In recent years, the evolution in hydrocarbon exploration from two- to three-dimensional seismic methods has resulted in improved resolution and better definition of the subsurface geological structure and prospects. Although the homogeneity of the media and the geometry of the experiment involved in marine unexploded ordnance (UXO) detection is relatively simple compared to the complexity of geologic earth models, the marine environment still comprises some degree of complexity considering the short wavelength of the seismic waves needed to yield sufficient resolution. The rugosity of the sea floor determines the coupling and the coherency of the seismic wavefield as it propagates into and out of the sediments, and therefore, the signal-to-noise ratio of the backscattered energy by the UXO. However, seafloor rugosity can scatter coherent energy into the sediments at angles larger than the critical angle. Biologic activity in the upper parts of the sediments may cause anaerobic conditions producing gas pockets that attenuate the seismic signal and constrain the maximum penetration of the waves. If free gas is present, it may produce attenuation particularly for the shorter wavelengths of the seismic signal. Therefore, research is needed to investigate whether an array of seismic sources and receivers can be used to increase seismic energy levels radiated into the seafloor and how the signal-to-noise ratio of the back-scattered seismic energy can be improved by beam-forming and focusing the energy onto the UXO target.
The objective of this project is to conduct a proof-of-principle numerical modeling study to demonstrate that seismic imaging and scattering techniques can be used to detect and locate buried UXO at underwater sites.
Seismic scattering approaches can be applied to environments where strong contrasts between a host medium and an embedded anomaly produce reflected fields with strong amplitudes. UXO buried at underwater sites represent a case where the associated impedance contrast is about an order of magnitude larger than that associated with common seismic exploration targets. Two- and three-dimensional numerical finite difference (FD) modeling will be conducted to investigate how source and receiver geometries can be optimized to enhance the interaction of seismic waves with buried UXO. In order to create a realistic numerical model, the required physical parameters will be derived from field measurements at the former Mare Island Naval Shipyard in Vallejo, California.
This technique, if successful, will improve the detection and discrimination of UXO in coastal areas. Using a combination of seismic techniques, the energy focused onto the UXO will be increased and the signal-to-noise ratio of the back-scattered energy will be improved such that deeper targets may be illuminated. Attempts also will be made to increase the resolution of the imaging technique.