Downhole logging tools

The Dipole Shear Sonic (DSI-2) tool combines high-speed telemetry with simultaneous, 12-bit dynamic range digitization of an eight-receiver array. The sonde incorporates both monopole and crossed-dipole transmitters with an eight-station array of electronically configurable hydrophones for monopole and dipole reception. Combining new dipole-based technology with the latest monopole developments into one system provides the best method available today for obtaining borehole compressional, shear and Stoneley slownesses (slowness is the reciprocal of velocity and corresponds to the interval transit time measured by standard sonic tools).



Dipole technology allows borehole shear measurements to be made in "soft" rock as well as "hard" rock formations. Limited by borehole physics, monopole tools can detect only shear velocities that are faster than the borehole fluid velocity -- or in hard rocks only. Dipole tools overcome this fluid velocity barrier.

The receiver array provides more spatial samples of the propagating wavefield for full waveform analysis. The arrangement of the transmitters and receivers allows measurement of wave components propagating deeper into the formation.

The DSI-2 tool differs from the DSI previously used in ODP by an upgraded receiver section. The upgrade improves the shear measurements in slow formations.

 

The DSI-2 tool has several data acquisition operating modes, any of which may be combined to acquire digitized waveforms over each 6-in. logging interval. For waveforms, eight channels are digitized simultaneously with a 12-bit dynamic range.

Upper and lower dipole modes
Eight dipole waveforms from firings of either of the dipole transmitters -- sampling every 40 microsec, 512 samples/waveform.

Crossed dipole mode
Standard acquisition of 32 total waveforms, in-line and cross-line from both transmitters -- sampling every 40 microsec, 256 samples/waveform.

Stoneley mode
Eight monopole waveforms from firings of the monopole transmitter driven with a low-frequency pulse -- sampling every 40 microsec, 512 samples/waveform.

P and S monopole mode
Eight monopole waveforms from firings of the monopole transmitter driven with a high-frequency pulse -- sampling every 10 microsec, 512 samples/waveform.

First-motion mode
Eight sets of monopole threshold-crossing data from firings of the monopole transmitter driven with a high-frequency pulse -- primarily for compressional first-arrival applications.

New fast tool bus and data reduction techniques have allowed double the maximum logging speed in most instances.

A switchable power regulator has enabled a one-third reduction in power needs, resulting in broader combinability with other tools.

Additional human-interface engineering has improved field acquisition quality and efficiency.

A new low-frequency transmitter driver improves signal-to-noise ratio and allows successful logging of extremely slow formations and greatly enlarged holes.

Improved waveform processing techniques have greatly improved vertical resolution.

New answer products utilize Stoneley slowness to evaluate fractures and indicate permeability.

In addition to the new dipole features, acquisition of the Stoneley wave velocity utilizes a low-frequency monopole energy pulse for highest-quality Stoneley measurements. Stoneley-derived permeability is useful for evaluating fractures as well as investigating deeply into the formation.

A new technique for detecting compressional wave arrival--digital first-motion detection (DFMD)--provides measurements that are compatible with previous sonic logs, in addition to a 6-in (15 cm) vertical resolution compressional sonic.

Processing with the MAXIS wellsite unit displays a full wave and its component characteristics. Its high-speed array processor uses the slowness-time-coherence (STC) method to determine compressional, shear and Stoneley slowness values. A choice of band-pass filters permits utilization of the optimum frequency range within a mode. The process reliably provides unambiguous transit times even in difficult borehole conditions. The resulting values are useful inputs for mechanical properties, formation evaluation and seismic applications.

For details on tool components and acoustic wave propagation click here.

Depth of Investigation/Eccentering Effects

Depths of investigation for sonic devices depend on the formation type, shear and compressional slowness, the transmitter-to-receiver spacing, wavelength of the wave considered and whether it is a head wave or a guided wave, the source frequency and signal types.

Frequency determines the wavelength that drives the depth of investigation of the measurement.

Typical sonic wavelengths at different frequencies and slownesses are shown in the "Additional Specifications" table. Low frequency penetrates deeper into the formation and helps read beyond altered zones.

Numerical simulations verified by measurements from scale models show that when eccentering is small compared to the borehole radius, there is little change in the character of the dipole waveforms or in the STC-processed slowness values. Large eccentering, on the order of 2 to 4 in (5-10 cm) in a 12-in (31 cm) borehole, increases the flexural wave amplitude relative to the compressional. For the DSI-2 tool, the variation in the shear slowness estimate is ± 2 percent over the normal slowness range.

Slowness or velocities can be plotted alongside resistivity, density, or image data. For an example of DSI data, click here.

Deployment Notes

Stuck/lost tool information

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