Detailed Imaging of Deepwater Hydrate Geology With Horizontal Arrays of Seafloor Sensors

Abstract

Abstract This paper describes new procedures for processing data acquired with 4C seismic sensors distributed as horizontal arrays on the seafloor and seismic sources positioned either at the sea surface or on the seafloor. Two types of images can be produced from 4C seismic data: a P-P image and a P-SV (converted-shear) image. For P-P imaging, the large elevation difference between deep-water seafloor sensors and air-gun sources at the sea surface allows P-P data to be processed with algorithms similar to those used to process vertical seismic profile (VSP) data. This novel VSP-type approach toprocessing deep-water seafloor sensor data provides optimal resolution of near-seafloor geology and does not appear to be documented in the literature. For P-SV imaging, low S-wave velocities in near-seafloor strata result in submeter wavelengths, even though signal energy from a surface-based air gun does not exceed 100 Hz. As a result, P-SV data image seafloor layering as thin as 1 m over subseafloor depths of several tens of meters. Combining high-resolution P-P and PSV images produced along horizontal seafloor arrays allows improved understanding of deep-water gas hydrate geology. P-P Imaging Conventional processing of deep-water 4-C seismic data for oil/gas exploration involves a wave-equation datuming step that transforms the data to a domain in which sources and receivers are on the same depth plane. This data transformation removes the water layer and allows seafloorsensor data to be processed as if both sources and receivers are on a flat seafloor. After this adjustment of source-receiver geometry, deep-water multicomponent seismic data can be processed using software developed for shallow-water environments where marine multicomponent data acquisitiontechnology was originally developed and applied. A new approach to P-P imaging of near-seafloor geology using deep-water multicomponent seismic data is not to eliminate the large elevation difference between sources and receivers but to take advantage of that elevation difference. This data-processing approach allows deep-water, horizontal-array, multicomponent data to be processed in a manner similar to the way vertical seismic profile (VSP) data are processed because VSP data acquisition also involves large elevation differences between sources and receivers (Fig. 1). Figure 1. Illustration of similar source-receiver geometries used for acquiring (a) VSP data and (b) deep-water, horizontal-array seismic data. (available in full paper) In this data-processing strategy, a seafloor hydrophone response (P) and a seafloor vertical-geophone response (Z) are combined to create downgoing (D) and upgoing (U) P-P wavefields using the expressionsD = P + Z/cos? andU = P - Z/cos? ? defines the incident angle at which the downgoing compressional wave arrives at the seafloor. Once this wavefield separation is done, deep-water multicomponent seismic data are defined in terms of downgoing and upgoing wavefields, just as VSP data are. Subseafloor P-P reflectivity can then be determined by taking the ratio U:D in the frequency domain, which is a deconvolution using the downgoing wavefield.