CGISS - Boise State University

CGISS - Boise State University

Attribute analysis of GPR and seismic data for material property determination

Attribute analysis is a large group of tools used to quantify variations in amplitude, frequency content, and phase of a reflected waveform, and to relate these quantities to the physical properties of the medium in which the signal is propagating. We focus on the application of attenuation analysis, amplitude variation with offset (AVO) analysis, and phase variation with offset (PVO) analysis. To make full use of the information extracted through attribute analysis, it is important to determine the spatial distribution of attribute anomalies. This requires a detailed understanding of wavefield kine-matics which depend strongly on the velocity field through which the signal propagates. Thus, an important component of our research involves the application of sophisticated velocity analysis and imaging tools, such as traveltime inver-sion and migration velocity analysis, to GPR and shallow seismic data. Not only do these tools allow us to pinpoint attribute anomalies, but the resulting images are used to characterize lithol-ogy, identify fracture zones, and locate buried objects.

There are a wide variety of applications for attribute analysis ranging from aquifer characteriza-tion to determination of snow properties. A major focus of our research is direct detection of groundwater contaminants using GPR. A wide variety of organic contaminants, including chlori-nated solvents and many common light hydrocarbons, have electrical properties that differ signif-

icantly from water. When these chemicals displace groundwater a zone of anomalous electrical properties is formed. This can lead to large GPR AVO gradients and high electromagnetic velocities (Figures 1 and 2). In snow research, we use GPR velocity and attenuation to determine meltwater content and snow density (Figure 3). Attribute analysis is an exciting area of research that has only recently emerged in shallow seismic and GPR studies. Continued development of these methodologies will serve to signifi-cantly enhance our ability to understand and characterize the shallow subsurface.

Figure 1. In this AVO gradient image with wiggle trace overlay, the large AVO gradient, associated with the top of a gasoline saturated sand, is clearly identified as a zone of red coincident with a high amplitude reflection.

Figure 2. Pre-stack depth migrated GPR image over a hydrocar-bon contaminant plume at Hill AFB, Utah. The velocity model is shown in the background as a color image. The high velocity anomaly just above the water table may be associated with a previ-ously unidentified hydrocarbon rich zone.

Figure 3. Acquisition of GPR data for a snow density study on Pika Glacier, Alaska Range, AK.