Communications – electrical: acoustic wave systems and devices – Signal transducers – Receivers
Patent
1991-04-10
1992-12-15
Eldred, J. Woodrow
Communications, electrical: acoustic wave systems and devices
Signal transducers
Receivers
367182, 367185, 181122, H04R 900
Patent
active
051723450
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a geophone system for seismic exploration with a plurality of geophones each consisting of a mechanical transducer with an electronic processing circuit, and connected with a central processing station by means of a transmission line.
For measuring seismic signals in such a system, use is made of seismometers or geophones. These geophones are, generally, passive analog sensors connected in series in groups and are connected with a measuring station. By a movement of the geophone, a voltage is induced in a coil which is movably suspended in the magnetic field of a permanent magnet. To achieve a high sensitivity, the mass of the magnet is large, which unfavourably influences the coupling at high frequencies between the geophone and the ground in which it is implanted. The analog connection between the geophones and the measuring station is sensible for disturbances by external electro-magnetic fields.
In the processing station, the analog output signal of the geophones is amplified, sampled and digitized. Because of the high demands put on the resolution, the analog/digital converter and the anti-alias filter required to that end are so sensitive to component tolerances that manufacturing in IC-technology is almost impossible.
Not properly operating geophones can cause much damage to the collected seismic data, which damage will become only apparent during data processing, and recovery thereof is, then, not possible anymore. In order to prevent this, geophones are tested in the field.
From EP-A0264509 (Schlumberger) a system for exploration of bore holes is known comprising transducers in which the position of an inertial mass is detected by optical means, the latter controlling, via an amplifier, the current through the coil of the transducer in question for repositioning the mass in the initial position.
However, the operation of such a transducer is dependent on its orientation in the gravitational field, which, moreover, leads to a wide measurement range. For stability reasons, the loop gain of the processing circuit must be low. Such a system is already not very suitable for seismic exploration in the field, but the small loop gain makes it impossible to digitalize such a system as, then, a high loop gain is required for suppressing the quantization noise. Digitalization is very favorable in view of suppression of signal disturbances in the line or lines toward the central data processing station and also has other advantages as to the data processing and the grouping of the various sensors in the overall system.
From an article by Z. Yin and M. J. Usher entitled "A High Resolution Wide Band Digital Feedback System for Seismometers", J.Phys. E. Sci. Instr. 21 (1988) 748-752, an acceleration transducer with a digital output is known. This transducer uses a capacitive sensor to detect the displacement of an inertial mass, an analog/digital converter to produce a digital signal, and a magnetic coil force-feedback transducer controlled by a digital/analog converter. By changing the parameters in a computer program, gain ranging and variation of the bandwidth can be achieved. To prevent instability of the transducer system, pole-zero cancellation is used. Pole-zero requires high component accuracy, and is not suitable for a system with a high loop gain and a large bandwidth which is to manufactured in IC-Technology. The reduction of quantization noise in this system is therefor comparatively small. Moreover this known system is not intended for seismic exploration in the field by means of a plurality of distributed sensors, but is intended for seismological surveys relating to earthquakes and similar geological phenomena, and this system comprises only one sensor in a laboratory setup which is directly connected with a data processing device.
From U.S. Pat. No. 3,429,189 (H. F. Krabbe) an accelerometer is known producing a digital output signal. The sensor assembly consists of a sensor element determining the position of an inertial mass and a drive coil exerting a repositioning forc
REFERENCES:
patent: 2776560 (1957-01-01), Erath et al.
patent: 3088062 (1963-04-01), Hudihac
patent: 3429189 (1969-02-01), Krabbe
patent: 4051718 (1977-10-01), Meckl et al.
patent: 4159464 (1979-06-01), Hall, Jr.
patent: 4412317 (1983-10-01), Asjes et al.
patent: 4517514 (1985-05-01), Howell
Journal of Physics E Sci. Instrument, vol. 21, No. 8, 1988, Z. Yen et al.: "A High-resolution wideband digital feedback system for seismometers", pp. 748-752.
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