Previous studies of crosswell seismic data have mostly used tomographic inversion operating on direct traveltimes to obtain velocity information about the medium between the boreholes. A study is presented here for the analysis and processing of the full waveform of crosswell seismic data. This work uses direct arrivals to obtain velocity information and reflected arrival to construct an image in depth.
Two data sets are considered through the development of the method; a synthetic data set and a field data set from the Midale field of southeastern Saskatchewan, Canada. The field data were acquired by Shell Development Company for Shell Canada Ltd. as part of their EOR monitoring studies. Raw field data showed a complex assortment of wave modes that included direct compressional and shear waves, head waves, converted transmitted waves, and reflected shear waves.
A traveltime inversion technique (layer-stripping via raytracing) is developed to obtain P- and S-wave interval velocities from their respective direct arrivals. Field data inversion results showed good agreement between the seismic and sonic velocities. Traveltimes generated from forward modeling using the estimated (1-D) velocity functions also showed favorable agreement when compared to the observed (picked) traveltimes. Error analysis gave a maximum relative error of 1.45% in one particular interval in the P- wave function. The estimated P and S velocities were in general agreement with measurements of Vp/Vs reported in the literature.
Techniques of processing crosswell seismic data to construct a subsurface image using the reflected wavefields are developed here. Most of the processing steps are conventionally used in VSP data processing. This allows processing of crosswell data to be carried out using existing software. The processing flow for crosswell data includes band-pass, median, and f-k filtering steps. The subsurface coverage of the crosswell geometry is derived based on a constant-velocity model and found to cover zones past the midpoint between the boreholes. From a reconstruction technique, called here XHLCDP, reflected wavefields are used to construct a reflected image similar to that of the VSP transformation procedure. A final section is produced by summing all the individual reflection images. This section provides a detailed description both laterally, in the subsurface between the boreholes, and vertically, with depth resolution on the order of 1 m.
Synthetic seismograms are used to interpret the final sections. Their use has led to the identification of many subsurface horizons at and around the zone of interest. In this case study, the crosswell images have shown higher resolution of strata than those depicted by the synthetic seismograms. Finally, the coverage of the crosswell experiment promises to complement and extend the interpretation of well logs.
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