Typically, electromagnetic sensors are applied using one of two modalities to detect buried or obscured targets: 1) lower frequency Electromagnetic Induction (EMI) sensors that enable detection of targets in the near-field; and 2) higher frequency wave propagation sensors, such as Forward Looking or Ground Penetrating Radar (FL/GPR) that enable detection of targets in the far-field. Each modality has advantages and limitations. EMI sensors enable deep penetration of overburden or structures that may obscure a target; however, sensitivity is typically limited to high conductivity targets (i.e., metals) due to the relatively low frequency of operation. Wave propagation sensors, such as GPR, enable detection of both conductive and non-conductive targets as a result of inherent dielectric contrast sensitivity; however, penetration into ground or structures is limited due to rapid attenuation of the propagating wave through lossy materials. In this paper, we present a concept for enhancing the target range capabilities of EMI sensors to extend sensitivity to lower conductivity targets. This concept incorporates an efficient transmitter driver design that extends the range of EMI operation into the High Frequency (HF) band while providing high power output. This ability to produce high frequency, high power output provides a sensor modality that bridges the gap between traditional EMI and wave propagating modalities. This High Frequency Transmitter (HFTX) concept could enable sensitivity to low conductivity targets (i.e., non-metals) while maintaining effective penetration through soil overburden or other materials that would typically impede GPR wave propagation.
This paper presents an active source Electromagnetic Induction (EMI) sensor that offers extended detection ranges (>
2m) with minimal sensitivity to magnetic geology. The Ultra Deep Search (ULTRA) EMI system employs a large (20 -
40m), stationary, surface-laid transmitter loop that produces a relatively uniform magnetic field within the search region.
This primary field decays slowly with depth due to the non-dipolar nature of the field within the search volume. An
array of 3-axis receiver cubes measures the time derivative of secondary field decays produced by subsurface metallic
objects. The large-loop transmitter combined with the vector sensing induction coil receivers produces a deep search
capability that remains robust in environments containing highly magnetic soils. In this paper, we assess the general
detection capabilities of the ULTRA system and present data collected over a set of standardized UXO targets.
Additionally, we evaluate the potential for target feature extraction through dipole fit analysis of several data sets.
Because the recovery of underwater munitions is many times more expensive than recovering the same items on dry
land, there is a continuing need to advance marine geophysical characterization methods. To efficiently and reliably
conduct surveying in marine environments, low-noise geophysical sensors are being configured to operate close to the
sea bottom. We describe systems that are deployed from surface vessels via rigid or flexible tow cables or mounted
directly to submersible platforms such as unmanned underwater vehicles. Development and testing of a towed
configuration has led to a 4 meter wide hydrodynamically stable tow wing with an instrumented top-side assembly
mounted on the stern of a surface survey vessel. An integrated positioning system combined with an instrumented cable
management system, vessel and wing attitude and wing depth measurements to provide sub-meter positional accuracy in
up to 25 meter water depths and within 1 to 2 meters of the seafloor. We present the results of data collected during an
instrument validation survey over a series of targets emplaced at measured locations. Performance of the system was
validated through analyses of data collected at varying speeds, headings, and height above the seafloor. Implementation
of the system during live-site operations has demonstrated its capability to survey hundreds of acres of marine or
lacustrine environment. Unique deployment concepts that utilize new miniaturized and very low noise sensors show
promise for expanding the applicability of magnetic sensing at marine sites.
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