Imaging of near‐borehole structure using full‐waveform sonic data

Abstract

The full waveforms recorded by an array of receivers in a modern borehole sonic tool contain secondary arrivals that are reflected from near‐borehole structural features. These arrivals are used to form an image of the near‐borehole structural features in a manner similar to seismic migration. Possible uses of this technique include horizontal well logging; structural dip and contour determination; fault, salt dome, pinnacle reef, and fracture zone imaging; and EOR steam‐flood monitoring. Since both the source and the receivers pass through structures that cross the borehole, the downdip structure and the updip structure can be imaged separately. The technique involves a backprojection of the recorded data into a matrix of accumulation bins representing distances radially out from the borehole and along the borehole axis. Separate matrices are formed for the updip and for the downdip raypaths. The basic technique is illustrated with synthetic data, generated to approximate the case of a sonic tool logging through a dipping bed boundary. Results are shown for a borehole experiment performed in Alaska. The data were acquired with a research sonic prototype tool and specially recorded with a long acquisition time—20 ms per trace instead of the normal 5 ms. This longer acquisition time enabled the acquisition of scattered P and S arrivals to be recorded after most of the direct signal had died out. Images are shown of near‐borehole structural features to a distance of 18 m from the borehole. The images are presented against an independently derived formation lithology analysis and a high‐resolution synthetic seismic display computed from the measured density and slowness logs.