82 Well seismic surveying and acoustic logging Robertson Geologing and Geovista have also developed a probe with hydrophones. The gap between the sources and receivers is 2 to 3 m and the frequency is in the range of 100 to 1,000 Hz. The source is a horizontal electromagnet that produces a pressure wave in the borehole fluid (electrodynamic source). At the borehole wall, this pressure wave is converted into P and S seismic waves that travel radially away from the wall of the hole. These waves are reconverted into pressure waves in the borehole fluid and detected by the receivers (vertical component for the P-wave; horizontal component for the S-wave). An acoustic tool is characterized by: • the type of system: – monopole: transmission frequency 10-40 kHz – dipole: transmission frequency 1-3 kHz • transmitter and receiver type: – magnetostrictive – piezoelectric • number of transmitters and receivers: – standard, with one or two transmitters and two receivers – receiving antenna with four to eight receivers • distance between receivers: from ten to fifty centimeters • transmitter offset relative to the first receiver: from one to five meters • mechanical characteristics: – rigid framework – flexible framework • time sampling interval: – 5 or 10 μs for a monopole tool; – 20 μs for a dipole tool • listening time: – 2 or 5 ms for refracted mode analysis; – 10 ms or more for reflected mode analysis Figure 3.1 left shows a monopole acoustic tool, that is flexible and has a small diameter (50 mm), which is used for geotechnical borehole studies but also for acoustic measurements in the oil sector. The transmitter is magnetostrictive (transmission frequencies: 17-22 kHz). It can be equipped with two pairs of receivers (both near receivers (1 - 1.25 m) and far receivers (3 - 3.25 m)). The acoustic examples presented in Chapter 5 were obtained using this tool, developed by P. Gaudiani. Figure 3.3 shows examples of acoustic tools.
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