PathFinder Sonic Services

Compressional slowness measurements

PathFinder CLSS compensated long-spaced sonic tool takes two compressional slowness measurements for natural fracture identification, seismic time-to-depth correlation, acoustic porosity determination, pore pressure evaluation, and formation strength and drillability information. Measurements are computed downhole using a semblance technique and are available for transmission to surface while drilling. For enhanced processing, full waveforms are stored in tool memory for retrieval when the tool is tripped to surface.

PathFinder CLSS collars have two transmitters, one 4 ft above and the other 4 ft below a four-receiver array with 1-ft spacings. This configuration provides significant measurement redundancy and the opportunity for borehole compensation. To indicate quality, an ultrasonic transducer at the center of the receiver array measures standoff.

Slimhole sonic tool

For logging in hole sizes as small as 6 in, the PathFinder SCLSS slim compensated long-spaced sonic tool provides compressional and shear porosity measurements. When combined with the PathFinder HDS-1L fixed-collar directional service and the PathFinder SCWR slim compensated wave resistivity tool, the PathFinder SCLSS tool supplies a complete set of formation data for 6-in holes.

Shear in slow formations

PathFinder eSONIC LWD tool expands the measurement range to include shear in slow formations. Two unipole transmitters excite compressional and shear body waves in hard rock and compressional and mixed borehole-guided waves in soft formations. The PathFinder eSONIC tool uses two frequencies:

• 15 kHz for compressional-arrival processing and shear analysis in hard rock
• 5 kHz to enhance the borehole-guided mode and process guided waves in softer rock

To compute shear slowness in slow formations, PathFinder data-computing centers complete the processing using an advanced dispersion correction algorithm. Recent remodeling of the unipole tool response enables dispersion correction using only compressional and guided wave slownesses, plus mud weight, as inputs to a look-up table. Some advantages of the new dispersion model are that density and caliper curves are no longer required, and that there is no need to try to isolate a pure flexural mode because the dispersion correction uses a mixed-mode model.