Borehole Seismic Methods: Estimating Anisotropy And The Distributed Acoustic Sensing (DAS) To Geophone Transformation

Several vertical seismic profile (VSP) methods are addressed with the aim of improving accuracy of the results or extracting new insights from the data. An effective horizontal transverse isotropy (HTI) framework was built to relate Thomsen’s parameters to fracture density and fracture fluid. Direct shear energy present on the horizontal components of vertical-vibrator zero-offset VSP data on two field data examples was used to characterize the fast-shear azimuth along well depth. Parametric wavefield decomposition was used for the strong anisotropy case and the rotation-correlation method was adapted for the weak anisotropy case. The results correlated with independent log measurements and studies. Synthetic signatures of azimuthal VSP data generated from weakly, moderately, and heavily fractured model show that fracture response of dry fractures is stronger than that of fluid-filled fractures. Similar signatures using a dipping interface within isotropic and fractured models show that isotropic subsurface structures can produce an apparent fracture response and they can distort the true fracture response. A structure-consistent orientation workflow was developed to correct for the structural effect and uncover the true fracture response. Beyond conventional VSPs, a transformation was developed to convert distributed acoustic sensing (DAS) measurements to conventional velocity/acceleration measurements. A theoretical framework was developed to show that DAS data are inherently filtered in spatial frequency and amplitude. Synthetic and field data examples are analyzed to show that DAS measurements have a detrimental effect on traveltime picking, Q-estimation, and imaging. These effects are negated by the DAS-to-velocity transform. The methods developed in this dissertation are being actively used to deliver commercial products in the field.