Improvement of Seafloor Coupling and Suppression of Flow-Induced Noise
Authors: Sullivan L.B. 
Published: October 2008

Marine seismic exploration using co-located hydrophores and geophones on the seafloor as acoustic detectors has become more common in recent years. This method of exploration offers many benefits, but has suffered from poor marine geophone performance. A method for improving this performance by stabilizing the geophone and adjacent wiring andcabling whi/e increasing the sensor response to seafloor particle motion has been developed and utilized successfully during exploration under hostile conditions and high currents.

INTRODUCTION

Seismic exploration in shallow water is a natural extension of exploration in marshes and swamps, and many of the methods employed in modern wetland and shallow water seismic surveying differ only slightly from those used for over four decades. Sources of seismic energy have evolved over the years from dynamite charges to vibratory mechanical devices to air guns, and all of these sources are still in use worldwide. Acoustic receivers deployed today in these types of exploration consist of hydrophores and geophones, sensors that are identical in principle albeit refined in design to those in use for over 50 years. Recording systems have advanced with advances in digital electronic technology, but regardless of whether the data are conducted from sensor to recorder via electrical, light or radio transmission, the acousticdetectors must be distributed spatially and attached to wires and cables before any exploration can begin.

Shallow water data acquisition, with hydrophone receivers located on the seafloor, has historically provided the means to extend land exploration seaward to the water depth where deep marine seismic vessels towing hydrophone streamers can maneuver safely. In recent years similar shallow water exploration techniques have allowed seismic operations to be performed in congested deeper water areas where obstructions such as wells and platforms prevent the safe operation of large seismic vessels. But water column reverberation, the repetitive reflection ofacoustic energy from the water surface and the seafloor, is always a drawback to this form of exploration, The first order reverberation of this type is known in the industry as the receiver ghost; and the ghost and subsequent reverberations destructively interfere with the frequency content of seismic data in a depth-dependent manner.

A few years ago a new operations technique to obtain geophysical data on land, referred to as the "patch method", was introduced that had the potential to provide good 3D seismic imaging in an economical manner through the use of large spatial arrays of sensors and relatively few source locations,1,2 At about the same time, a method to attenuate water column reverberation in marine data acquisition was introduced that offered a way to enhance the quality of seismic data acquired with seafloor-located sensors independent of depth. This attenuation, suggested by ecognition that reverberation effects on the frequency data from seafloor hydrophores and geophones were complementary, was achieved by collecting data from co-located sensors of both types and combiningthese data in processing to recover the missing requency components.

With the expectation that full frequency bandwidth data could now be obtained from shallow water seismic acquisition.