How multibeam sonar swath bathymetry is improving seismic

Topography, water density affect data

Kenneth J. Lambert
PGS Exploration (US)

• Multi-beam bathymetry survey acquired over 193 OCS blocks in the Garden Banks area, Gulf of Mexico. [54,021 bytes]

Echo sounding is a technique for measuring water depths by transmitting acoustic pulses from the ocean surface and listening for their reflection (or echo) from the seafloor. This technique has been used since the early twentieth century to provide depth input to charts that now map most of the world's water-covered areas.

Until the early 1960's, most depth sounding used single-beam echo sounders (fathometers). These devices make a single depth measurement with each acoustic pulse (or ping). Today, multi-beam sonar systems map more than one location on the ocean floor with a single ping and with higher resolution than those of conventional echo sounders. These systems map a contiguous area of the bottom, usually a strip of points in a direction perpendicular to the path of the survey vessel. Successive pings together in one vessel traverse will collect a swath of bathymetry data.

Multibeam systems consist of projector and hydrophone arrays, installed under the hull of the vessel, that transmit and receive sonar beams. Within the vessel is instrumentation that records and processes the data. Post-processing of the data accounts for roll, pitch, and yaw of the vessel as the bathymetric data is acquired, as well as final positioning (x, y, z). The data is then output to a grid to produce images of the seafloor with detail and accuracy. Geologic conditions of the seafloor such as fault scarps, landslides, mud volcanoes, and dramatic topographies are some typical features imaged.

Survey vessels today can acquire and achieve onboard real-time multi-beam processing and display of bathymetry data in a manner impossible only a few years ago. All indications are that multi-beam bathymetry data will have a wide variety of applications not only as a stand-alone product but also in support of integrated processes in seismic exploration and production. Below are examples of present and potential applications.

Hazards assessment

In the Gulf of Mexico, all oil companies are required to submit a shallow hazards report and obtain approval from the Minerals Management Service (MMS) for well sites and pipeline locations. This report identifies, maps, and assesses seafloor and sub-seafloor geologic hazards. It also identifies possible seep zones that might affect the permitting and drilling of exploratory wells.

An accurate mapping of the water bottom is necessary to define seafloor conditions. Multi-beam bathymetry provides accurate mapping of the seafloor through means independent of the bathymetry derived from 3D seismic data.

Recent interaction with MMS indicates that multi-beam bathymetry, coupled with high-resolution 3D seismic data, is preferred for deepwater hazard assessments. In this application multi-beam bathymetry data is most useful for engineering and production departments of oil companies and their drilling and pipeline contractors.

Water column velocity

One of the by-products of the multi-beam process is the generation of the velocity field of the water column. It is by this variable that a final and accurate depth value is derived. This field in itself can be valuable to exploration geophysicists.

Historically, in seismic processing, the velocity of the water column has always been applied as a constant. In fact, that is not the case. The velocity of sound in water is affected by pressure, temperature, and salinity (or density), and has some effect on ray bending and wave propagation.

In seismic processing, one can, with high confidence, resolve the sediment velocity at and beneath the seafloor. However, an error in the water column portion of the velocity function will result in mispositioned data at and beneath the seafloor. In one recently reported case, an apparent syncline was re-imaged as an anticline, once the correct water column velocity was applied.

Depth imaging is a process involving the ray tracing of thousands of rays through modeled layers. It is based on modeled velocity assumptions of the shallow layers. If these assumptions are inaccurate, including the water column, an accumulative error exists and the final output can be less than optimal.

Many processing parameters are applied as a function of the water bottom such as gains, mutes, deconvolution windows, etc. and therefore an effort is made to define a table of control points that map the water bottom. This can require considerable effort on the part of the seismic processor. With the availability of multi-beam bathymetry this effort can be reduced thus saving valuable processing time and producing superior results.

Gravity corrections

Bathymetry can have a significant effect on gravity work. Bathymetry data is used in the process of making Bouguer corrections. This variable, if incorrect, can result in an inaccurate definition of salt thickness and "base of salt."

Standard fathometer bathymetry does not provide a continuous bathymetric surface whereas multi-beam swath bathymetry does. It is a conclusion strengthened by more areal sampling and a statistical correction process.

Multi-beam swath bathymetry improves 3D Bouguer gravity corrections by as much as 70% in water depths less than 300 meters and by about 25% in water as deep as 1,000 meters. Using accurate multi-beam bathymetry will optimize gravity work.

To date, Petroleum Geo-Services, Inc has installed multi-beam sonar equipment on three of the vessels now working in the Gulf of Mexico and has acquired over 550 OCS blocks of bathymetry data. Most of this data was acquired using a Seabeam Instruments 2112 (12 kHz) system designed to operate in water depths ranging from 125 meters to 11,000 meters.

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