Abstract

Distributed acoustic sensing (DAS) using fiber-optic cables is a recent addition to seismic acquisition methods. However, a DAS “sensor” differs significantly from conventional, discrete sensing devices such as geophones or accelerometers. For one, DAS measures something akin to strain instead of particle velocity or acceleration. Other properties of the DAS system also aren't obvious at first. What is its instrument response, noise performance, and repeatability? How are DAS channels properly positioned, e.g., in case of a borehole deployment: depth calibrated? To better understand these issues and their impact on the DAS seismic method's application space, a field test was conducted in which three DAS vendors recorded the same survey using a borehole-installed fiber while recording simultaneously with a conventional downhole array. The results show that all DAS systems achieved good, repeatable signal integrity while exhibiting different noise characteristics. DAS noise can be addressed with well-established processing algorithms, but further benefits can be gained from DAS-specific algorithms. Where required, DAS seismic data can be processed to closely match the vector response of conventional geophones. DAS data converted in this way can assist in the up/down separation step without the need for dip filters. DAS VSP data can also be merged with conventional 3D and 4D seismic, adding value in situations such as undershooting of surface facilities in marine settings.

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