Reverse Time Migration of Vertical Cable Seismic Data to Image Hydrate-Bearing Sediments With High Resolution

Abstract

Marine vertical cable seismic (VCS) is a promising survey technique for submarine complex structure imaging and reservoir monitoring, which uses vertical arrays of hydrophones deployed near the seafloor to record seismic wavefields in a quiet environment. Recently, we developed a new type of distributed VCS system for exploration and development of natural gas hydrates preserved in shallow sediments under the seafloor. Using this system and air-gun sources, we accomplished a 3D VCS yield data acquisition for gas hydrates exploration in the Shenhu area, South China Sea. In view of the characteristics of VCS geometry, we implement reverse time migration (RTM) on a common receiver gather to obtain high-resolution images of marine sediments. Due to the unique acquisition method, it is asymmetrical for the reflection path between the sources and the receivers in the VCS survey. Therefore, we apply accurate velocity analysis to common scatter point (CSP) gathers generated from common receiver gathers instead of the conventional velocity analysis based on common depth point gathers. RTM with this reliable velocity model results in high-resolution images of submarine hydrate-bearing sediments in deep water conditions. The RTM imaging section clearly shows the bottom simulating reflector (BSR) and also the reflection characteristics of the hydrate-bearing sediments filled with consolidated hydrates. Moreover, its resolution is relative to that of acoustic logging curves from the nearby borehole, and this imaging section is well consistent with the synthetic seismogram trace generated by the logging data. All these results reveal that VCS is a great potential technology for exploration and production of marine natural gas hydrates.

Hydrate-Bearing Sediment Imaging of Ghost Reflection in Vertical Cable Seismic Data Using Seismic Interferometry

“…Data Processing. In 2017, we obtained a 3D VCS dataset natural gas hydrate exploration in Shenhu area, South China Sea, using the distributed vertical cable seismic (DVCS) acquisition system [4], conducted by the Guangzhou Marine Geological Survey Bureau [15]. This dataset consists of 12 Here, we mainly use receiver ghost reflection from the shallowest hydrophone to image hydrate-bearing sediments across the line 39 (Figure 2), which is the closest line to the VC.…”

“…Vertical cable seismic (VCS) exploration method makes use of vertical hydrophone arrays (VHA) to image subsurface target in geologically complex area [1][2][3]. Generally, several hydrophones are assigned on the seismic cable with equal interval, and this cable was vertically suspended in seawater under the joint action of buoys and anchor [4]. Actually, vertical cable is moored on the seafloor so that it can avoid some common marine noise such as swell in deep seawater.…”

“…Actually, vertical cable is moored on the seafloor so that it can avoid some common marine noise such as swell in deep seawater. Consequently, VCS can provide high-quality seismic images beneath the seabed [4]. As vertical cable is deployed on the seabed, various marine sources can be used to shoot along seismic surveying line with any azimuth [5].…”

“…Derived from defence research, the VCS method was successfully applied to conduct offshore seismic survey, for highquality subsalt imaging [3,7], for low-angle fault scarp imag-ing and assessment of remaining exploration potential [8], for hydrothermal deposit exploration [5], for seafloor massive sulphide survey [9][10][11][12], and for natural gas hydrate exploration [4,[13][14][15]. All these applications reveal that the VCS technique is an effective approach to exploiting and developing the complex offshore oil and gas fields, which can provide superior seismic image compared with conventional marine seismic method.…”

“…Accurate velocity fields can be estimated by performing velocity analyzing on common scatter point (CSP) gathers. In order to overcome the drawback of the irregular field geometry for VCS, the accurate velocity model can be directly applied to image marine hydrate-bearing sediments by reverse time migration [4]. Besides, VSP processing method can be used for VCS data processing [17] because of its geometry similar to the VSP.…”