Summary

Wide-azimuth, far-offset, refracted first breaks are too often eschewed as an alternative, redundant, inexpensive ocean-bottom seismic (OBS) positioning technology. I document their viability despite adverse bathymetry and anisotropy

Introduction

There are many ways to position Ocean Bottom Seismic (OBS) receivers. Dedicated, high-frequency, positioning acoustics are probably the most common way ... and the most expensive in time and equipment. Seismic airgun water-arrival, first break positioning lines are also possible. Extra time is required, but no extra equipment. Unfortunately, the first-break observable is much cruder than the dedicated acoustic observable and first-break picking delays are difficult to calibrate. Therefore, direct water-arrival first breaks are not the same as dedicated acoustics. A third technique is to use wide-azimuth, faroffset production seismic data. This is the cheapest way since no positioning lines are required. Vastly more data are available than in water-arrival first break positioning, so the statistics of large numbers make up for the coarse quality of the first-break observation. Because data are observed at all azimuths and offsets, picking delays are easily calibrated. Far-offset seismic data arrive through the water and also through one or more faster refractors. Therefore, refracted first breaks are subject to geological velocity gradients that must be calibrated. Is this the reason that refracted first-break OBS positioning has lost popularity since the 1990s?

At the time of writing this paper FairfieldNodal were conducting a multi-client Ocean Bottom Node (OBN) survey in about 2200 meter depth in Walker Ridge in the Gulf of Mexico. OBNs in this survey have more than 300,000 source events in 12-second records. These copious data can be divided into multiple, mutually-exclusive, amply-populated zones (rings) to investigate the repeatability of a wide-azimuth, far-offset, first-break positioning algorithm that calibrates the velocity gradient in the refractors. This paper reports on that investigation.

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