Experiments were performed comparing the response of fiber optic distributed acoustic sensing (DAS) to vertical geophones installed on the surface. The DAS consisted of an optical interrogator attached to an optical fiber. The fiber was part of an optical cable that was installed at depths of 0.3 to 0.76 meters in a coastal environment composed of unconsolidated sand. Seismic signals generated with an impact hammer were recorded simultaneously with both systems and directly compared. Experiments were performed with two different configurations, broadside and end-fire, between the source and the fiber optic cable. The seismic signals recorded in the two configurations and with the two sensor systems were processed identically using the Spectral Analysis of Surface Wave method. The results demonstrate the suitability and limitations of using DAS for near-surface seismic measurements.

The U.S. military has developed and currently uses composite material munitions. These composite munitions are typically comprised of carbon fiber and, because of their low electrical conductivity, have a much lower electromagnetic induction signature, which makes them difficult to detect using traditional metal detecting methods. The term intermediate electrically conductive (IEC) is used to describe these lower conductivity materials, with conductivity, σ, typically in the range 10 < σ < 105 S/m. The electromagnetic induction (EMI) relaxation response of carbon fiber munitions peaks in the low megaHertz range (<15 MHz), but above 100 kHz. Thus, detecting and characterizing these munitions for remediation on military ranges is problematic using available geophysical EMI sensors. This paper describes initial efforts in the development of a prototype frequency-domain EMI sensor with the goal of extending the measurement frequency range to 15 MHz.

Intermediate electrical conductivity (IEC, 10(0)-10(5) S/m) objects are increasingly important to properly detect and classify. For the US Military, carbon fiber (CF) "smart bomb" unexploded ordnance (UXO) are contaminating training ranges. Home-made explosives (HME) may also fit in this conductivity range. Objects in this conductivity range exhibit characteristic quadrature response peaks at high frequencies (100 kHz-15 MHz). Previous efforts towards electromagnetic induction (EMI) sensing of IEC targets have required single-turn, small-diameter transmitter (Tx) and receiver (Rx) loops. These smaller loops remain electrically short in the high frequency EMI (HFEMI) range (100 kHz-15 MHz), a necessary feature, but provide low signal-to-noise ratio (SNR), especially at low frequencies (1000 Hz-10 kHz). We propose a modification to our pre-production HFEMI instrument which has a hybrid low frequency/high frequency transmit coil. This hybrid system uses many turns in the traditional range, and a single wire turn at HFEMI frequencies, to maximize SNR across a wider EMI band. The turns which are not current carrying in high frequency mode must have negligible inductive coupling to the single current-carrying turn, low enough that any coupling is suitable for background subtraction. This is enforced by mutually disconnecting every turn from every other turn. The instrument uses the same calibration techniques as previously introduced,1 namely background subtraction and ferrite compensation. This paper discusses engineering tradeoffs, compares results to numerical models and actual data from an advanced induction sensor, shows improvement in signal-to-noise ratio (SNR) at traditional EMI frequencies, and shows the same ability to detect IEC targets in the HFEMI band.

We introduce a frequency-domain electromagnetic induction (EMI) instrument for detection and classification of objects with either high ( $\sigma > 10^{5}~\textrm {S/m}$ ) or intermediate ( $1 < \sigma < 10^{5}~\textrm {S/m}$ ) electrical conductivity. While high conductivity metallic targets exhibit a quadrature peak response for frequencies in a traditional EMI regime under 100 kHz, the response of intermediate conductivity objects manifests at higher frequencies, between 100 kHz and 15 MHz. Materials such as carbon fiber or conducting salt solutions exhibit conductivities in this intermediate range and are undetectable by traditional low-frequency EMI instruments. To detect these intermediate conductivity targets, we developed a high-frequency EMI (HFEMI) instrument with a frequency range extended to 15 MHz. The HFEMI instrument requires novel hardware considerations, coil design, and data processing schemes. Most importantly, the wire lengths of transmit and receive coils are shorter than those of traditional frequency EMI sensors, so that the phase on the transmit and receive coils is nearly constant. We present the hardware and software aspects of the HFEMI instrument along with preliminary data, demonstrating its ability to detect intermediate conductive objects.

Depleted uranium (DU) is a byproduct of the uranium enrichment process and contains less than 0.3 % of the radioactive U-235 isotope. Since, the natural uranium has about 0.72 % of the uranium U-235 isotope, the enrichment produces large quantities of low-level radioactive DU. The non-fissile uranium U-238 isotope constitutes the main component of DU and makes it very dense. With 19.1 g/cm3 density, the DU is about 68.4 % denser than lead. Because of its high density, the DU has been used for as armor-piercing penetrators by the U.S. army. There are at least 30 facilities where munitions containing DU have been evaluated or used for training. These evaluation studies have been conducted with and without catch-boxes and have left a legacy of DU contamination. Thus, there are needs for rapid and cost-effective approaches to detect and locate subsurface DU munitions and to assess large contaminated areas. In this paper, a new ultra-wideband (from 10s of Hertz up to 15 Megahertz) geophysical instrument is evaluated for sensing subsurface DU munitions and DU materials related to contaminations in soil. Namely, full electromagnetic induction (EMI) responses are investigated using computational and experimental data for a DU rod, dart, and three samples of Yuma Proving Ground (YPG) soils. Numerical data are obtained via the full 3D EMI solver based on the method of auxiliary sources. The EMI signals sensitivity with respect to DU size, orientations, and material composition are illustrated and analyzed. Comparisons between computational and experimental studies are demonstrated. The studies show that the new ultra-wideband EMI sensor measures the complete polarization relaxation response from the DU rod and dart, and is able to sense relative DU contamination levels in soil.

A ground penetrating radar (GPR) and root excavation study were conducted to determine the efficacy of GPR for estimating subsurface tree root volume. The survey was conducted in sandy soil, which is favorable for GPR imaging. The tree was a loblolly pine (Pinus taeda) that was isolated from other trees to minimize outside influences. GPR antenna frequencies of 450 MHz, 900 MHz, and 1200 MHz were used to map subsurface profiles over a 9-m(2) sample grid, and six 1-m(2) cells of the root system were subsequently excavated to a depth of 1 m for verification. The 900 MHz GPR was successful at mapping the larger tree roots (>2 cm) and some smaller roots (<2 cm). A simple approach based on average signal strength was used to estimate root volume from the GPR data. The total root volume estimate based on the GPR data compared well with the total root volume determined from the in situ measurements when information from all of the excavated cells was used (<2% error). However, accuracy reduced significantly (30% error) when only three verification cells were used, representing near, intermediate, and far distances from the tree. Matching the GPR root volume to a particular in situ root volume did not improve the root volume estimation.

Ultrawide band electromagnetic induction (EMI) instruments have been traditionally used to detect high electric conductivity discrete targets such as metal unexploded ordnance. The frequencies used for this EMI regime have typically been less than 100 kHz. To detect intermediate conductivity objects like carbon fiber, even less conductive saturated salts, and even voids embedded in conducting soils, higher frequencies up to the low megahertz range are required in order to capture characteristic responses. To predict EMI phenomena at frequencies up to 15 MHz, we first modeled the response of intermediate conductivity targets using a rigorous, first-principles approach, the Method of Auxiliary Sources. A newly fabricated benchtop high-frequency electromagnetic induction instrument produced EMI data at frequencies up to that same high limit. Modeled and measured characteristic relaxation signatures compare favorably and indicate new sensing possibilities in a variety of scenarios.

Electromagnetic induction (EMI) instruments have been traditionally used to detect high electric conductivity discrete targets such as metal unexploded ordnance (UXO). The frequencies used for this EMI regime have typically been less than 100 kHz. To detect intermediate conductivity objects like carbon fiber, even less conductive saturated salts, and even voids embedded in conducting soils, higher frequencies up to the low megahertz range are required in order to capture characteristic relaxation responses. In this context, nonconducting lastic landmines can be considered a void plus small metallic parts such as the firing pin. To predict EMI phenomena at frequencies up to 15MHz, we modeled the response of conducting and nonconducting targets using the the Method of Auxiliary Sources. Our high-frequency electromagnetic induction (HFEMI) instrument is able to acquire EMI data at frequencies up to that same high limit. Modeled and measured characteristic relaxation signatures compare favorably and indicate new sensing possibilities in a variety of scenarios including the detection of voids and landmines.

Intermediate electrical conductivity (IEC) materials (10(1)S/m < sigma < 10(4)S/m), such as carbon fiber (CF), have recently been used to make smart bombs. In addition, homemade improvised explosive devices (TED) can be produced with low conducting materials (10(-)4S/m < sigma < 1S/m), such as Ammonium Nitrate (AN). To collect unexploded ordnance (UXO) from military training ranges and thwart deadly IEDs, the US military has urgent need for technology capable of detection and identification of subsurface IEC objects. Recent analytical and numerical studies have showed that these targets exhibit characteristic quadrature response peaks at high induction frequencies (100kHz - 15MHz, the High Frequency Electromagnetic Induction (HFEMI) band), and they are not detectable with traditional ultra wideband (UWB) electromagnetic induction (EMI) metal detectors operating between 100Hz 100kHz. Using the HFEMI band for induction sensing is not so simple as driving existing instruments at higher frequencies, though. At low frequency, EMI systems use more wire turns in transmit and receive coils to boost signal-to-noise ratios (SNR), but at higher frequencies, the transmitter current has non-uniform distribution along the coil length. These non-uniform currents change the spatial distribution of the primary magnetic field and disturb axial symmetry and thwart established approaches for inferring subsurface metallic object properties. This paper discusses engineering tradeoffs for sensing with a broader band of frequencies ever used for EMI sensing, with particular focus on coil geometries.

For the past several years, Quantum Technology Sciences (QTSI) and U.S. Army Engineering Research and Development Center (ERDC) have been developing a system to actively sustain present and future artillery ranges at zero unexploded ordnance (UXO) gains. With the Department of Defense (DoD) using over two million high-explosive (HE) munitions per year with a significant fraction as UXO, reducing costly range remediation and environmental restoration efforts will offer significant savings. The developed Seismic Acoustic Impact Monitoring Assessment (SAIMA) system is not designed for past ranges, but as a complementary technology to detect, locate within two meters, and classify UXO in near real-time to aid existing cleanup technologies. Feasibility and descriptions of system components have been previously provided ( VanDeMark et al., 2009 , 2010 , 2013 ). The current system is composed of multiple buried seismic arrays encircling a mortar or artillery impact area, communications from the arrays to a central processing station, and a processing unit that employs an algorithm suite based in the seismology and statistical analysis disciplines to detect, locate, and classify the HE or UXO impact. Recent deployments of the SAIMA system have demonstrated hardware maturity and algorithm refinements to nearly enable the goal of locations within two meters. A field deployment at Ft. Sill, Oklahoma, in June 2012 demonstrated acoustic locations at a large range ( QTSI, 2012 ). Subsequent systems tests with five arrays using a synthetic UXO source (kinetic source only; no acoustic phases) on a small field (80 m by 80 m) resolved locations within 0.5 m of ground truth with coverage ellipses at 0.1 m 2 (time and azimuth). On a small mortar field, approximately 365 m by 480 m, simulated UXO (inert rounds) were located within an average mislocation distance of 4.1 m and confidence ellipses on the order of 5.8 m by 3.8 m. Scheduled field testing in the near future will validate the system.

: This report presents the results of a geophysical study performed to determine the location of buried utilities beneath or in the vicinity of the levees on the south shore of Lake Pontchartrain approximately 8 km (5 miles) north of downtown New Orleans, LA. There was concern that utilities located beneath or buried near the toe of the levees could act as a water conduit during flooding events. If a water-filled utility fails, it is possible that it may cause the levee to fail either by piping material from within the levee or cause slope stability problems. It is also possible that buried utilities can act as potential seepage paths through the levee during high water events. In this case, the buried pipe would not have to fail to cause a problem. The utilities needed to be accurately located so that they could be rerouted, removed, or abandoned and grouted-in. Electromagnetic, total field magnetic, and ground penetrating radar systems were assessed to determine the best method for detecting the buried utilities in this area. The Geonics EM31 electromagnetic induction instrument was considered the most effective for detecting the utilities. EM31 anomalies, presumed to be the locations of buried utilities, were mapped and their coordinates tabulated for further interrogation.

The Department of Defense (DoD) uses over two million rounds of high-explosive (HE) munitions per year ( Defense Science Board Task Force, 2003 ). A small percentage does not explode, thus generating unexploded ordnance (UXO) in current range areas at a substantial rate. As these ranges are closed, the DoD becomes responsible for the environmental restoration of the affected properties. Current methods of UXO remediation are costly because of high false alarm rates. Our current research is to develop a complementary technology that will alleviate false alarm rate by detecting, classifying, and locating UXO in near real time (less than 1 minute) as a munition impacts the range. This technology will utilize an array of buried seismic sensors in a calibrated range area, along with a set of algorithms based on theoretical and applied seismology and statistical analysis.Initial field tests at three sites focused on developing concepts of the seismic and acoustic location of ordnance impacts. Our research program developed from these initial field tests has four primary objectives: 1) fully implement a wired seismic-acoustic ordnance impact location system for live fire ranges; 2) develop a system capability to discriminate high-order (HE), low-order (partially exploded), and zero-order (UXO) events; 3) reduce location error to a stringent program metric of 1–2 m; and 4) investigate the feasibility of developing a wireless implementation of the technology.This paper describes the procedures and results from follow-on tests that were conducted in two locations at the U.S. Army Aberdeen Proving Ground (APG), Maryland. These tests were used to evaluate potential seismic-acoustic methods and system configurations for a Seismic-Acoustic Impact Monitoring Assessment (SAIMA) system for mitigating UXO hazards. Significant results from this work include: 1) seismic impulses from low-order impacts were detected at distances up to 1,000 meters; 2) classification features based on measurements of the amplitude of acoustic and seismic phases produce clear discrimination between HE and UXO impacts; 3) calculated location solutions for HE and UXO impacts yield an average location error of 10–20 meters; and 4) empirical observation and waveform modeling demonstrated that surface waves dominate the signal at all distances and therefore should be the primary phase used for all components of analysis. Furthermore, these tests demonstrated the current system design, allowing further enhancements, is capable of meeting the initial research objectives (1) and (2). Future research will focus on improving system performance with refinement of the sensor-layout geometry and the detection and location algorithms through system error analyses and follow-on field testing.

Prior to 1990, UXO were generally modeled or approximated as compact, ferrous objects; the model was effectively a uniformly magnetized sphere of iron at a specified or an unknown distance from the magnetic sensor. Correlations were developed between various UXO, represented as compact masses of iron, and magnetic anomaly signature features such as maximum positive value, peak-to-peak value, and wavelength. The uniformly magnetized sphere, equivalent to a point dipole model external to the sphere, cannot account for magnetic phenomenology of actual UXO, which exist in forms ranging from approximately spherical to highly elongated, with elongations as large as 5 (ratio of length to diameter). UXO are generally ferrous, with large magnetic permeability, although some can contain aluminum or other non-magnetic metals. This paper reviews the phenomenology of models applied to simulation of UXO magnetic anomalies. The multipole expansion solution of the prolate spheroid model in earth's magnetic field is highlighted, as it replicates most of the phenomenology of UXO magnetic anomalies, and is about the most complicated model for which practical geophysical inversion can be achieved (8-parameter model vector, plus magnitude and orientation of the earth's magnetic field). While the prolate spheroid model works well for the larger UXO (e.g., 60-mm mortars and larger) at distances (burial depth plus sensor height) greater than the length of the target or model, it has not been tested for close distances (less than the target length) and for the smaller UXO (e.g., 20-mm to 40-mm projectiles). Test stand magnetic anomaly measurements for these small UXO at distances equal to the length or less from the sensor are compared to model calculations. The importance of including the octupole component is demonstrated for small ordnance at close distances, and the differences in modeling and inversion results for UXO physical dimension versus UXO ferrous component dimension are presented.

Thomas F. VanDeMark, Quantum Technology Sciences, Inc., Cocoa Beach, FL Ray Conner, Quantum Technology Sciences, Inc., Cocoa Beach, FL Lars B. Johnson, Quantum Technology Sciences, Inc., Cocoa Beach, FL Jay Bennett, U.S. Army Engineer Research and Development Center, Vicksburg, MS Janet E. Simms, U.S. Army Engineer Research and Development Center, Vicksburg, MS Don E. Yule, U.S. Army Engineer Research and Development Center, Vicksburg, MS

: This effort focused on characterizing the noise floor of giant magnetoresistive (GMR) sensors and evaluating their performance in detecting small (81 mm) munitions. A GMR sensor allows data acquisition over a broadband frequency spectrum in a single coverage, thus providing the capability of measuring responses presently requiring both a magnetometer and frequency domain or time domain electromagnetic induction sensor. Both the Honeywell and NVE GMR sensors studied have similar operating range, sensitivity, and noise levels. The additional features available on the Honeywell sensors make them the GMR sensors of choice. The performance characteristics of a single GMR sensor were studied, both as a static magnetic field sensor and time domain electromagnetic induction field sensor. In passive mode, the GMR sensor, which has a resolution of less than 10 nT, performed similarly to a cesium vapor magnetometer. When tested in dynamic mode for measuring the response of a single-turn copper coil, multi-turn copper coil, and solid copper sphere, the GMR sensor measured a distinct decay response for each target.

Wetlands are natural resources that are protected under federal regulations; therefore, the delineation of wetlands is necessary to ensure their protection. Standard methods used for delineating wetlands can be time consuming, or a wetland could be problematic, i.e., lacking hydrophytic vegetation or hydric soil indicators, or periodically lacking hydrologic indicators. A magnetic susceptibility study could be an additional technique used to aid in the delineation process. A study using magnetic susceptibility was undertaken in central Mississippi to identify the transitional zone or boundary between non-hydric (uplands) and hydric (wetlands) soils. The soils were silt loam with a minor percentage of sand. A survey line that traversed the transitional zone between wetland and upland on each end of the transect was revisited four times during a single year and once two years later. One survey was conducted a few weeks after the winter inundation (moderately wet soil conditions), one was conducted several months after inundation but immediately after some heavy rainfall (moderately wet soil conditions), and two were conducted several weeks or months after inundation or significant rainfall (dry soil conditions). There were measurable differences between the magnetic susceptibility values collected in the upland and wetland regions during each survey. One transitional zone was easily identified using magnetic susceptibility, exhibiting a sharp decrease in susceptibility values between the upland and wetland. The other transitional zone contained an intermediate ridge, which made demarcation of the zone less obvious. The measured magnetic susceptibility values were comparable for the respective upland, transition, and wetland regions, and the characteristics of the curves were similar for all time-periods. Overall, magnetic susceptibility proved to be a successful method for delineating a wetland in this area.

: The spectral response of quartz is a phenomenon of interest to countermine technologies because of suspected disruption in spectral signature from disturbed soil during emplacement of landmines in the subsurface. The research was divided into two studies: data analysis using spectral information from the Advanced Spaceborne Thermal Emission and Reflection (ASTER) spectral library and a laboratory study using a Fourier transform infrared (FTIR) spectrometer. Spectral and mineralogical data for five major soil orders (Alfisol, Aridisol, Entisol, Mollisol, and Inceptisol) were acquired from the spectral library. Using these data, the relationship of the percentage of reflectance around the 8.2-micron quartz peak to the percentage of quartz in a soil was used to develop a linear regression model. The equations from this model predict the expected quartz peak reflectance for a soil of given quartz percentage. The predictive equations determined from the ASTER database can be modified based on quartz species and grain size to enable better prediction of expected reflectance from natural soils. The FTIR system was used to measure the spectral response of different forms of quartz, clays, and quartz-clay mixtures. The spectral results revealed that different species of quartz exhibit different reflectance spectra, suggesting that the quartz reflectance response of a given soil type will vary depending on its impurities and depositional history. Grain-size fraction plays an important role in influencing the magnitude of reflectance. From the pure quartz studied, as grain size decreased from particle diameters of 1180 microns to less than 300 microns, FTIR reflectance increased according to a logarithmic relationship.

Levees are a fundamental part of many flood-damage reduction projects that protect life and property and their condition and performance in emergency flooding situations is of utmost importance. The U.S. Army Engineer Research and Development Center (ERDC) is conducting research related to levee condition evaluation and assessment. A primary objective of this research is to develop the capability to obtain information about levee conditions and rapidly convey the data to decision-makers during emergency operations, particularly in cases where levee failure is possible. A field study was conducted along a 9-km stretch of the landside levee toe of the Feather River south of Marysville, CA. The purpose of the investigation was to determine the potential of rapidly assessing levee foundations using a variety of geophysical instruments: a Geophex GEM-2 and Geonics EM31 and EM34 electromagnetic (EM) induction tools, a Geometrics OhmMapper capacitively coupled and a Scintrex SARIS direct-current (dc) resistivity system. Electrically non-conductive sleds were built to tow the EM instruments behind a vehicle. The capacitively coupled resistivity system and a combination of the EM instruments provided useful and definitive information for assessing foundation conditions.