Positioning Ocean-Bottom Seismic Cables
Authors: Partridge Simon , Zinn Noel 
Published: October 2008

Due to advantages of flexibility in acquisition geometry (especially around obstructions), greater surface consistency, resistance to layer reverberations, reduced noise and better coverage, ocean bottom cable (OBC) seismic surveys are gaining popularity in deeper and deeper water. Seismic detector positioning methods are not yet standardized in the industry. We discuss two common techniques that employ very different technology: the use of high-frequency acoustic sensors attached to the detectors and positioned independently of the seismic survey and the use of multiple occasions of the onset of seismic energy (first breaks) as surveying observations in a positioning algorithm. These techniques are both competitive and complementary.

An acoustic transceiver, mounted on a survey vessel, sequentially interrogates groups of transponders attached to ocean-bottom cables within acquisition range. By measuring in hardware the two-way travel times and having knowledge of the speed of sound in water, precise ranges are determined from all quadrants around the transponder. Knowing the vessel's position at the time of each interrogation allows transponder positions to be computed in real time or in post processing.

Seismic energy arrives at the detector by a variety of refracted or reflected paths. The fastest arrival (first-break) can be picked by automated methods that choose a significant change in the amplitude or inflection of the seismic energy. With some empirically-determined knowledge of the velocity field, the time of a first-break pick can be related to distance. Distances can be processed in a positioning algorithm such as batch or sequential least-squares estimation.

We present a case study from South East Asia that demonstrates the excellent agreement of these disparate technologies.

Overview of Ocean-Bottom and Transition-Zone Seismic Surveying

Reflection seismology is the primary geophysical method employed in the search for hydrocarbon accumulations. Seismic data are collected on land, at sea and in the transition zones in between land and sea. On land, seismic geophone sensors embedded in the earth and energy sources under the earth or at the surface are positioned conventionally or with the Global Positioning System (GPS). At sea, often called deep-marine seismic, seismic energy sources (typically air guns) and seismic hydrophone sensors towed behind a vessel are positioned by a variety of devices that include GPS, magnetic compasses, underwater acoustics, lasers and optical shaft encoders. In the transition zone of shallow surf, mud and alligators, we use any combination of these technologies that gets the job done.

In the marine environment, ocean bottom cable(OBC) surveying, in which the seismic cable is laid on the bottom, not towed near the surface, is gaining popularity. Some of the advantages of OBC over towed streamer surveys are the flexibility of acquisition geometry, greater surface consistency (i.e., more combinations of source and detector at different azimuths for a given midpoint, useful for resolving static delays and for amplitude compensation), more flexibility in working around obstructed zones, the use of dual sensors to remove ghosts and layer reverberations, reduced noise by eliminating cable vibration and strumming caused by towing and surface weather conditions, and better coverage due to the elimination of cable feather caused by currents.