Graduate Thesis Or Dissertation
 

Applications of source signature deconvolution to airgun seismic profiling and the measurement of attenuation from reflection seismograms

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  • Deterministic source signature deconvolution is applied to the processing of marine wide angle and vertical profiler data with air-gun sources. Optimum results are obtained with a source signature measured by stacking the signal reflected from a relatively homogeneous abyssal plain sedimentary environment. This eliminates the need for the unstable inverse source-receiver ghost filter. Improved resolution of reflection event timing allows the computation of more reliable interval velocities by the T² - X² method, provided the layer thickness limitation of the method is not exceeded. Accurate timing of primary reflection events in the deconvolved vertical profiler data permits computation of frequency dependent attenuation by univariate least-squares regression in the Fourier transform domain. The technique successfully extracts input amplitude attenuation functions from model reflection coefficient sequences with additive random noise. This success is attributed to the stability of singular value analysis in solving the least-squares regression model. Statistical tests on the solution vectors for model and field data give criteria for evaluating their reliability. The model data studies suggest that multiple and primary events not included in the regression may be considered part of the noise term without seriously affecting the accuracy of the computed spectral ratios. The method is tested on field data from the following sedimentary environments off the coast of Oregon and northern California: a continental shelf basin, an abyssal plain environment, the base of the continental slope and two locations on the Astoria sea fan, one near the Cascadia sea channel and one north of DSDP site 174. Velocity versus depth and frequency dependent spectral ratio plots are determined for each environment. The computed surface layer interval velocity of 1.77 km/sec over a thickness of 455 m for the station north of DSDP site 174 is in good agreement with the average material type found in the drill core (sandy-silt with greater than 60% sand). Maximum attenuation coefficients are estimated from the spectral ratios for the upper sediment intervals of the study areas using typical acoustic impedance values of surface sediment types determined from nearby piston cores. Some maximum attenuation coefficients are too high suggesting the possibility of a stratigraphic component. The maximum attenuation in the upper interval for SB 46 over the Tufts abyssal plain where fine-grained material (silts and clays) is expected is 0.025 dB/m at 127 Hz compared with 0.004 dB/m at 80 Hz for the upper interval of the turbidite environment north of DSDP site 174.
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  • File scanned at 300 ppi (Monochrome, 256 Grayscale, 24-bit Color) using Capture Perfect 3.0.82 on a Canon DR-9080C in PDF format. CVista PdfCompressor 4.0 was used for pdf compression and textual OCR.
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