Depth Calibration of DAS VSP Channels - A New Data-Driven Method

Summary Distributed Acoustic Sensing has revitalized VSP acquisition in recent years, but an issue that concerns all DAS data is depth calibration — i.e., establishing an accurate relationship between DAS channel depths and formation depths. While the depth of DAS channels is quite accurately known along the optical fiber, the position of the fiber with respect to the formation can be uncertain for a number of reasons. We propose using receiver scalars derived in DAS VSP processing for direct referencing of DAS channel depths to geology. This works because there is a relationship between DAS amplitudes and the medium velocity and density. An absolute depth calibration requires sonic and density logs to be available for at least a part of the DAS fiber depth range. In time-lapse VSP applications, the DAS channel scalar curves can be used for depth alignment of DAS VSP vintages even in the absence of an absolute depth reference such as well logs. Additional envisioned applications include compaction effect studies and in-situ detection of hardening/softening around the well.