DSI Dipole Shear Sonic Imager

Combined Monopole and Dipole Sonic Acquisition

The DSI dipole shear sonic imager combines monopole and dipole sonic acquisition capabilities. The transmitter section contains a piezoelectric monopole transmitter and two electrodynamic dipole transmitters perpendicular to each other. An electric pulse at sonic frequencies is applied to the monopole transmitter to excite compressional- and shear-wave propagation in the formation. For Stoneley wave acquisition a specific low-frequency pulse is used. The dipole transmitters are also driven at low frequency to excite the flexural wave around the borehole.

The tool is made up of three sections— acquisition cartridge, receiver section, and transmitter section. An isolation joint is placed between the transmitter and receiver sections to prevent direct flexural wave transmission through the tool body.

The receiver section has an array of eight receiver stations spaced 6 in [15.24 cm] apart and 9 ft [2.74 m] from the monopole transmitter, 11 ft [3.35 m] from the upper dipole transmitter, and 11.5 ft [3.50 m] from the lower dipole transmitter. Each receiver station consists of two pairs of wide-band piezoelectric hydrophones aligned with the dipole transmitters. Summing the signals recorded by one pair of hydrophones provides the monopole waveform, whereas differentiating them cancels the monopole signal and provides the dipole waveform. When a dipole transmitter is fired, the hydrophone pair diagonally in line with the transmitter is used. Four sets of eight waveforms can be acquired from the four basic operating modes fired in sequence.

A special dipole mode enables recording both the inline and crossline (perpendicular) waveforms for each dipole mode. This mode, called both cross receivers (BCR), is used for anisotropy evaluation.

The optional S-DSI modification to the DSI tool uses a special slow sleeve to extend the slowness measurement to 1,200 ms/ft [3,937 ms/m] from the standard 700 ms/ft [2,296 ms/m].

As part of ABC analysis behind casing suite of services, the DSI tool can also provide a measurement of slowness behind the casing by using recently developed acquisition strategies and BestDT automated sonic waveform processing.

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