Ocean Bottom Receiver Location: A Model-Based Solution To Locating Receivers On The Ocean Bottom By Using Geophysical Data

Abstract

ABSTRACT The ocean bottom cable method (OBC) of seismic data acquisition involves placing stationary receivers on the ocean floor and using an air gun array towed by a separate boat as an energy source. The location of each receiver group after deployment is a function of water depth, prevailing water currents and the dynamics of the cable falling through the water. Ocean Bottom Receiver Location (OBRL) positions the receivers accurately by utilizing first break information, estimated water and water bottom velocities, shot locations and water depths. The major advantages of the OBRL method are speed and accuracy of the solutions and that no additional equipment is required for locating the receivers. In addition, the near real time processing of the data allows for adjustments in the shooting and deployment patterns to assure proper subsurface coverage. INTRODUCTION The ocean bottom cable (OBC) method and its use for 2-D and 3-D surveys has been discussed previously (Rigs by et al., 1987). This method is currently used in water depths ranging from very shallow to 100 meters and is not impeded by obstructions such as drilling rigs, production platforms, or boat traffic. Figure 1 depicts a recording boat anchored on prospect and attached to the receivers on the water bottom via a telemetry cable. The gun boat towing a short air gun array is separate. This configuration allows for work in obstructed areas and versatility with the acquisition geometry. The minimum configuration of an OBC crew is:The source boat towing only an air gun array and thus Highly maneuverablethe cable boat, responsible for cable deployment and retrieval,the recording boat carries the dog house and all recording Instrumentationa chase boat, used as a water taxi and picket duty along The deployed cable. Stationary receivers on the water bottom allow for versatility with the acquisition geometry. Patch, swath or radial designs are commonly used in 3-D surveys. The swath technique involves laying out approximately 24 km of bottom cable in one or two lines and recording multiple shot lines parallel to the eceiver cable as shown in figure 2. Figure 3 depicts a patch geometry in which several receiver cables (in this case 4) are laid out with multiple shot lines orthogonal to the receiver lines. To make full use of the acquisition geometry, receiver and shot positions are predetermined on a map before the survey commences. The cable boat deploys the receivers at these locations, however, currents and other factors cause the receiver positions on the water bottom to be different than that of the surface drop position. It is necessary in an OBC survey to locate the receivers on the water bottom. OBRL locates the receivers accurately by using first break analysis recorded by the receiver groups and a single air gun or air gun array as a source