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.

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.

: The purpose of this U.S. Army Engineer Research and Development Center- Geotechnical and Structures Laboratory (ERDC-GSL) technical note is to identify and review effective technologies related to the remote monitoring of earthen structures (dams and levees).

: Millions of acres of former and currently used military training and testing ranges are potentially contaminated by surface and buried unexploded ordnance (UXO), giving rise to requirements for UXO environmental restoration of formerly used sites and for sustainable use and active range cleanup. Geophysical surveys are required to map the location of buried UXO. The major cost driver of current cleanup and restoration is the inability to discriminate between buried false alarm and UXO targets. Excavation of false alarm targets is the major cost driver of UXO cleanup. Application of complementary geophysical sensor systems increases the potential for discrimination of UXO targets from false alarm targets. Development of new and innovative data integration methods and cooperative geophysical inversion algorithms allows enhanced discrimination and gives potential for target classification.

: This report summarizes an investigation of the seismic response of Corps powerhouse structures. Research results include (1) the characterization and organization of the range in geometry of Corps powerhouse substructures for generator bays and service/erection bays; (2) development of representative, idealized substructure geometries for these types of structures; (3) construction of a series of finite element models of the substructures and subjecting these substructures to earthquake acceleration time-histories representing central/eastern United States ground motions and ground motions of the Pacific Northwest of the United States; (4) development of a database of first-mode periods of the typical powerhouse substructures; (5) computation of top-of-substructure acceleration time-histories and conversion of these time-histories into response spectra; (6) development of amplification factor curves (as a function of frequency) that quantify the magnitude of the amplification of ground shaking by the substructures; (7) development of a methodology for performing a simplified seismic analysis of the superstructure (or of the equipment on top of the substructure) that takes into account, in an approximate sense, the effect of the substructure on the response of the superstructures in the decoupled seismic structural analysis.

: At the request of the Charleston District, the Engineer Research and Development Center (ERDC) conducted a seismic stability evaluation of the St. Stephen Hydropower Plant, SC. Enclosure 1 summarizes the details of this seismic engineering evaluation of the powerhouse superstructure. A performance-based approach, as outlined in the FEMA document 356 (ref a), was used for the seismic evaluation to determine the risk of the superstructure collapse during a major earthquake. Collapse prevention performance requires that collapse of the structure be prevented regardless of the level of damage inflicted by the earthquake on the superstructure.

Results of a foundation investigation for the dynamic loading of a radar tower are presented. Geophysical tests comprised of surface refraction and borehole seismic tests were performed. Results from geotechnical in situ tests, which included plate bearing tests, are also presented. The elastic properties, Young’ s modulus, Poisson’ s ratio, shear modulus and constrained modulus were determined as a function of depth for the foundation soils. These properties are necessary to design a stable foundation that will experience dynamic loading. Foundation materials encountered at this site were coral sands silts and gravels to a depth of 60 ft. The upper 14 ft of foundation soils are a hydraulically placed sand fill. The shear modulus ran es from 7.7x(103) psi at 93 the surface to 22x(10 ) psi at the bottom of t e 5 profile. Youn$s modulus shows a range of 22x(10 ) psi to 66x(10 ) psi. Poisson’ s ratio varies from 0.40 for unsaturated to 0.49 for saturated soil. A method was presented to estimate moduli under the expected load from present unloaded in situ moduli values using the K2 parameter. An estimate of the moduli for the expected foundation load was presented for the upper 30 ft of the foundation. A comparison of the plate bearing test and seismic derive 4 Young’ s modul s Y shows good agreement, 123x(10 ) psi and 78x(10 ) psi, respectively.

: This report documents the results of seismological, geological, laboratory, field, and analytical investigations conducted to evaluate the liquefaction potential of two earth embankment sections of the Alben Barkley Lock and Dam Project, Kentucky. these sections are representative of those of the main embankment and powerhouse/switch-yard areas. The design earthquake, from the New Madrid Seismic Zone, had a body-wave magnitude of 7.5. Of particular interest in this study was the evaluation of the liquefaction potential of silty sands in the foundation.