Direct measurement of water velocity and tidal variations for improved 4D repeatability in marine seismic acquisition

Abstract

Geophysicists concur that the quality of marine seismic data is often constrained by geometric variation in source and receiver locations, as well as water velocity uncertainty, caused by the multitude of environmental effects that prevail at offshore field locations. Such uncertainties make significant contributions to the nonrepeatability of marine time-lapse seismic data, and affect the quality of 4D data processing and interpretation. In order to reduce some of these uncertainties we have developed a seafloor instrument called a Pressure Inverted Echo Sounder (PIES). This instrument measures tidal variation and average water velocity at regular intervals. When placed at multiple locations across the survey area the instrument enables determination of the spatial and temporal variations in water velocity throughout each seismic survey and across multiple surveys as necessary. The resulting data can be used to reduce the seismic direct arrival inversion process to a very accurate positioning and timing exercise. In this article, the authors aim to describe the process of measuring tidal variation and water velocity including a description of the instruments used, an examination of the data processing steps required, and an analysis of the results provided by various deployments at Shell-operated fields.