conceivable little piece of metal then generates a signal on the metal detector and must be opened up with the same care as would be exercised for a landmine by the so-called ‘‘spade man’’ (Joynt, 1998). MECHEM is a military research company that undertakes basic development work for the South African National Defence Force and has designed vehicles that are protected against AP and AT blasts (Joynt, 1998). MECHEM started landmine clearing contracting in 1991 and adopted a combination of techniques to operate in typical field conditions. They combined mechanical mine clearance with manual clearance by men using metal detectors, as well as vapor detection and sniffer dogs to do area reduction (Joynt, 1998). The mechanical mine clearance technique employs a Casspir armoured vehicle fitted with steel wheels that is used to detonate landmines by driving over them and simultaneously clearing the way in densely vegetated areas. This works well in the case of AP mines but AT mines still cause damage to the vehicle that has to be repaired at considerable cost. Even though the demining process is sped up through the removal of vegetation and creation of ‘‘safe’’ tracks, the manual stage of detecting and lifting the remaining undetonated mines still has to be performed. The actual rate of progress (and cost of demining) is still determined by the ‘‘spade man’’, who is responsible for opening each detected signal (Joynt, 1998).
The above operational procedure is representative of the humanitarian demining industry and illustrates the improvements that need to be made, namely: • A method (or combination of methods) has to be found to discriminate between scrap metal and mines. • A practical way still has to be found to localize minimum metal/plastic mines under all soil conditions. • Being an industrial venture, new techniques should not raise the cost of demining above the current 0.4–0.6 US $ per square meter (Joynt, 1998) (which implies that techniques should not only be accurate, but also efficient). • The equipment must be able to withstand true field conditions and be simple enough to be operated and maintained by trained deminers.
3. ‘‘New’’ technologies to meet the challenges The humanitarian demining industry has led to numerous scientists, universities and other institutions becoming involved in the search for the ultimate landmine detector. A brief summary of techniques under investigation follows below, together with some advantages and disadvantages related to them. The review list has been compiled from the proceedings of the ‘‘Detection of abandoned land mines’’ conference held in Edinburgh (Scotland) in October 1998, and is taken to be representative of the different disciplines. Most of these methods are aimed at detecting non-metallic mines or at reducing the false alarm rate associated with metal detection. It is also a very common assumption that a combination of methods/instruments would be used in any specific operation to achieve the optimum results. 3.1. Radar/microwave surveys These methods help to define geometric shapes of buried objects based on their dielectric properties, operating at megahertz and gigahertz frequencies. They are very sensitive to noise and the depth of investigation is adversely affected by high soil conductivities. Research focuses on finding the optimum frequency bands, antenna configurations, image processing and filtering techniques to present the data in an unambiguous manner. Some very good results have been obtained in sand pits and controlled environments, depending on different systems. The United States Army’s handheld standoff mine detection system (HSTAMIDS) which is a combination of metal detector, infrared camera and ground penetrating radar, is a working field prototype and the results look very promising. Extensive field testing in different environments is the only way to know whether this system will achieve the required clearing standard at an economic rate of clearance.
M. Combrinck / African Earth Sciences 33 (2001) 693–698
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3.2. Infrared/thermal images
3.7. Seismo-acoustic techniques
This method is based on the temperature difference between soil and landmines and gives very good results for shallow landmines (<5 cm) in desert regions where the background temperature distribution is very homogeneous. It is important to remember that although AP mines are usually buried at depths of less than 7 cm, that may not be where they are found after 30 years of erosion and floods, as for example in Mozambique where severe recent floods have relocated and reburied many mines.
These techniques can either be used to image the subsurface, based on reflected waves (resolution and energy are limitations), or to analyze the frequency content of the reflected signals. The frequency content of the signals could be used to pick up the natural resonant frequencies of the landmines, provided that these frequencies are determined beforehand and accessible through a real-time database. The alternative to developing new technologies is to improve existing instruments and data analysis. In the remainder of this paper I will give a review on how pulse induction (PI) metal detection can be improved by analogy to the transient electromagnetic method used in geophysical exploration.
3.3. Nuclear quadrupole response and other nuclear techniques Nuclear techniques are very promising for reducing false alarm signals, because they detect the chemical elements used in explosives (most often focusing on the nitrogen content). The neutron–neutron detector is available as a hand-held system, but is also susceptible to false alarms created from wood and metal objects. The neutron–gamma device is too slow and bulky for continuous use, but boasts a 100% detection rate for explosives and no false alarms for metal and plastic mines. 3.4. Flail/mechanical devices Used to physically detonate or unearth the landmines (e.g., the modified Casspir used by MECHEM), these devices are very helpful, but can miss smaller mines and are vulnerable to larger ones. 3.5. X-ray backscatter This instrument consists of an X-ray source and receiver that senses photons that are reflected back from a landmine. The depth of investigation has been tested up to 10 cm. Unfortunately this system is also too bulky, at present, for hand-held operation. A big advantage in terms of cost and maintenance is that all the components used in these instruments already are commercially available. 3.6. Dogs Dogs smell explosives and can also locate tripwires, but due to the migration of the chemical elements through soil as time elapses, the mines cannot be pinpointed more accurately than 5–7 m. The dogs are most often used to reduce search areas, based on the absence of explosives, rather than in locating the landmines themselves.
4. Transient electromagnetic (TEM) techniques in the exploration industry: an aid to deminers? Except for the coincidence that both are looking for ‘‘mines’’, the geophysical mineral exploration and landmine demining industries have much in common. In both cases one of the major objectives is the ability to find and characterize hidden targets without physically penetrating the earth’s surface. This need for efficient gathering of subsurface information is addressed through the earth science discipline of geophysics in the mineral and oil exploration industries. There are just as many different techniques for finding significant geological targets as there are for finding landmines, and not surprisingly, many of them are based on the same scientific principles. In essence, physical or chemical measurements are taken on the earth’s surface (e.g., magnetic and electromagnetic fields, acoustic/sound waves or radioactive element counts) and interpreted to give subsurface information. As is the case with demining, there is no one instrument or technique that provides an ultimate solution. Since a target can only be detected if it shows a measurable contrast with the background, the best method (or preferably combination of methods) to use in each situation will depend on both the target and background (host rock or soil) properties. This problem of detectability is often encountered where metal detectors are used to find landmines in areas with high soil conductivity. If no adjustments are made to the metal detector sensitivity, false alarms are continuously generated by the conductive soil alone. The metal detectors can be adjusted to have a lower sensitivity, but at some stage the sensitivity will be too low to pick up the landmines. The equivalent of this problem is experienced in geophysics where conductive ore bodies are buried beneath conductive overburden. Due to high conductivity of the background it is easy to
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miss the contribution of another conductive body such as the ore deposit itself. In geophysics this problem has been partially solved by the application of transient (time domain) electromagnetic (TEM or TDEM) techniques, which have been used for decades in the mineral, oil and gas industries. The method is based on inducing electrical currents in subsurface conductors, and measuring the accompanying magnetic fields (secondary decay signal) generated through this process. The amplitude and rate-of-decay properties of these magnetic fields are analyzed and interpreted to give information on subsurface features. Time domain methods have the advantage over their frequency domain counterparts of not only penetrating deeper (depending on the input current and time of measurement), but also being able to ‘‘see through’’ conductive overburden. Furthermore, by calculating the decay constants of the overburden and expected targets, it is possible to quantify the detectability of a specific target for specific background conditions. If the decay constant of the target were larger than that of the overburden, the target will be detectable at a late time of measurement. The decay constant is merely a value determined by the conductivity and geometry of a body that indicates how rapidly an electromagnetic field (caused by this body) will decay with time. The equivalent of TEM equipment in the demining industry is the PI metal detector. It can be considered as a single time-channel, vertical component, miniature, transient electromagnetic system. It measures only the ‘‘width’’ of the secondary decay signal; the wider the signal, the higher the target decay constant (Sower and Cave, 1995), but it does not provide information to quantitatively determine the decay constant. Compared to the geophysical TEM equipment, which measures up to 32 time channels and three components (Fig. 1) of the decaying magnetic fields, PI metal detectors measure very little of the available information. If more channels and components were measured with metal detectors, it would be possible to apply geophysical modeling algorithms to the data and end up with a system indicating more than just the presence of metal. Although frequency domain metal detectors that can discriminate
Fig. 1. TEM receiver loop orientations relative to the transmitter loop to measure the x (radial), y (tangential) and z (vertical) components of the decaying secondary magnetic field (B). Traditional metal detectors only measure the vertical component.
between different types of metal and indicate depths already exist, this is not yet done with PI detectors (web site 1). As far as the hardware is concerned, three ‘‘miniTEM’’ systems, measuring a number of time channels, have already been manufactured. They are devoted to the very shallow (<10 m) subsurface investigation, with civil engineering investigations, pollution management and unexploded ordinance as target application fields. These are the VETEM (very-early TEM) from the United States Geological Survey, the nanoTEM from Zonge Engineering and the EM-63 from Geonics. Although these are not handheld systems, the gap is definitely being closed between geophysical electromagnetic equipment and traditional metal detectors. The question that remains is how to process and interpret the data in the most accurate and efficient way. It is at this point that some of the differences between demining and geophysical mineral exploration become evident. A mineral exploration company can afford to miss some targets with their initial exploration approach as long as they discover enough economic ore deposits to make a profit. In the demining industry, however, it is essential to pick up all targets (or at least 99.6% of them). Real-time, automated data processing and interpretation is something else that the geophysicist would like to see, but it is certainly not a prerequisite for a successful mineral exploration program. In the demining industry this is exactly what is needed in the manual stage of demining. Five main ways of automated data interpretation can be considered: 1. Curve matching. 2. Conductivity depth imaging (CDI). 3. Stationary current imaging (SCI – trademark of Geoterrex-Dighem Pty Limited). 4. Equivalent current least-squares inversion. 5. General inversion of TEM data 4.1. Curve matching Curve matching requires that a number of representative curves be calculated for known bodies. These curves are stored in a library and can then be matched to any curve generated from field data. Sower and Cave (1995) used this approach to look for a characteristic decay curve and compared it to curves stored in a library. Problems arose from the fact that the response curve depends on several factors, e.g., the orientation of the metallic object and the exact metal type, and that the matching is done only with objects known a priori.
1
http://www.treasurenet.com/misc/howmetaldetectorswork.html.
M. Combrinck / African Earth Sciences 33 (2001) 693–698
4.2. Conductivity depth imaging (CDI) This technique has been developed by Macnae and Lamontagne (1987) and is routinely used in processing airborne TEM data. A forward transformation (application of a mathematical operator or sequence of operators) is performed on the data, determining a depth related to each time channel as well as the corresponding cumulative conductance. The result is a conductivity vs depth section that can be generated automatically, as fast as the data are acquired, with no prior input needed from the operator. The only assumption made in this process is that the earth is one-dimensional (that is layered, with no lateral variations). Unfortunately, landmines are three-dimensional objects, which will lead to inaccurate calculations if CDI is applied. 4.3. Stationary current images (SCI) This technique is better suited than the CDI for the specific three-dimensional application. This method highlights the edges of conductors, gives an indication of dip and allows a qualitative estimate of the conductance of localized conductors (Wolgram et al., 1998). 4.4. Equivalent current least-squares inversion Barnett (1984) described a method in which a thin, finite conductor is replaced by an equivalent current filament. This filament approximation is used in the design of a least-squares inversion procedure that fits circular or rectangular current filaments to observed current distributions. The inversion procedure provides a rapid, precise means of estimating the position, size and attitude of a conductor. This method was developed for a single conductor in a resistive environment and would have to be adapted for multiple conductors in conductive environments (not a trivial exercise when mutual conductances have to be calculated between a number of conductive bodies). 4.5. General inversion The most general (and intensive) way of interpretation is to do inversion of the observed data. Inversion is described as: ‘‘Deriving from field data a model to describe the subsurface that is consistent with the data. Determining the cause from observation of effects’’ (Sheriff, 1991). Due to the complexity of the mathematical calculations involved in these procedures, severe constrictions had been placed on the formulation of models in the past. The interpreter had to simulate complex three-dimensional geological features with only a limited number of spheres, plates, cylinders or wires, mostly limited to two-dimensional solutions. With the
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constant improvements in computer technology, as well as focusing on the development of faster numerical algorithms, it is now possible to do true three-dimensional electromagnetic forward modeling using arbitrary models. The procedures are still very time intensive (depending on the complexity of the model) and care has to be taken when choosing the algorithm to be used. Some algorithms, for example, will only give reliable results with conductivity contrasts of less than 1:100, while others can only facilitate conductors confined to one layer of a layered model. The biggest challenge at this stage is to be able to do inversion in real-time. This depends on the model, the algorithm used and the speed of the computer, but 3 h for a single inversion is common today in the mineral exploration environment (S. MacInnes, Zonge Engineering and Research Organization, pers. comm.). In many ways, though, the landmine problem is simpler than the mineral exploration problem. The target conductivity is known, the background (small areas of soil) is relatively homogeneous and the area of investigation is very small. This could drastically reduce the time for doing inversion calculations. A relatively good initial model is essential in inversion procedures, and the curve matching, CDI, SCI or equivalent filament methods may prove very efficient for an initial guess. Another important factor to investigate is the choice of forward modeling algorithm. There are many possibilities, including finite difference, finite element and hybrid solution algorithms, with the best one ultimately depending on the complexity and nature of the model.
5. Conclusions Despite the continuing demining efforts of many organizations and people worldwide, landmines planted indiscriminately during years of warfare remain a problem. They result not only in the loss of life and severe injuries but also lay waste to large areas of land that would otherwise be used for mining, agriculture or social infrastructure. The objective of finding and characterizing hidden targets without physically penetrating the earth’s surface is one shared by geophysics applied to the mineral and oil exploration industries, although on a different scale. Due to similarities in the instrumentation and data acquisition, the results of research done by the mineral exploration industry may turn out to be beneficial to the demining industry and vice-versa. From a mathematical point of view, the landmine detection problem appears to be less complicated than mapping vast areas of complex three-dimensional geology. This implies that algorithms developed for geological applications might
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well prove to give superior results when applied to the landmine model and this will be the subject of continuing research. References Amazeen, C.A., Locke, M.C., 1998. Developmental status of the US Army’s new handheld standoff mine detection system (HSTAMIDS). In: Conference on the Detection of Abandoned Landmines, Edinburgh, October 1998. Conference Publication No. 458, pp. 193–197. Barnett, C.T., 1984. Simple inversion of time-domain electromagnetic data. Geophysics 49 (7), 925–933. Blagden, P.M., 1998. The changing scene of mine clearance. In: Conference on the Detection of Abandoned Landmines, Edinburgh, October 1998. Conference Publication No. 458, pp. 19–22. Chignell, R.J., 1998. ‘‘MINEREC’’ a development platform for antipersonnel mine detection and recognition. In: Conference on the Detection of Abandoned Landmines, Edinburgh, October 1998. Conference Publication No. 458, pp. 64–67.
Joynt, V.P., 1998. Mobile metal detection: a field perspective. In: Conference on the Detection of Abandoned Landmines, Edinburgh, October 1998. Conference Publication No. 458, pp. 14–18. Macnae, J., Lamontagne, Y., 1987. Imaging quasi-layered conductive structures by simple processing of transient electromagnetic data. Geophysics 52 (4), 545–554. Office of International Security and Peacekeeping Operations (OISPO), 1994. Hidden killers, The global landmine crises. Report to the US Congress on the Problem with Uncleared Landmines and the United States Strategy for Demining and Landmine Control. Department of State Publication 10225. Sheriff, R.E., 1991. Encyclopedic Dictionary of Exploration Geophysics, third ed. Society of Exploration Geophysicists. Sower, G.D., Cave, S.P., 1995. Detection and identification of mines from natural magnetic and electromagnetic resonances. In: Proceedings of SPIE (The International Society for Optical Engineering), Orlando, Florida, vol. 2496, pp. 1015–1024. Trevelyan, J., 1998. Technology transfer in humanitarian demining. In: Conference on the Detection of Abandoned Landmines, Edinburgh, October 1998. Conference Publication No. 458, pp. 23–27. Wolgram, P., Hyde, M., Thomson, S., 1998. How to find localised conductors in GEOTEM data. Exploration Geophys. 29, 665–670.
Transient electromagnetic exploration techniques: can they be applied to the landmine discrimination problem? M. Combrinck Department of Earth Sciences, University of Pretoria, Pretoria 0002, South Africa Received 31 March 2001; accepted 9 July 2001
Abstract The use of metal detectors in the humanitarian demining industry closely resembles the use of electromagnetic techniques in the mineral exploration industry to find conductive orebodies. An overview is given for the landmine problem and some of the current technologies used in the demining process. The similarities of metal detection to geophysical transient electromagnetic data acquisition, processing and interpretation are used to suggest ways in which more information than just the ‘‘presence of metal’’ may be obtained when using pulse induction (PI) metal detectors. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Landmines; Mineral exploration industry; Metal detection
1. Introduction Despite efforts of nations around the world to reduce the total number of buried landmines, more landmines are deployed in armed conflict every year than are removed by mine clearance personnel. According to statistics laid before the US Congress by the Office of International Security and Peacekeeping Operations (OISPO, 1994), the world was littered with an estimated 80–110 million anti-personnel (AP) landmines in 64 countries, which maim or kill an estimated 500 people every week – mostly civilians. Angola and Mozambique are estimated to have more than one million landmines each. In addition to the lives that are lost due to explosions of abandoned landmines, the mere suspicion that landmines may be present in an area lays waste to large areas of land that could otherwise be used for mining, agriculture or social infrastructure. Removing landmines from a designated area to such an extent that civilian populations can continue their normal existence is known as humanitarian demining (OISPO, 1994). The most widely used standard for these operations is the United Nations standard of 99.6% clearance (Blagden, 1998). This is a very different problem compared to military countermine operations in which the object is mainly to
E-mail address: [email protected] (M. Combrinck).
detect and avoid mined areas during times of conflict (Trevelyan, 1998). The level of detection and discrimination in the humanitarian demining industry therefore has to be superior to that used in the military operations. When mines are used in proper military operations the positions of minefields and individual mines are mapped and the danger area is clearly marked. New developments also include self-deactivating and selfdestruct mines that can limit innocent lives to be claimed once conflict situations have been resolved. The greatest challenge for deminers occurs in countries which have been subject to years of guerilla warfare in which landmines have been planted indiscriminately (OISPO, 1994). This is the case in many African countries where independence and civil wars have been raging for decades.
2. Landmines and demining techniques There are many types of landmines developed in countries all over the world, consisting of different materials and activated in different ways. One can, however, distinguish between AP and anti-tank (AT) landmines based on their preferred application, destructive potential, approximate size, depth of burial and triggering mechanism. AP mines are approximately 7.5 cm in diameter and between 2.5 and 5 cm thick (Chignell, 1998), are buried at depths less than 7.6 cm
0899-5362/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 9 - 5 3 6 2 ( 0 1 ) 0 0 0 8 6 - 0
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M. Combrinck / African Earth Sciences 33 (2001) 693–698
(Amazeen and Locke, 1998), triggered by pressure, tripwire or command and are designed to kill or maim a person within a radius of up to 50 m (OISPO, 1994). AT mines are approximately 20–30 cm in diameter, buried at depths less than 20 cm (Amazeen and Locke, 1998) and are aimed at demobilizing armored vehicles. Landmines are traditionally constructed from or at least contain measurable quantities of metal, and metal detectors are used with great success (if not always speed) to find these. A more recent development is the minimal metal or plastic mines, e.g., the Chinese Type 72 AP, the PMA-2 and R1M1 mines (Joynt, 1998). In areas with high soil conductivity these mines cannot be detected reliably with metal detectors and have given rise to a number of new technological innovations that have shown different measures of success under different conditions. In considering humanitarian demining these plastic mines still constitute less than 5% of the total world occurrence (Joynt, 1998). In reality, the operational mine detection process has evolved very little in technological terms since World War II. Metal detection still forms the basis for clearing operations and works very well under most circumstances. There is, however, one very pronounced drawback and that is the number of false alarms generated by a metal detector sensitive enough to detect minimum metal mines. Every fuse-holder, spent case, nail, piece of shrapnel and any other conceivable little piece of metal then generates a signal on the metal detector and must be opened up with the same care as would be exercised for a landmine by the so-called ‘‘spade man’’ (Joynt, 1998). MECHEM is a military research company that undertakes basic development work for the South African National Defence Force and has designed vehicles that are protected against AP and AT blasts (Joynt, 1998). MECHEM started landmine clearing contracting in 1991 and adopted a combination of techniques to operate in typical field conditions. They combined mechanical mine clearance with manual clearance by men using metal detectors, as well as vapor detection and sniffer dogs to do area reduction (Joynt, 1998). The mechanical mine clearance technique employs a Casspir armoured vehicle fitted with steel wheels that is used to detonate landmines by driving over them and simultaneously clearing the way in densely vegetated areas. This works well in the case of AP mines but AT mines still cause damage to the vehicle that has to be repaired at considerable cost. Even though the demining process is sped up through the removal of vegetation and creation of ‘‘safe’’ tracks, the manual stage of detecting and lifting the remaining undetonated mines still has to be performed. The actual rate of progress (and cost of demining) is still determined by the ‘‘spade man’’, who is responsible for opening each detected signal (Joynt, 1998).
The above operational procedure is representative of the humanitarian demining industry and illustrates the improvements that need to be made, namely: • A method (or combination of methods) has to be found to discriminate between scrap metal and mines. • A practical way still has to be found to localize minimum metal/plastic mines under all soil conditions. • Being an industrial venture, new techniques should not raise the cost of demining above the current 0.4–0.6 US $ per square meter (Joynt, 1998) (which implies that techniques should not only be accurate, but also efficient). • The equipment must be able to withstand true field conditions and be simple enough to be operated and maintained by trained deminers.
3. ‘‘New’’ technologies to meet the challenges The humanitarian demining industry has led to numerous scientists, universities and other institutions becoming involved in the search for the ultimate landmine detector. A brief summary of techniques under investigation follows below, together with some advantages and disadvantages related to them. The review list has been compiled from the proceedings of the ‘‘Detection of abandoned land mines’’ conference held in Edinburgh (Scotland) in October 1998, and is taken to be representative of the different disciplines. Most of these methods are aimed at detecting non-metallic mines or at reducing the false alarm rate associated with metal detection. It is also a very common assumption that a combination of methods/instruments would be used in any specific operation to achieve the optimum results. 3.1. Radar/microwave surveys These methods help to define geometric shapes of buried objects based on their dielectric properties, operating at megahertz and gigahertz frequencies. They are very sensitive to noise and the depth of investigation is adversely affected by high soil conductivities. Research focuses on finding the optimum frequency bands, antenna configurations, image processing and filtering techniques to present the data in an unambiguous manner. Some very good results have been obtained in sand pits and controlled environments, depending on different systems. The United States Army’s handheld standoff mine detection system (HSTAMIDS) which is a combination of metal detector, infrared camera and ground penetrating radar, is a working field prototype and the results look very promising. Extensive field testing in different environments is the only way to know whether this system will achieve the required clearing standard at an economic rate of clearance.
M. Combrinck / African Earth Sciences 33 (2001) 693–698
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3.2. Infrared/thermal images
3.7. Seismo-acoustic techniques
This method is based on the temperature difference between soil and landmines and gives very good results for shallow landmines (<5 cm) in desert regions where the background temperature distribution is very homogeneous. It is important to remember that although AP mines are usually buried at depths of less than 7 cm, that may not be where they are found after 30 years of erosion and floods, as for example in Mozambique where severe recent floods have relocated and reburied many mines.
These techniques can either be used to image the subsurface, based on reflected waves (resolution and energy are limitations), or to analyze the frequency content of the reflected signals. The frequency content of the signals could be used to pick up the natural resonant frequencies of the landmines, provided that these frequencies are determined beforehand and accessible through a real-time database. The alternative to developing new technologies is to improve existing instruments and data analysis. In the remainder of this paper I will give a review on how pulse induction (PI) metal detection can be improved by analogy to the transient electromagnetic method used in geophysical exploration.
3.3. Nuclear quadrupole response and other nuclear techniques Nuclear techniques are very promising for reducing false alarm signals, because they detect the chemical elements used in explosives (most often focusing on the nitrogen content). The neutron–neutron detector is available as a hand-held system, but is also susceptible to false alarms created from wood and metal objects. The neutron–gamma device is too slow and bulky for continuous use, but boasts a 100% detection rate for explosives and no false alarms for metal and plastic mines. 3.4. Flail/mechanical devices Used to physically detonate or unearth the landmines (e.g., the modified Casspir used by MECHEM), these devices are very helpful, but can miss smaller mines and are vulnerable to larger ones. 3.5. X-ray backscatter This instrument consists of an X-ray source and receiver that senses photons that are reflected back from a landmine. The depth of investigation has been tested up to 10 cm. Unfortunately this system is also too bulky, at present, for hand-held operation. A big advantage in terms of cost and maintenance is that all the components used in these instruments already are commercially available. 3.6. Dogs Dogs smell explosives and can also locate tripwires, but due to the migration of the chemical elements through soil as time elapses, the mines cannot be pinpointed more accurately than 5–7 m. The dogs are most often used to reduce search areas, based on the absence of explosives, rather than in locating the landmines themselves.
4. Transient electromagnetic (TEM) techniques in the exploration industry: an aid to deminers? Except for the coincidence that both are looking for ‘‘mines’’, the geophysical mineral exploration and landmine demining industries have much in common. In both cases one of the major objectives is the ability to find and characterize hidden targets without physically penetrating the earth’s surface. This need for efficient gathering of subsurface information is addressed through the earth science discipline of geophysics in the mineral and oil exploration industries. There are just as many different techniques for finding significant geological targets as there are for finding landmines, and not surprisingly, many of them are based on the same scientific principles. In essence, physical or chemical measurements are taken on the earth’s surface (e.g., magnetic and electromagnetic fields, acoustic/sound waves or radioactive element counts) and interpreted to give subsurface information. As is the case with demining, there is no one instrument or technique that provides an ultimate solution. Since a target can only be detected if it shows a measurable contrast with the background, the best method (or preferably combination of methods) to use in each situation will depend on both the target and background (host rock or soil) properties. This problem of detectability is often encountered where metal detectors are used to find landmines in areas with high soil conductivity. If no adjustments are made to the metal detector sensitivity, false alarms are continuously generated by the conductive soil alone. The metal detectors can be adjusted to have a lower sensitivity, but at some stage the sensitivity will be too low to pick up the landmines. The equivalent of this problem is experienced in geophysics where conductive ore bodies are buried beneath conductive overburden. Due to high conductivity of the background it is easy to
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miss the contribution of another conductive body such as the ore deposit itself. In geophysics this problem has been partially solved by the application of transient (time domain) electromagnetic (TEM or TDEM) techniques, which have been used for decades in the mineral, oil and gas industries. The method is based on inducing electrical currents in subsurface conductors, and measuring the accompanying magnetic fields (secondary decay signal) generated through this process. The amplitude and rate-of-decay properties of these magnetic fields are analyzed and interpreted to give information on subsurface features. Time domain methods have the advantage over their frequency domain counterparts of not only penetrating deeper (depending on the input current and time of measurement), but also being able to ‘‘see through’’ conductive overburden. Furthermore, by calculating the decay constants of the overburden and expected targets, it is possible to quantify the detectability of a specific target for specific background conditions. If the decay constant of the target were larger than that of the overburden, the target will be detectable at a late time of measurement. The decay constant is merely a value determined by the conductivity and geometry of a body that indicates how rapidly an electromagnetic field (caused by this body) will decay with time. The equivalent of TEM equipment in the demining industry is the PI metal detector. It can be considered as a single time-channel, vertical component, miniature, transient electromagnetic system. It measures only the ‘‘width’’ of the secondary decay signal; the wider the signal, the higher the target decay constant (Sower and Cave, 1995), but it does not provide information to quantitatively determine the decay constant. Compared to the geophysical TEM equipment, which measures up to 32 time channels and three components (Fig. 1) of the decaying magnetic fields, PI metal detectors measure very little of the available information. If more channels and components were measured with metal detectors, it would be possible to apply geophysical modeling algorithms to the data and end up with a system indicating more than just the presence of metal. Although frequency domain metal detectors that can discriminate
Fig. 1. TEM receiver loop orientations relative to the transmitter loop to measure the x (radial), y (tangential) and z (vertical) components of the decaying secondary magnetic field (B). Traditional metal detectors only measure the vertical component.
between different types of metal and indicate depths already exist, this is not yet done with PI detectors (web site 1). As far as the hardware is concerned, three ‘‘miniTEM’’ systems, measuring a number of time channels, have already been manufactured. They are devoted to the very shallow (<10 m) subsurface investigation, with civil engineering investigations, pollution management and unexploded ordinance as target application fields. These are the VETEM (very-early TEM) from the United States Geological Survey, the nanoTEM from Zonge Engineering and the EM-63 from Geonics. Although these are not handheld systems, the gap is definitely being closed between geophysical electromagnetic equipment and traditional metal detectors. The question that remains is how to process and interpret the data in the most accurate and efficient way. It is at this point that some of the differences between demining and geophysical mineral exploration become evident. A mineral exploration company can afford to miss some targets with their initial exploration approach as long as they discover enough economic ore deposits to make a profit. In the demining industry, however, it is essential to pick up all targets (or at least 99.6% of them). Real-time, automated data processing and interpretation is something else that the geophysicist would like to see, but it is certainly not a prerequisite for a successful mineral exploration program. In the demining industry this is exactly what is needed in the manual stage of demining. Five main ways of automated data interpretation can be considered: 1. Curve matching. 2. Conductivity depth imaging (CDI). 3. Stationary current imaging (SCI – trademark of Geoterrex-Dighem Pty Limited). 4. Equivalent current least-squares inversion. 5. General inversion of TEM data 4.1. Curve matching Curve matching requires that a number of representative curves be calculated for known bodies. These curves are stored in a library and can then be matched to any curve generated from field data. Sower and Cave (1995) used this approach to look for a characteristic decay curve and compared it to curves stored in a library. Problems arose from the fact that the response curve depends on several factors, e.g., the orientation of the metallic object and the exact metal type, and that the matching is done only with objects known a priori.
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4.2. Conductivity depth imaging (CDI) This technique has been developed by Macnae and Lamontagne (1987) and is routinely used in processing airborne TEM data. A forward transformation (application of a mathematical operator or sequence of operators) is performed on the data, determining a depth related to each time channel as well as the corresponding cumulative conductance. The result is a conductivity vs depth section that can be generated automatically, as fast as the data are acquired, with no prior input needed from the operator. The only assumption made in this process is that the earth is one-dimensional (that is layered, with no lateral variations). Unfortunately, landmines are three-dimensional objects, which will lead to inaccurate calculations if CDI is applied. 4.3. Stationary current images (SCI) This technique is better suited than the CDI for the specific three-dimensional application. This method highlights the edges of conductors, gives an indication of dip and allows a qualitative estimate of the conductance of localized conductors (Wolgram et al., 1998). 4.4. Equivalent current least-squares inversion Barnett (1984) described a method in which a thin, finite conductor is replaced by an equivalent current filament. This filament approximation is used in the design of a least-squares inversion procedure that fits circular or rectangular current filaments to observed current distributions. The inversion procedure provides a rapid, precise means of estimating the position, size and attitude of a conductor. This method was developed for a single conductor in a resistive environment and would have to be adapted for multiple conductors in conductive environments (not a trivial exercise when mutual conductances have to be calculated between a number of conductive bodies). 4.5. General inversion The most general (and intensive) way of interpretation is to do inversion of the observed data. Inversion is described as: ‘‘Deriving from field data a model to describe the subsurface that is consistent with the data. Determining the cause from observation of effects’’ (Sheriff, 1991). Due to the complexity of the mathematical calculations involved in these procedures, severe constrictions had been placed on the formulation of models in the past. The interpreter had to simulate complex three-dimensional geological features with only a limited number of spheres, plates, cylinders or wires, mostly limited to two-dimensional solutions. With the
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constant improvements in computer technology, as well as focusing on the development of faster numerical algorithms, it is now possible to do true three-dimensional electromagnetic forward modeling using arbitrary models. The procedures are still very time intensive (depending on the complexity of the model) and care has to be taken when choosing the algorithm to be used. Some algorithms, for example, will only give reliable results with conductivity contrasts of less than 1:100, while others can only facilitate conductors confined to one layer of a layered model. The biggest challenge at this stage is to be able to do inversion in real-time. This depends on the model, the algorithm used and the speed of the computer, but 3 h for a single inversion is common today in the mineral exploration environment (S. MacInnes, Zonge Engineering and Research Organization, pers. comm.). In many ways, though, the landmine problem is simpler than the mineral exploration problem. The target conductivity is known, the background (small areas of soil) is relatively homogeneous and the area of investigation is very small. This could drastically reduce the time for doing inversion calculations. A relatively good initial model is essential in inversion procedures, and the curve matching, CDI, SCI or equivalent filament methods may prove very efficient for an initial guess. Another important factor to investigate is the choice of forward modeling algorithm. There are many possibilities, including finite difference, finite element and hybrid solution algorithms, with the best one ultimately depending on the complexity and nature of the model.
5. Conclusions Despite the continuing demining efforts of many organizations and people worldwide, landmines planted indiscriminately during years of warfare remain a problem. They result not only in the loss of life and severe injuries but also lay waste to large areas of land that would otherwise be used for mining, agriculture or social infrastructure. The objective of finding and characterizing hidden targets without physically penetrating the earth’s surface is one shared by geophysics applied to the mineral and oil exploration industries, although on a different scale. Due to similarities in the instrumentation and data acquisition, the results of research done by the mineral exploration industry may turn out to be beneficial to the demining industry and vice-versa. From a mathematical point of view, the landmine detection problem appears to be less complicated than mapping vast areas of complex three-dimensional geology. This implies that algorithms developed for geological applications might
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well prove to give superior results when applied to the landmine model and this will be the subject of continuing research. References Amazeen, C.A., Locke, M.C., 1998. Developmental status of the US Army’s new handheld standoff mine detection system (HSTAMIDS). In: Conference on the Detection of Abandoned Landmines, Edinburgh, October 1998. Conference Publication No. 458, pp. 193–197. Barnett, C.T., 1984. Simple inversion of time-domain electromagnetic data. Geophysics 49 (7), 925–933. Blagden, P.M., 1998. The changing scene of mine clearance. In: Conference on the Detection of Abandoned Landmines, Edinburgh, October 1998. Conference Publication No. 458, pp. 19–22. Chignell, R.J., 1998. ‘‘MINEREC’’ a development platform for antipersonnel mine detection and recognition. In: Conference on the Detection of Abandoned Landmines, Edinburgh, October 1998. Conference Publication No. 458, pp. 64–67.
Joynt, V.P., 1998. Mobile metal detection: a field perspective. In: Conference on the Detection of Abandoned Landmines, Edinburgh, October 1998. Conference Publication No. 458, pp. 14–18. Macnae, J., Lamontagne, Y., 1987. Imaging quasi-layered conductive structures by simple processing of transient electromagnetic data. Geophysics 52 (4), 545–554. Office of International Security and Peacekeeping Operations (OISPO), 1994. Hidden killers, The global landmine crises. Report to the US Congress on the Problem with Uncleared Landmines and the United States Strategy for Demining and Landmine Control. Department of State Publication 10225. Sheriff, R.E., 1991. Encyclopedic Dictionary of Exploration Geophysics, third ed. Society of Exploration Geophysicists. Sower, G.D., Cave, S.P., 1995. Detection and identification of mines from natural magnetic and electromagnetic resonances. In: Proceedings of SPIE (The International Society for Optical Engineering), Orlando, Florida, vol. 2496, pp. 1015–1024. Trevelyan, J., 1998. Technology transfer in humanitarian demining. In: Conference on the Detection of Abandoned Landmines, Edinburgh, October 1998. Conference Publication No. 458, pp. 23–27. Wolgram, P., Hyde, M., Thomson, S., 1998. How to find localised conductors in GEOTEM data. Exploration Geophys. 29, 665–670.