Communications – electrical: acoustic wave systems and devices – Signal transducers – Vibrator-type transmitter
Reexamination Certificate
1999-06-21
2001-01-30
Pihulic, Daniel I. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Signal transducers
Vibrator-type transmitter
Reexamination Certificate
active
06181646
ABSTRACT:
DESCRIPTION
The present invention relates to a geophysical exploration system having a seismic source which provides a composite sweep over a multi-octave frequency band. A composite sweep, as provided in accordance with the invention, is a FM sweep of sinusoidal or continuous wave (CW) signals which sweep over different portions of the frequency band during the same interval of time. The system provided by the invention may be used in marine environments or on land and is capable of producing acoustic transmissions of sufficient power into an earthen strata to detail the geophysical properties of the strata to a desired depth, usually for seeking evidence of petroleum reservoirs.
Acoustic transmissions for geophysical exploration are generally provided by an FM sweep of frequencies over a frequency band extending from a low frequency and covering a bandwidth of typically 3 to 6 octaves. The sweep may have a duration of the order of 10 seconds (e.g. from 5 to 20 seconds) and is usually repeated as the platform which moves over the land or the boat deploying the acoustic transmitting system progresses along a survey line. Acoustic reflections from the earthen strata are received by a hydrophone in the case of marine environments or a geophone in the case of land surveys. An array of such hydrophones or geophones may be used. The signal is recorded and analyzed as by means of correlation processing and seismograms depicting the earthen strata are generated. The low frequencies in the transmission are important for deep penetration of the acoustic waves in the earth, while the high frequencies are important for resolution of the interfaces between strata having different signal propagation characteristics.
Seismic vibrator sources have been proposed which use different sources for different portions of the bandwidth. See Mifsud, U.S. Pat. No. 4,295,213 issued Oct. 13, 1981 and Ward, U.S. Pat. No. 4,823,326 issued Apr. 18, 1989. It has also been proposed to transmit different portions of the frequency band successively in successive time increments. See Mayne, U.S. Pat. No. 4,004,267 issued Jan. 18. 1977. It has also been proposed, in order to reduce the sweep time from the time required for discrete sweep signals which have been transmitted successively, to transmit those signals at the same time as a combined signal. However, in no case are the amplitudes of the signals adjusted in order to provide a flat spectrum of transmitted energy, that is, to create a constant amplitude acoustic pressure or spectral level. Moreover, each spectral portion has been processed separately and not as a composite signal extending over the entire frequency band.
Where a sweep has extended over a large frequency band and is generated in a single source or where separate sources have been used for the low frequencies, such sources have incorporated large radiating pistons (radiators) which can vibrate over large strokes. To maintain a constant acoustic energy flux density over the sweep, the ability for the source to handle large, physical amplitudes of motion, to cover the low frequencies of the band, has been required. More power is also needed to drive the vibrator at low frequencies and high stroke levels, which, in the case of hydraulically driven vibrator sources, requires large pumps in order to handle the flow to support the large strokes.
It is the principal object of the present invention to provide an improved geophysical exploration system and particularly to improve such system by improving the seismic vibrator source therein so that a composite sweep can be transmitted with generally the same energy spectrum (a generally constant spectrum) over the entire frequency band of multiple octaves while significantly reducing the peak power (and flow demand in case of hydraulic sources) and the size of equipment associated with the sources.
The invention provides a vibrator source having a composite sweep that enables the minimization of source size and radiator stroke for a given sweep frequency range and acoustic source level requirement. In a system embodying the invention, the spectral energy density is made essentially constant by utilizing different amplitudes for the lower and higher frequency sweeps which sweep at different rates (rate of change of frequency) over the same time interval to provide the composite sweep.
Specifically, the amplitude of the sweep which covers the lower, (approximately the first octave) of the bandwidth is reduced with respect to the amplitude of the sweep extending above the first octave, in proportion to the square root of the ratio of the sweep rate of the higher frequency sweep to the sweep rate of the lower frequency sweep. In other words, since the sweep periods are equal, the lower frequency sweep amplitude is reduced by the square root of the bandwidth of the upper frequency sweep to the ratio of the bandwidth of the lower frequency sweep.
It will be understood that the sweeps which cover the lower and upper frequency portions of the band can change frequency in the same direction, say upwardly or downwardly or one sweep can change in frequency upwardly while the other changes downwardly. In all cases, a composite sweep signal is generated and transmitted by the seismic vibrator.
Consider for example the generation of a composite sweep in accordance with the invention extending from 5 Hz to 200 Hz. This frequency band is divided into two sweep components which run concurrently. In this example, the first sweep component covers the lowest octave, 5 to 10 Hz and the second sweep component covers the remaining range of the bandwidth from 10 Hz to 200 Hz. Now if both sweeps are linear and extend over a typical time duration of 10 seconds, the rate of the sweep for the first sweep component is 5 Hz/10 SEC or ½ Hz per second while the rate of the second sweep component is 190 Hz /10 SEC or 19 Hz per second. Since the spectral energy density transmitted by the vibrator is proportional to the square of the acoustic pressure per unit frequency (P
2
/Hz ), different sweep rates require different acoustic pressure levels to achieve the same spectral level of interest. For a flat spectrum (essential constant) the sweep component from 5 to 10 Hz must have a an acoustic level reduced by the square root of the ratio of the second sweep bandwidth to the first sweep bandwidth or the square root of 190/5 (or 16 dB) relative to the amplitude of the sweep component extending from 10 to 200 Hz. For an equivalent spectral level, a single linear sweep from 5 to 200 Hz would therefore require a spectral amplitude approximately 16 dB higher in the 5 to 10 Hz band than the composite sweep case.
Since the radiation displacement growth over the octave from 10 Hz to 5 Hz , for constant acceleration, is 12 dB, the 16 dB reduction achieved by the above exemplary composite sweep format enables the 5 to 10 Hz sweep to require an amplitude less than the amplitude required at the 10 Hz end of the 10 to 200 Hz sweep. Accordingly, the composite sweep enables a significant reduction in the maximum stroke required by the vibrator source, thereby significantly reducing its size, weight and input power demands, while maintaining a constant spectral level at the low end of the desired frequency spectrum. The composite sweeps are not limited to two components nor do any of the components have to extend over a certain frequency range (such as an octave). The invention provides a composite set of sweeps which provide a spectral energy level of interest while enabling source size displacement amplitudes and power supply, (for example, pump and flow requirements) to be minimized.
REFERENCES:
patent: 4004267 (1977-01-01), Mayne
patent: 4295213 (1981-10-01), Mifsud
patent: 4339810 (1982-07-01), Nicholas et al.
patent: 4458339 (1984-07-01), Wason
patent: 4823326 (1989-04-01), Ward
Bouyoucos John V.
Hollinger David P.
Hyroacoustics Inc.
Lukacher M.
Pihulic Daniel I.
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