The DSI 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.

The DSI combines new dipole-based technology with the latest monopole developments into one system, providing 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 DSI tool is a multireceiver tool with a linear array of eight receiver stations, a monopole transmitter and two dipole transmitters. 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.

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. Low frequency penetrates deeper into the formation and helps read beyond altered zones.

Tool operating modes

The DSI 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.

The available modes are:

• Upper and lower dipole modes
• Crossed dipole mode
• Stoneley mode
• P and S mode
• First-motion mode

Tool Specifications

Temperature Rating	350° F (175° C)
Pressure Rating	20 kpsi (13.8 kPa)
Tool Diameter	3 5/8 in (9.2 cm)
Tool Length	51 ft (15.5 m)
Sampling Interval	6 in (15.24 cm)
Max. Logging Speed	900-3,600 ft/hr
Minimum Hole Size	5.5 in (13.9 cm)
Maximum Hole Size	21 in (53.3 cm)