Borehole acoustic reflection surveys (BARS) are not new to the industry; however, since the introduction of Schlumberger’s Sonic Scanner acoustic scanning platform in late 2005, the

Hydro needed to improve its definition of the Brent formation in the Troll area of the Norwegian North Sea. The high-resolution result shows a 5° dipping event that is visible for at least 45 ft (13.7 m) into the formation. The dip of the event is in agreement with the expected local geology at the well location. The high-resolution event can be correlated to a 3.2-ft (1-m) thick coalbed at the same depth position, which is indicated by the petrophysical logs. The coalbed is interpreted as the Top Etive formation. (Images courtesy of Schlumberger)
quality of the recorded raw waveforms has taken significant step forward. Hence, the potential of obtaining high-resolution images around the well bore has increased accordingly. The objective of BARS is to capture the energies transmitted by the three monopoles and reflected at either bed interfaces, fractures or possible faults and attenuate the (primary) compressional, shear and Stonely waves.

The Sonic Scanner tool has eight banks of 13 receivers; every monopole “shot firing” provides 104 waveforms. It implies that at every depth position, there are 3 by 104 waveforms available and all can be used for the BARS survey. The BARS acquisition does not require a separate run. The only requirement is that the logging speed is adjusted to allow for storage of the data at each point. Depending on geometry, one monopole (MF, MU or ML) may contain more reflections then the other.

The Sonic Scanner in BARS mode typically uses a source frequency of 8 kHz. This provides the basis for high frequency results. The depth (range) of investigation is smaller in BARS compared to surface and borehole seismic survey, but the resolution is significantly higher. Where resolutions for surface and borehole seismic are between 33 ft and 330 ft (10 m and 100 m), possible resolution obtained from a BARS image can be less then 3 ft (1 m), coming close to log resolutions.

BARS can now help analysts do the following:
• Obtain high-resolution directional images around and away from the borehole;
• Confirm well placement relative to formation topography;
• Identify high resolution sub-seismic interbeds, faults or fractures;
• Analyze reservoir structure and characterization;
• Establish distance to reservoir boundaries; and
• In productions wells, when provided with a turn-around time as short as 24 hours, may help identify reservoir features of importance for the well completion.

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