Spectral Analysis of Thinning Beds Using Ground Penetrating Radar

Abstract

Ground Penetrating Radar (GPR) is a near surface geophysical method that has been used for applications including archaeological sites, groundwater contamination, and geological mapping. Though GPR has been used extensively, advancements on data processing had a great impact on data resolution. GPR is frequently used for shallow investigations because of the high resolution near the surface; however, it has limited depth of penetration and vertical bed resolution. Vertical resolution is proportional to frequency. The thickness of beds in the subsurface is conventionally resolved to one-fourth the wavelength of the central frequency. The vertical resolution at a central frequency of 200 MHz in a beach environment is approximately 17 cm; however, that value does not accurately represent fine-scale lamina and pinching out of beds, which can be an order magnitude or more than the current resolution. Complex trace analysis and spectral analysis have been used in seismic reflection for characterizing structures and stratigraphy. These "attributes" have been used to indicate hydrocarbon presence in industry. The same concept was applied to a theoretical GPR model and tested against actual data. The theoretical GPR model was created to simulate a case in which two ideal 0 degree phase Ricker wavelets merge. The wavelets constructively "add" together to create a composite wavelet with double amplitude. Applying a spectral analysis reveals that an attribute in the form of instantaneous phase and instantaneous frequency can be used to image the beds merging. The spectral analysis was applied to field data from North Padre Island National Seashore, Texas, to image "pinch-outs". Multiple survey arrays were collected using a 200 MHz frequency antenna to image internal dune structures. The results showed anomalous features at merging beds and contacts between interfaces. The results directly influence sedimentological and geomorphological interpretations of internal dune structure and can be used to better understand erosional processes in coastal sedimentary environments