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.

Shear‐wave imaging using cross‐dipole acoustic logging tool

“…The imaging application in the past primarily focused on using compressional (P) waves from a typical full-wave monopole acoustic tool (e.g., Hornby, 1989;Li et al, 2002). Because the monopole tool is unable to determine the azimuth of a reflector, directional acoustic sensing techniques need to be developed.…”

Application of the equivalent offset migration method in acoustic log reflection imaging

“…The acoustic imaging technique was developed in the 1980's and has improved with the advances of logging instruments and processing methods in recent years (Hornby 1989). By utilizing the wave energy radiating into the formation from the borehole, the acoustic reflection image log can detect the wave fields reflected from near-borehole fractures or small structures.…”

Experimental study of the azimuthal performance of 3D acoustic transmitter stations

“…The acoustic waves reflected by the near-borehole interfaces with non-continuous acoustic impedance are obtained through acoustic reflection logging (Hornby 1989;Ellis et al 1996;Esmersoy et al 1997Esmersoy et al , 1998Chang et al 1998;Yamamoto et al 1998;1999). Migration imaging techniques that are similar to those used in seismic exploration are then employed to visualize small-scale geologic structures from a few meters to dozens of meters away from boreholes (Yamamoto et al 2000;Tang 2004;Pistre et al 2005;Li et al 2008;Chai et al 2009;Tang et al 2007).…”