Full Wave Form Inversion of High Frequency OBH-Data from the Yaquina Basin off Peru
Abstract
The lower boundary of the methane hydrate stability zone in continental margin sediments is often marked by a strong, phase reversed reflection subparallel to the seafloor, called the bottom simulating reflection (BSR). High resolution multichannel seismic (MCS) data from the Yaquina Basin offshore Peru at 8 deg S show a BSR that is varying laterally in amplitude as well as in continuity. The amplitudes of the reflections above the BSR also vary with the appearance of the BSR. Where the BSR is strong, the reflections above it are weaker than where the BSR is weak. Also although the strong part of the BSR is underlain immediately by strong reflections, reflections several hundred meters beneath the BSR appear weaker than those where the BSR is weak. This variation indicates significant heterogeneity in the distribution of gas and gas hydrate on this area. Chemoherms occurring in the Yaquina Basin show the presence of free gas in the sediments up to the seafloor. The presence of gas and gas hydrate within the sediment significantly influences the p-wave velocity in the affected layers, so a detailed analysis of velocity variations can help us to understand the apparently different conditions for the formation of gas hydrate along the BSR and the migration paths of the free gas. Ocean bottom hydrophone (OBH) data from profiles coincident with the MCS data can provide such detailed velocity depth information. These data were acquired using two 1.7l airguns with a spatial sampling of 20m. The signal contains frequencies of up to 250Hz, which leads to an expected vertical resolution of about 2m. One-dimensional full waveform inversion using reflectivity synthetic seismograms in the intercept-time-slowness-domain has proved an effective method for extracting detailed velocity information from BSRs (e.g. Singh et al., 1993). We present preliminary results of the application of this technique to selected OBH profiles from the Yaquina Basin where lateral variability is sufficiently small for the one-dimensional approach to be valid. Singh, S.C., Minshull, T.A., Spence, G.D., 1993, Velocity Structure of a Gas Hydrate Reflector, Science, Vol.260,204-207
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.T22B1161N
- Keywords:
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- 8105 Continental margins and sedimentary basins