Gary R. Olhoeft

Dielectric properties of the first 100 meters of the Moon

Reviewing 92 measurements of lunar sample dielectric constant versus density at frequencies above 100 kHz, gives the relationK′ = (1.93 ± 0.17)p by regression analysis, where K′ is the dielectric constant of a soil or solid at a density ofpg/cm3. This formula is the geometric mean between the dielectric constant of vacuum (1) and the zero porosity dielectric constant of lunar material. Similarly, the loss tangent (D) can be described byD = [(0.00053 ± 0.00056) + (0.00025 ± 0.00009)C]p whereD is the loss tangent at densitypg/cm3 withC percent of total FeO + TiO2 (approximately proportional to ilmenite content). Using the density versus depth relations derived from lunar surface core tubes, and from laboratory studies of lunar soil compression gives a model of the dielectric properties as a function of depth in the lunar regolith. The dielectric constant increases smoothly with depth, as a function of the soil compaction only. The loss tangent, however, is more sensitive to the ilmenite content than it is to density. Neither dielectric constant nor loss tangent varies significantly with the temperature observed in a lunar day.

Maximizing the information return from ground penetrating radar

Ground penetrating radar data is not always easy to acquire, and sometimes the acquisition may be constrained by equipment availability, weather, legal or logistical constraints, safety or access considerations. Examples of these include archaeological or geotechnical sites about to be excavated, contaminated lands undergoing remediation, hazardous areas such as unexploded ordnance lands or active volcanoes, and difficult to visit locations such as Antarctica or the surface of Mars. These situations may result in only one chance at acquiring data. Thus, the data need to be acquired, processed and modeled with the aim of maximizing the information return for the time, cost and hazard risked. This process begins by properly setting up the survey with the expectation of the site conditions but allowing for flexibility and serendipity in the unknown. Not only are radar data acquired, but also calibration, orientation, location and other required parameters describing the equipment and survey are recorded. All of these parameters are used in the processing and modeling of the data. The final results will be not just a radar image as a pseudo-cross-section, but a corrected geometric cross-section, interpreted electrical and magnetic properties of the ground, location, orientation, size and shape of subsurface objects, and composition of the ground and objects as inferred density, porosity, fluid saturation, and other relevant material occurrence properties.

Surface and borehole ground-penetrating-radar developments

During the past 80 years, ground-penetrating radar (GPR) has evolved from a skeptically received glacier sounder to a full multicomponent 3D volume-imaging and characterization device. The tool can be calibrated to allow for quantitative estimates of physical properties such as water content. Because of its high resolution, GPR is a valuable tool for quantifying subsurface heterogeneity, and its ability to see nonmetallic and metallic objects makes it a useful mapping tool to detect, localize, and characterize buried objects. No tool solves all problems; so to determine whether GPR is appropriate for a given problem, studying the reasons for failure can provide an understanding of the basics, which in turn can help determine whether GPR is appropriate for a given problem. We discuss the specific aspects of borehole radar and describe recent developments to become more sensitiveto orientation and to exploit the supplementary information in different components in polarimetric uses of radar data. Multicomponent GPR data contain more diverse geometric information than single-channel data, and this is exploited in developed dedicated imaging algorithms. The evolution of these imaging schemes is discussed for ground-coupled and air-coupled antennas. For air-coupled antennas, the measured radiated wavefield can be used as the basis for the wavefield extrapolator in linear-inversion schemes with an imaging condition, which eliminates the source-time function and corrects for the measured radiation pattern. A handheld GPR system coupled with a metal detector is ready for routine use in mine fields. Recent advances in modeling, tomography, and full-waveform inversion, as well as Greens function extraction through correlation and deconvolution, show much promise in this field.

Automatic processing and modeling of GPR data for pavement thickness and properties

A GSSI SIR-8 with 1 GHz air-launched horn antennas has been modified to acquire data from a moving vehicle. Algorithms have been developed to acquire the data, and to automatically calibrate, position, process, and full waveform model it without operator intervention. Vehicle suspension system bounce is automatically compensated (for varying antenna height). Multiple scans are modeled by full waveform inversion that is remarkably robust and relatively insensitive to noise. Statistical parameters and histograms are generated for the thickness and dielectric permittivity of concrete or asphalt pavements. The statistical uncertainty with which the thickness is determined is given with each thickness measurement, along with the dielectric permittivity of the pavement material and of the subgrade material at each location. Permittivities are then converted into equivalent density and water content. Typical statistical uncertainties in thickness are better than 0.4 cm in 20 cm thick pavement. On a Pentium laptop computer, the data may be processed and modeled to have cross-sectional images and computed pavement thickness displayed in real time at highway speeds.

Temperature dependence of electrical conductivity and lunar temperatures

Numerous investigations of the electrical conductivity of lunar and terrestrial materials as a function of temperature have been performed to date in an attempt to provide data on which to base lunar interior temperatures from magnetometer-derived lunar conductivity profiles (Schwereret al., 1971, 1972, 1973; Dubaet al., 1972 and others). There are several pitfalls inherent in the extrapolation of lunar temperatures from laboratory measurements of electrical conductivity. These include the choice of representative material for the lunar interior, appropriate environmental conditions (pressure, fugacity, etc.) and the various measurement difficulties.

Frequency and temperature dependence in electromagnetic properties of Martian analog minerals

[1] Ground-penetrating radar (GPR) has the potential to image the Martian subsurface to give geological context to drilling targets, investigate stratigraphy, and locate subsurface water. GPR depth of penetration depends strongly on the electromagnetic (EM) properties (complex dielectric permittivity, complex magnetic permeability, and DC resistivity) of the subsurface. These EM properties in turn depend on the mineralogical composition of the subsurface and are sensitive to temperature. In this study, the EM properties of Martian analog samples were measured versus frequency (1 MHz-1 GHz) and at Martian temperatures (180–300 K). Results from the study found the following: gray hematite has a large temperature-dependent dielectric relaxation, magnetite has a temperature-independent magnetic relaxation, and JSC Mars-1 has a broad temperature-dependent dielectric relaxation most likely caused by absorbed water. Two orbital radars, MARSIS and SHARAD, are currently investigating the subsurface of Mars. On the basis of the results of our measurements, the attenuation rate of gray hematite is 0.03 and 0.9 dB/m, magnetite is 0.04 and 1.1 dB/m, and JSC Mars-1 is 0.015 and 0.09 dB/m at MARSIS and SHARAD frequencies, respectively, and at the average Martian temperature of 213 K. With respect to using GPR for subsurface investigation on Mars, absorbed water will be a larger attenuator of radar energy as high concentrations of magnetite and gray hematite are not found globally on Mars. Citation: Stillman, D., and G. Olhoeft (2008), Frequency and temperature dependence in electromagnetic properties of Martian analog minerals, J. Geophys. Res., 113, E09005, doi:10.1029/2007JE002977.

Ground-penetrating radar evaluation of railway track substructure conditions

railroad track substructure condition on a continuous top-ofrail nondestructive basis. In this study, 1 GHz radar data were acquired between concrete and wood ties as well as from the ballast shoulders beyond the ends of the ties, and with multiple antenna orientations and polarizations. Automatic processing of the data was developed to quickly generate hard copy sections of radar images and for input into railroad track performance monitoring software such as ORIM. Substructure conditions were observed such as thickness of the ballast and sub ballast layers, variations in layer thickness along the track, pockets of water trapped in the ballast, and soft subgrade from high water content. In addition, locations and depths of subsurface drainage pipes, trenches, and utilities were quickly and continuously mapped. GPR data were acquired and processed from a hirail vehicle moving continuously at 10 miles per hour with radar resolution of a few inches horizontally and a fraction of an inch vertically to depths of more than six feet. The largest errors resulted from the positioning system used to locate the antennas along and across the track. Automatic modeling to determine density and water content is being developed but the uneven and rough (at radar wavelengths) air-ballast interface is a major problem in modeling the data.

Electrical properties of lunar soil dependence on frequency, temperature and moisture.

Abstract We have examined the dielectric constant and loss tangent of a lunar soil sample in the frequency range from 100 Hz to 1 MHz. These results suggest that there is very little dispersion in the dielectric properties and that the loss tangent values are nearly a factor of 10 less than those measured by earlier studies. The d.c. conductivity is very low, around 10 −14 to 10 −15 Ω −1 /m at room temperature and is strongly temperature-dependent with an activation energy in the range of 0.4 – 0.9 eV. The introduction of atmospheric air has a profound influence on the electrical properties. The dielectric constant and loss tangent increase at frequencies below 10 kHz due to the presence of the moisture. The loss tangent increases by nearly a factor of 50 at the lower frequencies and the d.c. conductivity increases by 4 orders of magnitude. In order to make measurements on samples that represent lunar conditions it is essential to take great precautions to remove all residual moisture.

Nonlinear Complex-Resistivity Survey for DNAPL at the Savannah River Site A-014 Outfall

Nonlinear complex-resistivity (NLCR) cross-hole imaging of the vadose zone was performed at the A-014 Outfall at the Savannah River Site, Aiken, SC. The purpose of this experiment was to fieldtest the ability of this method to detect dense nonaqueous phase liquids (DNAPLs), specifically tetrachloroethene (PCE), known to contaminate the area. Five vertical electrode arrays (VEAs) were installed with ~15-ft (3 m) separations in and around the suspected source zone to depths of 72 ft (22 m), and measurements were carried out at seven nearest-neighbor panels. Amplitude and phase data were edited for quality and then inverted to form three-dimensional (3D) images. The comparatively small magnitude of the nonlinear resistivity Hilbert distortion allowed approximate linearized imaging of the 3D distribution of this quantity as well. Laboratory analysis of nearby soil contaminated in situ indicated that the NLCR response to the PCE-clay reaction is maximized near 50 mHz, leading to the development of a metric involving the phase and resistivity Hilbert distortion to infer the 3D distribution of PCE. Variations in PCE content were independently detailed at three drilling locations within the NLCR survey area using direct penetration-based soil-collection tools. Approximately 400 soil samples were collected and analyzed for chlorinated solvent mass composition at 1-ft (0.3-m) vertical intervals to compare with the NLCR-predicted distribution of DNAPL. The optimum performance for 1,000 mg/kg PCE was ~80% detection (true positives) with ~30% false alarms (false positives) at an effective resolution of 4 ft (1.2 m), or ~1/4 of the interwell separation. When smoothed to 12-ft (3.7 m) resolution (comparable to well spacing), detection was 100% with just 12% false alarms. NLCR successfully predicted the general distribution of PCE at parts-perthousand soil-mass fractions, specifically widespread near-surface contamination and a zone of discontinuous stringers and pods below the source.

Improving GPR Image Resolution in Lossy Ground Using Dispersive Migration

As a compact wave packet travels through a dispersive medium, it becomes dilated and distorted. As a result, ground-penetrating radar (GPR) surveys over conductive and/or lossy soils often result in poor image resolution. A dispersive migration method is presented that combines an inverse dispersion filter with frequency-domain migration. The method requires a fully characterized GPR system including the antenna response, which is a function of the local soil properties for ground-coupled antennas. The GPR system response spectrum is used to stabilize the inverse dispersion filter. Dispersive migration restores attenuated spectral components when the signal-to-noise ratio is adequate. Applying the algorithm to simulated data shows that the improved spatial resolution is significant when data are acquired with a GPR system having 120 dB or more of dynamic range, and when the medium has a loss tangent of 0.3 or more. Results also show that dispersive migration provides no significant advantage over conventional migration when the loss tangent is less than 0.3, or when using a GPR system with a small dynamic range.