Methods and apparatus for use in generating and transmitting acoustic signals
First Claim
1. For use in the transmission of a train of repetitive or quasirepetitive acoustic signals into a medium of finite tensile strength in response to an input control signal, the method of minimizing the negative dynamic pressure or stress excursions of said acoustic signals with respect to the average pressure or stress present in the absence of said signals and enabling the maximization of the power of said transmission in a given frequency band, said method comprisingcontacting said medium with a movable member, andapplying in direct response to said control signal forces to said member in opposite directions with respect to the average force on said member due to said average pressure or stress to place said medium alternately under compressive and tensile forces with the time duration of the application of the compressive force short compared with the time duration of the application of the tensile force and with the waveform of said tensile forces being approximately rectangular in shape, said compressive force time duration being in excess of one-half period of the upper frequency limit of said frequency band, the lower frequency limit of said band being approximately the repetition frequency of said train of repetitive or quasirepetitive acoustic signals.
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Abstract
Methods and apparatus describedmaximize, within a predetermined frequency band, the continuous or near-continuous transmission of broadband acoustic power into a medium having a finite tensile strength such as water or earth. Such methods and apparatus also minimize, for a given broadband acoustic power level in a given frequency band, the negative dynamic pressure or stress excursion of an acoustic signal with respect to the average pressure or stress present in the absence of the signal. Such signals may be transmitted for geophysical exploration purposes. The signals place the medium alternately under compressive and tensile stresses with respect to some average stress, where the time duration of the application of the compressive stress is short compared with the time duration of the application of the tensile stress. The tensile stress does not exceed the tensile strength of the medium or its interfaces. For highest efficiency, the minimum time duration of the compressive stress is in excess of one-half period of the upper frequency limit of the desired spectral band of the signal, while the maximum time duration of a contiguous tensile and compressive stress pair is approximately one period of the desired lower frequency limit of the band. The tensile stress waveform is preferably rectangular in shape.
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Citations
31 Claims
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1. For use in the transmission of a train of repetitive or quasirepetitive acoustic signals into a medium of finite tensile strength in response to an input control signal, the method of minimizing the negative dynamic pressure or stress excursions of said acoustic signals with respect to the average pressure or stress present in the absence of said signals and enabling the maximization of the power of said transmission in a given frequency band, said method comprising
contacting said medium with a movable member, and applying in direct response to said control signal forces to said member in opposite directions with respect to the average force on said member due to said average pressure or stress to place said medium alternately under compressive and tensile forces with the time duration of the application of the compressive force short compared with the time duration of the application of the tensile force and with the waveform of said tensile forces being approximately rectangular in shape, said compressive force time duration being in excess of one-half period of the upper frequency limit of said frequency band, the lower frequency limit of said band being approximately the repetition frequency of said train of repetitive or quasirepetitive acoustic signals.
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9. For use in generating a train of seismic signals, in response to a train of input control signals, and for transmitting said signals into a medium, either land or water, a seismic signal source which comprises
a housing having a chamber, a coupling member for radiating said signals into said medium, a piston connected to said coupling member, said piston being disposed in said chamber, said piston dividing said chamber into regions on opposite sides of said piston, means for applying a static force to said medium via said coupling member and for applying spring-like forces to said housing and coupling member for maintaining them at an equilibrium position with respect to each other, means responsive to said control signals for switching pressurized hydraulic fluid between said regions on opposite sides of said piston for applying switched forces, of duration corresponding to the duration of said control signals, via said piston to said coupling member during successive intervals of time one of which is short with respect to the other, which actuate said coupling member with greater acceleration toward said medium during said shorter period of time and with smaller acceleration away from said medium during said longer period of time, said periods of time being inversely proportional to said force durations whereby to generate and transmit said seismic signals with the positive pressure or stress of said seismic signals being larger in amplitude than the negative pressure or stress of said signals, with respect to the average pressure or stress in the medium in the path of transmission of said seismic signals therein.
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10. For use in generating, in response to a control signal, acoustic signals such as seismic signals having energy within a predetermined frequency band and for transmitting said signals into a medium, a source of said signals which is operated by a flow of pressurized hydraulic fluid which is at supply and return pressures, said source comprising
(a) at least one coupling member having a surface defining an interface with the medium, said member being movable in opposite directions at said interface; -
(b) means coupled to said coupling member for applying to said member a static force directed toward said medium; (c) hydraulically operated actuating means for said member including (i) at least one housing having a chamber for said pressurized hydraulic fluid, (ii) a piston in said chamber, said piston being connected to said coupling member and being movable in said opposite directions therewith, (iii) means for applying a force on said coupling member with respect to said housing for maintaining said housing at a equilibrium position with respect to said coupling member, (iv) said piston defining at least one variable volume region in said chamber, said piston having an area facing said region of such size that the force on said coupling member in the direction toward said interface when the hydraulic fluid in said region is at one of said supply and return pressures exceeds the force on said coupling member in the direction away from said interface when the hydraulic fluid in said region is at the other of said supply and return pressures, and (v) means responsive directly to said control signal for switching the hydraulic fluid pressure in said region to provide hydraulic fluid at said one of said pressures and at said other of said pressures during successive intervals of time, said interval of time when said hydraulic fluid in said region is at said one of said pressures being short as compared to said interval of time when said hydraulic fluid in said region is at the other of said pressures whereby the time duration of the force on said member in the direction toward said interface is less than the time duration of the force on said member in the direction away from said interface, said duration when the force on said coupling member is in the direction toward said interface being in excess of one-half period of the highest frequency in said band and the duration of the sum of said successive intervals of time being approximately one period of the lowest frequency in said band. - View Dependent Claims (11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 30)
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29. A seismic source operated by pressurized hydraulic fluid for transmitting seismic signals having n amplitude levels into a medium where n is an integer greater than one, said source comprising
a housing having a bore, a piston reciprocally movable in said bore, One of said housing and said piston having a coupling member at one end thereof, said member being arranged to contact said medium, said piston and bore each having V steps which are spaced longitudinally of each other, one of said piston and housing steps facing in a direction away from said medium, said steps defining with said bore and said piston wall V chambers in said bore, and V control valve means for applying said pressurized hydraulic fluid at supply and return pressures in predetermined sequence to said chambers, whereby the number of levels n, equals 2V.
Specification