NS21A-01
Geologic Controls on Geophysics for Tunnel Detection
Properties of soils are critical to using near-surface geophysical techniques to search for clandestine tunnels. We have constructed a database of soils sampled at sites on the northern (N) and southern (S) US borders and at sites in Iraq in conjunction with tunnel searches. Geologic materials at these sites consist of glacial gravels (N), volcanic tuff (S), and alluvial sands interbedded with marine clays (Iraq). The depth of interest for detecting clandestine tunneling is $<$ 30m, and as shallow as 2m at some locations. Mineral composition, grain size, moisture content, conductivity, permittivity, and magnetic susceptibility are critical for assessing the effectiveness of near-surface geophysical techniques. Values for these properties are consistent with soil stratigraphy and with vertical and lateral geologic variability. In some environments, in situ moisture content and the arrangement of conductive and resistive materials in the upper few meters limit significantly the depth of investigation using traditional near-surface techniques (electromagnetic induction, ground-penetrating radar). Geologic factors plus the small physical size of the targets limit the usefulness of commercial off-the-shelf techniques, and warrant an investment in new approaches.
NS21A-02
Physical Limits for the Detection of Clandestine Tunnels
The detection of clandestine tunnels is an extremely difficult geophysical problem. Lack of contrast, high material attenuation, limited instrument sensitivity and low signal-to-noise ratio control the quality of the geophysical data and limits the detectability of both engineered and non-engineered voids. Even in homogenous near surface environments, the footprint-to-depth ratios of these clandestine tunnels can be as low as one-tenth. This low profile restricts the successful application of seismic reflection, ground penetrating radar, electrical resistivity and gravity surveys in noisy environments. This paper presents limits in the detection of tunnels based on the sensitivity of geophysical instruments, frequency of excitation source and properties of the near surface soils. These limits will provide geophysicists and law-enforcement agencies with boundaries to help in the selection of appropriate techniques and to understand the boundaries where detection is practically impossible using traditional geophysical measurements.
NS21A-03 INVITED
Electromagnetics for Detecting Shallow Tunnels
Detecting tunnels by geophysical means, even very shallow ones, has been difficult, to say the least. Despite heavy R\&D funding from the military since the early 70s, geophysicists have not produced tools that are simple and practical enough to meet the military needs. The initial interest and R\&D funding on the subject perhaps started with the Vietcong tunnels in the 60s. Tunnels in the Korean DMZ, first found in the mid 70s, sharply escalated the R\&D spending. During the 90s, covert tunnels along the US-Mexico border have kept the topic alive but at a minimal funding level. Most recent interest appears to be in the terrorism-related shallow tunnels, more or less anywhere in the regions of conflict. Despite the longstanding effort in the geophysical community under heavy public funding, there is a dearth of success stories where geophysicists can actually claim to have found hitherto unknown tunnels. For instance, geophysics has not discovered a single tunnel in Vietnam or in Korea! All tunnels across the Korean DMZ were found from human intelligence. The same is true to all illicit tunnels found along the southwestern border. The tunnels under discussion are clandestine, which implies that the people who built them do not wish others to succeed in finding them. The place around the tunnel, therefore, may not be the friendliest venue for surveyors to linger around. The situation requires tools that are fast, little noticeable, and hardly intrusive. Many geophysical sensors that require ground contacts, such as geophones and electrodes that are connected by a myriad of cables, may not be ideal in this situation. On the other hand, a sensor that can be carried by vehicle without stopping, and is nothing obviously noticeable to bystanders, could be much more acceptable. Working at unfriendly environment also requires forgoing our usual practices where we collect data leisurely and make pretty maps later. To be useful, geophysical tools must be able to process observed data and translate them into actionable results. They may in forms of audio (similar to the beeper of a landmine detector), strip chart, or even a 2D graphic display on a computer screen. In short, the tool must be able to declare a contact, audibly or graphically, in real time or shortly thereafter. In summary, we have two questions here. The first one is if any of the available geophysical tools can detect tunnels. If the answer is yes, then the next question is if any of them are able to perform fast in an unfriendly environment. Electromagnetic sensors may be able to meet the operational requirements: under what circumstances it can find tunnels would be another outstanding question.
<a href='http://www.geophex.com'>http://www.geophex.com</a>
NS21A-04 INVITED
Monitoring of Tunneling Activities with Electrical Resistivity Imaging Methods
We demonstrate the effectiveness of electrical resistivity imaging methods for monitoring of tunneling activities with numerical modeling. The sharp resistivity contrast between an air-filled tunnel and surrounding materials presents an excellent opportunity for resistivity imaging methods to locate a tunnel. However, the subsurface inhomogeneity and varying moisture conditions produce strong resistivity anomalies that sometimes overwhelm the tunnel signature. Therefore, tunnel detection with resistivity imaging methods is feasible under a favorable condition but very challenging under complex subsurface conditions. Our studies indicate that resistivity imaging methods are more effective in monitoring of tunneling activities than in tunnel detection. We use a difference inversion algorithm for time lapse resistivity data processing. The systematic errors such as numerical errors and errors in electrode layout tend to cancel each other in a difference inversion. The subsurface inhomogeneity and varying moisture conditions also cancel each other in a difference image. Thus, fewer artifacts appear in time lapse inversion results. These characteristics make the time lapse resistivity monitoring a robust pure-anomaly method. Our modeling studies show that the aspect ratio of the tunnel depth to the tunnel dimension (width or height) can be five or larger for a tunnel to be detectable in a time lapse monitoring application. The recent advances in resistivity data acquisition hardware for time lapse monitoring made remote and continuous monitoring possible. Commercial off-the-shelf resistivity imaging systems for unattended monitoring of tunneling activities are available and ready for practical applications.
<a href='http://www.agiusa.com/'>http://www.agiusa.com/</a>
NS21A-05 INVITED
Microgravity and Electrical Resistivity Techniques for Detection of Caves and Clandestine Tunnels
The Center for Cave and Karst Studies, CCKS, has been using microgravity to locate caves from the ground's surface since 1985. The geophysical subsurface investigations began during a period when explosive and toxic vapors were rising from the karst aquifer under Bowling Green into homes, businesses, and schools. The USEPA provided the funding for this Superfund Emergency, and the CCKS was able to drill numerous wells into low-gravity anomalies to confirm and even map the route of caves in the underlying limestone bedrock. In every case, a low-gravity anomaly indicated a bedrock cave, a cave with a collapsed roof or locations where a bedrock cave had collapsed and filled with alluvium. At numerous locations, several wells were cored into microgravity anomalies and in every case, additional wells were drilled on both sides of the anomalies to confirm that the technique was in fact reliable. The wells cored on both sides of the anomalies did not intersect caves but instead intersected virtually solid limestone. Microgravity also easily detected storm sewers and even sanitary sewers, sometimes six meters (twenty feet) beneath the surface. Microgravity has also been used on many occasions to investigate sinkhole collapses. It identified potential collapse areas by detecting voids in the unconsolidated material above bedrock. The system will soon be tested over known tunnels and then during a blind test along a section of the U.S. border at Nogales, Arizona. The CCKS has experimented with other geophysical techniques, particularly ground penetrating radar, seismic and electrical resistivity. In the late 1990s the CCKS started using the Swift/Sting resistivity meter to perform karst geophysical subsurface investigations. The system provides good depth to bedrock data, but it is often difficult to interpret bedrock caves from the modeled data. The system typically used now by the CCKS to perform karst subsurface investigations is to use electrical resistivity traverses followed by microgravity over suspect areas identified on the modeled resistivity data. Some areas of high resistivity indicate caves, but others simply indicate pockets of dry limestone, and the signatures looks virtually identical. Therefore, the CCKS performs microgravity over all suspect areas along the resistivity traverses. A low-gravity anomaly that corresponds with a high-resistivity anomaly indicates a cave location. A high-resistivity anomaly that does not also have a low- gravity anomaly indicates a pocket of dry limestone. Numerous cored wells have been drilled both into the anomalies and on both sides to confirm the cave locations and to establish that the technique is accurate. The September 11, 2001 World Trade Center catastrophe was the catalyst for the formation of a program within the CCKS to use the techniques for locating bedrock caves and voids in unconsolidated materials for search and rescue and for locating clandestine tunnels. We are now into our third year of a grant from the Kentucky Science and Technology Center to develop a robot that will measure microgravity and other geophysical techniques. The robot has the potential for detecting clandestine tunnels under the U.S. border as well as military applications. The system will soon be tested over known tunnels and then during a blind test along a section of the U.S. border at Nogales, Arizona.
NS21A-06
Optical Sensing and Shallow Tunnel Detection
This presentation will examine the role optical sensors can play in the detection of near surface objects, particularly tunnels. Tunneling and tunnel-related activities can be expected to provide a variety of observable phenomena. A range of sensor modalities are available for recording these phenomena. For optical sensors, these phenomena could encompass changes in soil characteristics (e.g., moisture) due to changes in the underlying near surface soil structure or the sudden appearance of subsurface materials on exposed terrain. Optical sensors thus provide a potentially new modality to be exploited for this purpose. A review will be provided of the relevant physical processes and phenomenology associated with tunnels and tunneling activity with an emphasis on those relevant to optical domains. Optical sensing techniques suitable for monitoring and recording these observables will be examined and an assessment of their applicability to the detection problem will be presented. This assessment will also include the role of optical sensors in near term applications and a description of the relevant technology development needed to meet longer term detection requirements.
NS21A-07 INVITED
Tunnel Detection Using Seismic Methods
Surface seismic methods have shown great promise for use in detecting clandestine tunnels in areas where unauthorized movement beneath secure boundaries have been or are a matter of concern for authorities. Unauthorized infiltration beneath national borders and into or out of secure facilities is possible at many sites by tunneling. Developments in acquisition, processing, and analysis techniques using multi-channel seismic imaging have opened the door to a vast number of near-surface applications including anomaly detection and delineation, specifically tunnels. Body waves have great potential based on modeling and very preliminary empirical studies trying to capitalize on diffracted energy. A primary limitation of all seismic energy is the natural attenuation of high-frequency energy by earth materials and the difficulty in transmitting a high- amplitude source pulse with a broad spectrum above 500 Hz into the earth. Surface waves have shown great potential since the development of multi-channel analysis methods (e.g., MASW). Both shear-wave velocity and backscatter energy from surface waves have been shown through modeling and empirical studies to have great promise in detecting the presence of anomalies, such as tunnels. Success in developing and evaluating various seismic approaches for detecting tunnels relies on investigations at known tunnel locations, in a variety of geologic settings, employing a wide range of seismic methods, and targeting a range of uniquely different tunnel geometries, characteristics, and host lithologies. Body-wave research at the Moffat tunnels in Winter Park, Colorado, provided well-defined diffraction-looking events that correlated with the subsurface location of the tunnel complex. Natural voids related to karst have been studied in Kansas, Oklahoma, Alabama, and Florida using shear-wave velocity imaging techniques based on the MASW approach. Manmade tunnels, culverts, and crawl spaces have been the target of multi-modal analysis in Kansas and California. Clandestine tunnels used for illegal entry into the U.S. from Mexico were studied at two different sites along the southern border of California. All these studies represent the empirical basis for suggesting surface seismic has a significant role to play in tunnel detection and that methods are under development and very nearly at hand that will provide an effective tool in appraising and maintaining parameter security. As broadband sources, gravity-coupled towed spreads, and automated analysis software continues to make advancements, so does the applicability of routine deployment of seismic imaging systems that can be operated by technicians with interpretation aids for nearly real-time target selection. Key to making these systems commercial is the development of enhanced imaging techniques in geologically noisy areas and highly variable surface terrain.