Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Offshore prospecting
Reexamination Certificate
1997-10-31
2001-03-13
Barlow, John (Department: 2857)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Offshore prospecting
C367S021000, C367S013000, C367S024000, C367S135000, C367S043000, C367S911000, C181S110000
Reexamination Certificate
active
06201764
ABSTRACT:
TECHNICAL FIELD
This invention relates in general to seismic surveys and, more specifically, to a method and apparatus for matching outputs of hydrophones to output of geophones using digital signal processing filters.
BACKGROUND OF THE INVENTION
Known methods of performing seismic surveys for formations under a body of water must overcome some unique problems, such as reflections at the air/water interface. When water depths increase beyond 30 to 40 feet, the period of the received reflection from the air/water interface and subsequent reverberations in the water column become too large for deconvolution algorithms to remove. If this energy is not removed from the processed data, reflecting horizons are unnecessarily complicated and fault planes are obscured.
Known methods employed to remove the water-column reverberations use simultaneous data from both hydrophones (pressure) and geophones (velocity) sensors. At certain places on the ocean bottom, typically called stations, a hydrophone and geophone are placed close to each other as a pair. When a water column reverberation arrives at the geophone/hydrophone pair, the hydrophone reacts with a polarity opposite that of the geophone. Therefore, when the pressure and velocity signals are summed unwanted reverberations should be significantly eliminated. However, the signals are not effectively eliminated because of the difference between signals from geophones and hydrophones. For example, at low frequencies a problem is that a geophone is a second order system with a dB roll-off approximately two times the hydrophone's dB roll-off, which is a first order system. Therefore, combining the measured response of a hydrophone with the measured response of the geophone results in poor cancellation of the unwanted reverberations.
Known methods of causing the hydrophone to behave like a second order system require using an inductance or transformer. The transformer inductance resonates with the hydrophone capacitance to produce a second order response resembling the response of the geophone in its low frequency amplitude and phase characteristics. However, using the transformer introduces additional variables that add distortion to the signal, especially at low frequencies. For example, the transformer inductance changes with signal level, thus causing a signal phase shift.
Furthermore, known methods have increased errors because the capacitance of hydrophones vary from hydrophone to hydrophone. Accordingly, the response of one hydrophone is not identical to the response of another hydrophone, even though the two hydrophones are at the same temperature.
Therefore what is needed is a method and an apparatus for modifying the response of a hydrophone to match the response of a geophone and significantly reduce response variations between hydrophones caused by inherent characteristics.
SUMMARY OF THE INVENTION
Accordingly, a method and an apparatus are provided for matching the response of a hydrophone to the response of a geophone and significantly reducing response variations between hydrophones caused by inherent characteristics. To this end, the method includes the steps of determining a capacitance of the hydrophone in its operating environment, configuring a filter based on the hydrophone capacitance to produce a transfer function, and modifying the response of a hydrophone channel using the filter. The apparatus includes a processing unit for generating a seismic image, a recording unit coupled to the processing unit for storing data and the seismic image, a filter coupled to the processing unit for correcting data, and an acquisition unit coupled to the filter for measuring the response of the formation to the seismic energy.
An advantage of the present invention is that the effects of and errors due to variations between hydrophone are significantly reduced.
Another advantage of the present invention is that the first order response of the hydrophone is transformed to a second order response like that of a geophone, thereby significantly improving water reverberation cancellation.
REFERENCES:
patent: 4979150 (1990-12-01), Barr
patent: 5163028 (1992-11-01), Barr et al.
patent: 5235554 (1993-08-01), Barr et al.
G.M. Hoover et al., “The Influence of the planted geophone on seismic land data”, Geophysics, vol. 45, No. 8, pp. 1239-1253, Aug. 1980.
Christine E. Krohn, “Geophone ground coupling”, Geophysics, vol. 49, No. 6, pp. 722-731, Jun. 1984.
T. H. Tan, “Reciprocity theorem applied to the geophone-ground coupling problem”, Geophysics, vol. 52, No. 12, pp. 1715-1717, Dec. 1987.
Barr et al.—A Dual-Sensor, Bottom-Cable 3-D Survey in the Gulf of Mexico, Sep. 23, 1990.
Carroll Paul Ellington
Ragan Thomas Glen
Rice Charles Geoffrey
Barlow John
Haynes & Boone
Headley Tim
Input / Output Inc.
Jolly Anthony
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