Seismic Imaging of UXO-Contaminated Underwater Sites (Interim Report)

Gritto, Roland ; Korneev, Valeri ; Nihei, Kurt ; Johnson, Lane

Lawrence Berkeley National Laboratory

关键词: Signal-To-Noise Ratio;    Dimensions;    Energy Sources;    Sediments;    Seismic Waves;   

DOI  :  10.2172/861308
RP-ID  :  LBNL--56754
RP-ID  :  DE-AC02-05CH11231
RP-ID  :  861308

美国|英语

来源: UNT Digital Library

【 摘 要 】

Finite difference modeling with 2-dimensional models were conducted to evaluate the performance of source-receiver arrays to locate UXO in littoral environments. The model parameters were taken from measurements in coastal areas with typical bay mud and from examples in the literature. Seismic arrays are well suited to focus energy by steering the elements of the array to any point in the medium that acts as an energy source. This principle also applies to seismic waves that are backscattered by buried UXO. The power of the array is particularly evident in strong noise conditions when the signal-to-noise ratio is too low to observe the scattered signal on the seismograms. Using a seismic array, it was possible to detect and locate the UXO with a reliability similar to noise free situations. When the UXO was positioned within 3-6 wavelengths of the incident signal from the source array, the resolution was good enough to determine the dimensions of the UXO from the scattered waves. Beyond this distance this distinction decreased gradually while the location and the center of the UXO were still determined reliably. The location and the dimensions of two adjacent UXO were resolved down to a separation of 1/3 of the dominant wavelength of the incident wave, at which time interference effects began to appear. In the investigated cases, the ability to locate a UXO was independent on the use of a model with a rippled or a flat seafloor, as long as the array was located above the UXO. Nevertheless, the correct parameters of the seafloor interface were obtained in these cases. An investigation to find the correct migration velocity in the sediments to locate the UXO revealed that a range of velocity gradients centered around the correct velocity model produced comparable results, which needs to be further investigated with physical modeling.