Estimating Debye Parameters from GPR Reflection Data Using Spectral Ratios

In the GPR frequency range, electromagnetic wave attenuation is largely controlled by dielectric relaxation processes. A primary relaxation commonly occurs in the 10 – 100 MHz range for many earth materials in which the GPR signal propagates effectively. This relaxation leads to strong nonlinearity in the frequency dependent attenuation and occurs in a frequency range that is often used for groundwater investigations. This non-linearity complicates data analysis but also may provide additional material property information. I investigate inversion for Debye relaxation parameters directly from GPR reflection data, including increasing the bandwidth of the signal by summing the response from 25 MHz, 50 MHz, 100 MHz, and 200 MHz radar antennas. I compute the timefrequency distribution using spectral decomposition, then use the method of spectral ratios to measure the attenuation vs frequency curve for significant reflection events. I then fit the curve with the multiparameter Debye model. Using synthetic and field data I show that this approach provides reliable estimates of the primary relaxation time for a variety of realistic subsurface models. This approach has the potential to improve our understanding of aquifer material properties

2D Ground-Penetrating Radar AVO Response to a 3D Dielectric Permittivity Anomaly

To evaluate the amplitude vs offset response of GPR to small distributions of hydrocarbon contamination, I acquired multi-offset 450 MHz GPR data in TE and TM modes over a buried rectangular tank filled with gasoline saturated sand. All dimensions of the tank were less than one wavelength at the characteristic antenna frequency. The permittivity ratio at the moist sand/gasoline sand boundary, estimated by fitting the Fresnel equations to the observed amplitudes and by Brewster\u27s Angle analysis, differed from that obtained through migration velocity analysis by no more than 12%. 2D FDTD modeling reproduced amplitude characteristics for 3 o f 4 target/polarization combinations and explained some deviations from the Fresnel curves. Additional deviations may be caused by out-of-plane polarization effects or heterogeneity not included in the 2D model

Depth Characterization of Shallow Aquifers with Seismic Reflection, Part I—The Failure of NMO Velocity Analysis and Quantitative Error Prediction

As seismic reflection data become more prevalent as input for quantitative environmental and engineering studies, there is a growing need to assess and improve the accuracy of reflection processing methodologies. It is common for compressional-wave velocities to increase by a factor of four or more where shallow, unconsolidated sediments change from a dry or partially watersaturated regime to full saturation. While this degree of velocity contrast is rare in conventional seismology, it is a common scenario in shallow environments and leads to significant problems when trying to record and interpret reflections within about the first 30 m below the water table. The problem is compounded in shallow reflection studies where problems primarily associated with surface-related noise limit the range of offsets we can use to record reflected energy. For offset-to-depth ratios typically required to record reflections originating in this zone, the assumptions of NMO velocity analysis are violated, leading to very large errors in depth and layer thickness estimates if the Dix equation is assumed valid. For a broad range of velocity profiles, saturated layer thickness will be overestimated by a minimum of 10% if the boundary of interest is \u3c30 m below the water table. The error increases rapidly as the boundary shallows and can be very large\u3e(\u3e100%) if the saturated layer is \u3c10 m thick. This degree of error has a significant and negative impact if quantitative interpretations of aquifer geometry are used in aquifer evaluation such as predictive groundwater flow modeling or total resource\u3eestimates

Results of Millikan Library Forced Vibration Testing

This report documents an investigation into the dynamic properties of Millikan Library under forced excitation. On July 10, 2002, we performed frequency sweeps from 1 Hz to 9.7 Hz in both the East-West (E-W) and North-South (N-S) directions using a roof level vibration generator. Natural frequencies were identified at 1.14 Hz (E-W fundamental mode), 1.67 Hz (N-S fundamental mode), 2.38 Hz (Torsional fundamental mode), 4.93 Hz (1st E-Wovertone), 6.57 Hz (1st Torsional overtone), 7.22 Hz (1st N-S overtone), and at 7.83 Hz (2nd E-Wovertone). The damping was estimated at 2.28% for the fundamental E-W mode and 2.39% for the N-S fundamental mode. On August 28, 2002, a modal analysis of each natural frequency was performed using the dense instrumentation network located in the building. For both the E-W and N-S fundamental modes, we observe a nearly linear increase in displacement with height, except at the ground floor which appears to act as a hinge. We observed little basement movement for the E-W mode, while in the N-S mode 30% of the roof displacement was due to basement rocking and translation. Both the E-W and N-S fundamental modes are best modeled by the first mode of a theoretical bending beam. The higher modes are more complex and not well represented by a simple structural system

Reflection Waveform Inversion of Ground-Penetrating Radar Data for Characterizing Thin and Ultrathin Layers of Nonaqueous Phase Liquid Contaminants in Stratified Media

Accurately quantifying thin-layer parameters by applying a targeted reflection waveform inversion methodology to ground-penetrating radar (GPR) reflection data may provide a useful tool for near-surface investigation and especially for contaminated site investigation where nonaqueous phase liquid (NAPL) contaminants are present. We implemented a targeted reflection waveform inversion algorithm to quantify thin-layer permittivity, thickness, and conductivity for NAPL thin (≤ 1/2 dominant wavelength λ) and ultrathin (≤ 1/8λ) layers using GPR reflection data. The inversion used a nonlinear grid search with a Monte Carlo scheme to initialize starting values to find the global minimum. By taking a targeted approach using a time window around the peak amplitude of the reflection event of interest, our algorithm reduced the complexity in the inverse problem. We tested the inversion on three different synthetic data sets and four field data sets. In all testing, the inversion solved for NAPL-layer properties within 15% of the measured values. This algorithm provides a tool for site managers to prioritize remediation efforts based on quantitative assessments of contaminant quantity and location using GPR

The Need to Adapt the Exploration Model from the Oil Patch to Contaminated-Site Characterization: A Case from Hill AFB, Utah, USA

For decades, the oil industry has employed a working model for hydrocarbon exploration in which large-scale geophysical surveys are undertaken prior to a second phase of intensive, targeted drilling. This latter phase may be conducted in conjunction with further focused geophysical studies. The geophysical surveys provide lateral coverage and continuity that are used to drive placement of drilling locations. The reason for this approach is simple: wells are expensive relative to geophysical surveys. Also, practical limits on lateral coverage preclude optimization of exploration targets based on well information alone

Instantaneous Spectral Analysis: Time-Frequency Mapping via Wavelet Matching with Application to Contaminated-Site Characterization by 3D GPR

Spectral decomposition, by which a time series is transformed from the 1D time/amplitude domain to the 2D time/spectrum domain, has become a popular and useful tool in seismic exploration for hydrocarbons. The windowed, or short-time Fourier transform (STFT) was one early approach to computing the time-frequency (t-f) distribution. This method relies on the user selecting a fixed time window, then computing the Fourier spectrum within the time window while sliding the window along the length of the trace. The primary limitation of the STFT is the fixed window which prevents either time localization of high frequency components (if a long window is used) or spectral resolution of the low-frequency components (if a short window is used)

The Treadmill of Destruction in Comparative Perspective: A Panel Study of Military Spending and Carbon Emissions, 1961-2014

This article analyzes a unique panel data set to assess the effect of militarism on per capita carbon dioxide emissions. We extend previous research examining the effects of military expenditures on carbon emissions by including in our analyses over 30 years of additional data. In addition, we compare our preliminary results to those obtained from other estimation procedures. Specifically, we report and visually illustrate the results of 54 cross-sectional models (one for each year) and 36 unique panel regression models on both balanced and unbalanced panels. We assess how this relationship has changed over time by testing for interactions between military spending and time and by systematically re-analyzing our data across 180 panel regressions with varying time frames. A strong and enduring association between military spending and per capita carbon emissions is indicated in cross-sectional comparisons. Our panel analyses reveal a much weaker and varying relationship that has become stronger in recent decades. Moreover, we find that the effect of military spending on per capita carbon emissions is moderated by countries’ level of economic development, with military spending of more wealthy countries having relatively larger net effects on carbon emissions. We partially confirm previous findings on the temporal stability of the environmental impacts of militarism. Our analyses show, however, that this temporal stability has emerged relatively recently, and that the relationship between military expenditures and carbon emissions is weaker prior to the 1990s

Ground-Penetrating-Radar Reflection Attenuation Tomography with an Adaptive Mesh

Ground-penetrating radar (GPR) attenuation-difference analysis can be a useful tool for studying fluid transport in the subsurface. Surface-based reflection attenuation-difference tomography poses a number of challenges that are not faced by crosshole attenuation surveys. We create and analyze a synthetic attenuation-difference GPR data set to determine methods for processing amplitude changes and inverting for conductivity differences from reflection data sets. Instead of using a traditional grid-based inversion, we use a data-driven adaptive-meshing algorithm to alter the model space and to create amore even distribution of resolution. Adaptive meshing provides a method for improving the resolution of the model space while honoring the data limitations and improving the quality of the attenuation difference inversion. Comparing inversions on a conventional rectangular grid with the adaptive mesh, we find that the adaptively meshed model reduces the inversion computation time by an average of 75% with an improvement in the root mean square error of up to 15%. While the sign of the conductivity change is correctly reproduced by the inversion algorithm, the magnitude varies by as much as much as 50% from the true values. Our heterogeneous conductivity model indicates that the attenuation difference inversion algorithm effectively locates conductivity changes, and that surface-based reflection surveys can produce models as accurate as traditional crosshole surveys

Wave Field Migration as a Tool for Estimating Spatially Continuous Radar Velocity and Water Content in Glaciers

Normal-moveout velocity analysis can lead to significant overestimates of the velocity structure of temperate glaciers since most englacial reflectors approximate point scatters and violate the assumption of planar flat lying reflectors. Migration velocity analysis (MVA) is a tool that does not depend on the assumption of flat lying reflectors. MVA can provide laterally and vertically continuous velocity estimates from conventional common-offset radar sections. In a study of temperate Bench Glacier, Alaska, we used MVA coupled with dielectric modeling to estimate the distribution of water content along a cross-section of the glacier. We found the glacier has two layers, an upper layer with relatively low water content, and lower layer with relatively high water content. The ability to quantify hydrostratigraphy is important in understanding water storage and routing within glaciers