Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data

摘要

We have estimated the seismic attenuation in gas hydrate and free-gas-bearing sediments from high-resolutionP-cable 3D seismic data from the Vestnesa Ridge on the Arctic continental margin of Svalbard. P-cabledata have a broad bandwidth (20–300 Hz), which is extremely advantageous in estimating seismic attenuation ina medium. The seismic quality factor (Q), the inverse of seismic attenuation, is estimated from the seismic dataset using the centroid frequency shift and spectral ratio (SR) methods. The centroid frequency shift methodestablishes a relationship between the change in the centroid frequency of an amplitude spectrum and theQ value of a medium. The SR method estimates the Q value of a medium by studying the differential decayof different frequencies. The broad bandwidth and short offset characteristics of the P-cable data set are usefulto continuously map the Q for different layers throughout the 3D seismic volume. The centroid frequency shiftmethod is found to be relatively more stable than the SR method. Q values estimated using these two methodsare in concordance with each other. The Q data document attenuation anomalies in the layers in the gas hydratestability zone above the bottom-simulating reflection (BSR) and in the free gas zone below. Changes in theattenuation anomalies correlate with small-scale fault systems in the Vestnesa Ridge suggesting a strong structuralcontrol on the distribution of free gas and gas hydrates in the region. We argued that high and spatiallylimited Q anomalies in the layer above the BSR indicate the presence of gas hydrates in marine sediments in thissetting. Hence, our workflow to analyze Q using high-resolution P-cable 3D seismic data with a large bandwidthcould be a potential technique to detect and directly map the distribution of gas hydrates in marine sediments