Abstract: In a two-stage, time-division multiplexing method and apparatus, a plurality of sensors are disposed at equal intervals along a plurality of transmission channels. An interrogation signal is transmitted from a recording station to all of the sensors along each channel. Because of the spatial arrangement of each sensor along the telemetric system and different propagation path lengths along each channel, the output signals from the sensors are time-division multiplexed along each channel. The multiplexed signals along each channel are time-division multiplexed a second time upon arrival at the recording station where the signals from each channel are interleaved with each other.

Abstract: A dual mode vibrator assembly for selectively generating either pressure wave or shear wave signals in the earth has a single inertia mass with a plurality of vertically parallel offset cylinder bores and at least one horizontal cylinder bore extending therethrough. Each cylinder bore reciprocably retains a piston member and is in fluid communication with a fluid source. The lower ends of the vertical piston members are connected to a base plate to generate pressure waves. A foot member is connected to each end of the horizontal piston member and interconnected by anti-roll rails in sliding relationship with the inertia mass. Cleats are connected to the bottom of each foot member to engage the earth for generating shear waves. A manifold directs fluid from a servo-valve either to passages leading to the vertical cylinder bores for pressure wave operation or to the horizontal cylinder bore for shear wave operation.

Abstract: An apparatus for preventing thermally-induced stress fatigue in a vibrator support housing consists of a stress compensator that absorbs thermal dimensional changes between major structural components made of dissimilar materials. The stress compensator consists of a hydraulic clamp wedged between the free end of a vibrator actuator piston shaft and the supporting housing. Actuated by the substantially-constant system hydraulic pressure, the hydraulic clamp maintains a substantially constant clamping force between the actuator piston shaft and the supporting housing. Thermally-induced stresses are absorbed by the hydraulic system.

Abstract: An optical telemetric system for use in a borehole consists of a bidirectional optical fiber to which are coupled a plurality of acousto-optical seismic sensors. The sensors consist of an optical cavity that becomes resonant at certain wavelengths depending upon parameters of cavity length and index of refraction. Those parameters are capable of being modified on the basis of static and dynamic pressure differences within the borehole. A swept-wavelength laser chirp pulse is launched into the bidirectional optical fiber. The static pressure at each sensor establishes a resonant wavelength that serves as a carrier signal. Dynamic pressure changes due to seismic waves, modulate the carrier signal. The modulated carrier signals from each sensor are reradiated through the bidirectional optical fiber in a wavelength-division multiplexed format. The multiplexed signals are received by and demultiplexed by a suitable signal receiving apparatus.

Abstract: A seismic data recording system includes a central station and a plurality of remote seismic data recording units that are deployed within a desired region relative to the central station. The differential locations of the remote recording units relative to the central station are determined by receiving and recording electromagnetic positioning signals at the central station. A portion of the electromagnetic positioning signals is concurrently received and recorded at the respective remote recording units. The portion of the positioning signals recorded at each remote recording unit is combined with the recorded positioning signals from the central station to determine the required differential locations.

Abstract: A method for maintaining a substantially constant tension on a towed seismic cable, wherein the instantaneous towing force and the instantaneous cable displacement relative to a fixed shipboard reference are continously measured and sampled. A microprocessor associated with a servo controlled capstan device, uses those measurements to pay out cable or to retrieve cable in response to an increase or decrease in the instantaneous towing force. The microprocessor also tries to minimize the average cable displacement.

Abstract: A quantity of a solution of a long-chain polymer is injected into the water around an air gun. The resulting dilute solution of the long-chain polymer tends to reduce turbulence surrounding the air bubble that is created when the gun is fired.

Abstract: An acoustic borehole tool consists of two components: A first component containing power supplies, signal processing and control electronics and a sidewall locking mechanism. A second component includes a seismic sensor and a clamping device for pressing the sensor against the sidewall of a borehole. The weight of the second component is a small fraction of the weight of the first component so that the force exerted by the clamping device is less than a preselected limiting value and so that the clamping force/mass ratio may reach large value without danger of destroying the borehole.

Abstract: A system for maintaining a substantially constant tension on a towed seismic cable, wherein the instantaneous towing force and the instantaneous cable displacement relative to a fixed shipboard reference are continuously measured and sampled. A microprocessor associated with a servo controlled capstan device, uses those measurements to pay out cable or to retrieve cable in response to an increase or decrease in the instantaneous towing force. The microprocessor also tries to minimize the average cable displacement.

Abstract: A seismic vibrator for shaking the ground is driven by an input sweep signal. A control signal, which is a function of the output force level of the vibrator, modulates the sweep signal to limit the output force level to a value that will prevent ground decoupling. At the beginning of a sweep, an adaptive preset signal over-rides the control signal during a desired initial time portion of the sweep signal. The preset signal for a given sweep is derived from a sample of the control signal that existed during a specified portion of an immediately previous sweep.

Abstract: A method for optimizing the acoustic signal-to-noise ratio during a useful maximal-length recording cycle within the time frame of a minimal-length recording episode. To implement the method, during an intermittent-tow seismic operation, the noise level due to cable manipulation during an entire recording episode is monitored. A quiet-window of a desired length is established by adjusting the length of the interval during which the seismic cable is at rest. The initiation of a recording cycle is adjusted relative to the quiet window to take maximum advantage of the quiet period.

Abstract: A hydrophone streamer that includes several arrays of optical fiber pressure sensors. Each array consists of at least three sensors symmetrically disposed around the inside of the streamer skin to form a vertically-disposed array. Each sensor modulates a coherent light beam in accordance with the instantaneous ambient water pressure. The output signals of the sensors include an AC component due to seismic waves and a DC component due to hydrostatic pressure difference between the sensors of an array. Means are provided to resolve the AC and DC components to determine the arrival direction of the received seismic waves.

Abstract: Selected sections of a seismic streamer cable are provided with an acoustically-triggered compressed gas supply. The acoustic triggering devices associated with each compressed gas supply may be individually addressed by a coded signal. In the event that the streamer cable is severed, the gas supplies of the severed sections of the streamer are used to inflate the jackets of those severed sections so that they will float to the water surface for salvage.

Abstract: The compound accelerometer is made by combining two simple accelerometer units. The simple accelerometer units consist of a hollow container closed at each end by a piezo-electric transducer. The two transducers are oppositely polarized and are selected to have a desired combined series capacitance value. The container is partially filled with a volume of heavy liquid and the accelerometer is excited. During excitation, the output voltage is continuously measured and the volume of liquid is microadjusted until the output voltage equals a prescribed level. The outputs of the individual transducers are measured, the residual voltage difference is tabulated and the dominant transducer is identified. Two simple accelerometer units having the same residual voltage difference are secured together with the dominant transducers facing oppositely with respect to each other.

Abstract: A hermaphroditic connector-half for coupling together adjacent streamer-cable sections consisting of an outer plastic shell that can be slid over a stress plate that carries a plurality of multicontact connector plugs. For maintenance purposes, the outer shell can be removed so that free access may be had to the contacts of the connector plugs and to the multiple conductors in the streamer cable.

Abstract: In accordance with an embodiment of this invention, voltages are received from the pilot valve torque motor, the main servo valve and the actuator indicating the value of the torque motor current, spool position and actuator displacement. The received voltages are compared to a preselected threshold value. When one or more of the voltages exceed a threshold value, an attenuator control voltage is generated. An attenuator circuit is coupled between the reference driver signal generator and the DC servo amplifier. The attenuator control voltage, if present, is applied to the attenuator circuit and scales down the reference driver signal as a function of the error signal level thereby to limit the torque motor current and consequently the displacements of the main servo valve spool and actuator shaft.

Abstract: A method for manufacturing a compound accelerometer that is maximally sensitive to a desired acceleration vector to the exclusion of orthogonal vectors. The compound accelerometer is made by combining two simple accelerometer units. The simple accelerometer units consist of a hollow container closed at each end by a piezo-electric transducer. The two transducers are oppositely polarized and are selected to have a desired combined series capacitance value. The container is partially filled with a volume of heavy liquid and the accelerometer is excited. During excitation, the output voltage is continuously measured and the volume of liquid is microadjusted until the output voltage equals a prescribed level. The outputs of the individual transducers are measured, the residual voltage difference is tabulated and the dominant transducer is identified.

Abstract: An air gun of the reciprocating shuttle type includes a split-ring sleeve valve that is expandable against the inner wall of a firing chamber. The sleeve valve encircles the shuttle body. The shuttle body and the sleeve valve include a set of air release orifices that are alignable with a corresponding set of air exhaust ports of the firing chamber.

Abstract: A spring-suspended mass has top and bottom reflectors. A laser light beam is divided into separate beams that are reflected from the top and bottom reflectors into photo detectors. At the photo detectors, a reference beam from the same laser is recombined with the two reflected beams to produce two interference fringe patterns in response to seismic disturbances of the mass. A count of the number of fringes and fractions thereof that are detected over a unit time interval is proportional to velocity of the mass in terms of the wavelength of the laser beam. The phase difference between the two interference fringe patterns is a measure of polarity of the seismic disturbances.

Abstract: A cross country vehicle for seismic exploration is provided wherein a first drive means, such as wheels can be conveniently interchanged, in the field, for a second drive means having a different configuration such as endless tracks. The interchange is enabled by use of a pre-existing lifting means associated with a seismic acoustic source.