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: A mixed device for successively generating in the earth, at the same location, longitudinal and shear waves is disclosed. The device comprises in association a target coupled to the earth surface and having a first striking face substantially parallel thereto and at least one second striking face substantially perpendicular to said first striking face. A guide tube is adapted to be positioned above the first striking face and perpendicularly thereto, and a movable weight is slidable in the guide tube and adapted to strike the first striking face. At least one hammer is articulated to a support rigidly secured to the guide tube and adapted to strike one second striking face of the target. Directional coupling means connect the guide tube to the target in such a manner that the weight sliding in the guide tube directly impinges the first striking face, but such that the guide tube is substantially released from the effect of the hammer impact on a second striking face.

Abstract: An air gun is mounted in a reaction mass having an impulse-transmitting ground-contact plate movably mounted below the mass. This mass is in face-to-face contact with the plate over the major portion of the entire upper surface of the contact plate. Confined liquid is not employed. The air gun is charged with pressurized gas and fired, discharging the pressurized gas into the narrow horizontal interface region between reaction mass and plate. Rapid expansion of the discharged gas between mass and plate causes an intensely powerful seismic impulse to be transmitted into the earth. The air gun has a firing chamber for holding a charge of pressurized gas, a movable shuttle for suddenly discharging the pressurized gas, including upper and lower pistons with a hollow member connecting these pistons. A pedestal assembly carrying said shuttle is mounted from above to the reaction mass and extends down into the hollow shuttle member.

Abstract: A target member of the seismic energy source is driven into ground surface engagement, as the vehicle, to which the seismic energy source is pivotally connected, moves, due to the target member geometry, that is by resolving the tractive force so that the force driving the target member into the ground is a vector directed at an angle less than 90.degree. to the direction of the vehicle movement. Impulses from an impulse generator are transmitted to the target member after the latter has been driven into the ground, during the vehicle movement or at rest. The impulses are transmitted to propagate in a plane parallel to the ground surface and perpendicularly to the direction of the target member movement.

Abstract: A method and system are provided for well velocity shooting and vertical seismic profiling utilizing mobile seismic land sources for transmitting powerful seismic energy impulses into the earth. A plurality of nesting pairs of tanks are transported on a single vehicle to a well site where the tanks are unloaded and arranged in a suitable array, for example, in a linear array or uniformly spaced around the mouth of the well. The tanks each have a displaceable end portion serving as a diaphragm placed in contact with the ground near the well. Then the tanks are filled with a noncompressible liquid. The total effective mass of the liquid may be increased by adding density-increasing materials thereto. Air guns are suspended in the tanks submerged in the liquid therein. The air guns are fired in a predetermined manner, abruptly releasing pressurized gas into the liquid contained in the tanks for providing powerful seismic impulses which are transmitted into the ground by the displaceable diaphragms.

Abstract: The improved device for generating acoustic shear waves in the earth comprises a target member adapted to be coupled with the earth and provided with two lateral striking surfaces on which impinges a hammer pivotably mounted on a platform; the target member is connected through damping means to a cage supporting the platform, the hammer and its actuation means, said cage being displaceable with respect to the frame of a carrying vehicle between a position of contact with the earth of the target member and a lifted position thereof, the striking surfaces being inner walls of the target member.

Abstract: A carrier mounted seismic vibrator system that is primarily adapted for generation of high force, high frequency seismic energy into an earth medium. The apparatus includes first and second vibrators (12F and R) as supported by first and second lift systems (28F and R) disposed in tandem juxtaposition generally centrally in said vehicle (10), and the lift systems (28F and R) are designed to maintain equal hold-down force on the vibrator coupling baseplates (32F and R) without exceeding the weight of the carrier vehicle. The juxtaposed vibrators are then energized in synchronized relationship to propagate increased amounts of higher frequency seismic energy into an earth medium.

Abstract: An over-the-road type vehicle for transport and powering of a seismic hydraulic vibrator which is adapted to operate in either one of two modes of operation. A first mode is the conventional mode wherein the vehicle is operated over large distances at varying road speeds, obtained by use of multiple gear ratios, and varying engine speeds. The second mode of operation is in a single gear ratio, with engine under control of a governor, to operate at constant high speed, and the movement of the vehicle is by successive short traverses. In the second mode of operation the single engine alternately powers the hydraulic vibrator, then with the engine still at high speed, moves the vehicle to a new vibration point a short distance away.

Abstract: A vibratory seismic energy source capable of generating significant energy over a broad frequency band. The vibrating baseplate and associated structure are designed to have minimum weight while still retaining sufficient structural integrity to permit the use of high actuator forces. This, coupled with a large reaction mass results in the generation of significant energy levels in the earth at high frequencies.

Abstract: An underwater shear wave vibrator apparatus has a housing with a vibratory generating system mounted inside the housing for generating a force along an axis substantially parallel to the underwater seabed surface. A coupling apparatus which has an attachment surface and an underside surface for engaging the seabed is connected through a pivotal arm to the housing. A hydraulic apparatus is used for positioning the seabed engaging apparatus normal to the force axis of the vibratory generating apparatus by movement about the pivotal attachment so as to maintain contact of the seabed engaging apparatus with the seabed as the vibratory generating apparatus applies force against the seabed. Apparatus is also included for maintaining orientation of the underside surface of the positioning apparatus as the positioning apparatus moves the seabed engaging means.

Abstract: A variable frequency seismic pulse is produced by releasing a weighted piston within a partially evacuated cylinder. The piston compresses residual air in the cylinder before striking a base plate held firmly in contact with the ground. Controlling the pressure of the gas remaining in the cylinder controls the frequency and amplitude of the seismic pulse applied to the ground. The seismic pulse can be applied at various angles from vertical from a thumper mounted on a vehicle or from a hand-held thumper.

Abstract: An improved portable seismic energy source including a base member having cylindrical sidewalls and an open lower end for resting on the earth's surface, a horizontal baseplate secured to the base member upper end, the baseplate having a central opening therein, a gun mounted on the horizontal baseplate for firing a projectile through the opening therein to impact the earth's surface and generate a seismic signal, a spatter plate mounted to the base member such as by means of a vertical cylinder secured to the lower surface of the baseplate with the lower end of the cylinder being affixed to the spatter plate, the spatter plate having an opening therein in register with the cylinder so that a projectile fired passes through the opening, the spatter plate being positioned above the earth's surface, and a flexible diaphragm secured at its periphery to the lower circumferential surface of the base member and having a central opening therein, the diaphragm being stretched over the top of the spatter plate, the sp

Abstract: A releasable heavy mass is provided which is capable of producing by impact with the ground a shock which can be used in seismic prospecting.This mass is characterized by the fact that it comprises very many individual heavy elements connected together in a way loose enough to allow deformation of the mass without loss of cohesion on impact, and in that its base is of considerable area relative to the surface of the whole mass and is maintained substantially plane at least after release and up to impact. The mass is, in seismic prospecting, released from a helicopter.

Abstract: The remote control switch apparatus has an instruction switch provided on a steering wheel to project toward the inside of a passenger compartment away from an instrument panel of the car so as to indicate a driving condition. A transmitter is installed at the steering wheel to generate a radio wave signal or radiation signal in response to the instruction switch and a receiver, provided on the instrument panel, receives the radiation signal and generates a control signal.

Abstract: A transportable, acoustic pulse generator is described employing an impact mass suspended above a freely movable target plate positionable on the ground, the impact mass being forceably propelled downwardly to strike the target plate to generate an acoustic pulse adapted for seismic purposes. The generator housing is adapted to be mounted on the rearwardly extending rails of a conventional transport truck. In order to eliminate bouncing of the truck due to generator housing recoil on firing, the housing itself is jacked up by lift cylinders extending upward from the target plate so as to minimize or eliminate the spring suspension of the truck. Upstanding sleeve guides carried on the generator housing cooperate with the lift means to provide lateral stability and to establish a precise vertical separation between the impact mass and the target plate, thus insuring repeatable signal energy content and timing.

Abstract: A mobile generator for producing waves for seismic exploration. A hammer, mounted on an elongated boom, is dropped to strike an anvil embedded in the ground to create a ground wave. The boom is pivotal between positions selected according to the nature of the wave to be created. A pair of hydraulic piston actuators move the boom between its selected positions. A tensioning device is connected to the cable which drops the hammer in such a manner as to prevent the hammer from rebounding after an impact.

Abstract: Apparatus for incremental cylinder length control in hydraulic seismic vibrators having increased frequency range. The vibrator apparatus (10) of the type having a reaction mass (38) and axial cylinder bore (42) utilizes hydraulically movable end sleeves (86 and 96) in each end of the cylinder bore (42) so that the end sleeves (86 and 96) can be controllably positioned thereby to alter the cylinder volume and adjust the hydraulic drive system (18) for maximum efficiency relative to operating frequency.

Abstract: Seismic exploration method in arctic regions involving the generation of a seismic disturbance in the water beneath the ice in areas where conventional marine and land exploration methods are functionally inadequate. Seismic disturbances are generated by an air gun assembly which automatically executes lowering air guns through apertures in the ice and retrieving them while carrying out preventive measures against freeze-ups. Seismic sensing and recording equipment are positioned within an appropriate range to detect seismic data in the form of reflective or diffractive signals generated in response to the seismic disturbance after actuating the air gun array, wherein the seismic data is indicative of sub-surface structural formations existing below the body of water.

Abstract: A portable seismic energy source having a base for resting on the earth's surface, a gun providing a means of firing a projectile to impact the earth and generate a seismic signal and a ball and socket to mount the gun on the base to permit the projectile fired by the gun to strike the surface of the earth at a selectable angle.

Abstract: An over-the-road type vehicle for transport and powering of a seismic hydraulic vibrator which is adapted to operate in either one of two modes of operation. A first mode is the conventional mode wherein the vehicle is operated over large distances at varying road speeds, obtained by use of multiple gear ratios, and varying engine speeds. The second mode of operation is in a single value of road speed, with engine under control of a governor, to operate at constant high speed, and the movement of the vehicle is by successive short traverses at constant speed. In this second mode of operation the single engine alternately powers the hydraulic vibrator, and with the engine still at high speed, then moves the vehicle to a new vibration point a short distance away. A high energy efficiency is provided by the use of a mechanical, rather than hydraulic drive system, and a high time efficiency is provided by a novel style of clutch that can start the vehicle from rest with the engine at maximum speed.

Abstract: An improved seismic energy source having a base with an opening therethrough and having a gun mounted on the base for firing a projectile to impact on the earth's surface and generate a seismic signal, the improvement including a spatter plate and a flexible diaphragm affixed at its periphery to the base lower end, the diaphragm having an opening through which the projectile fired by the gun passes, the diaphragm serving to keep the base chamber free of compacted soil kicked up when the projectile impacts the earth and to contain and quiet the muzzle blast.

Abstract: A wide seismic source for use in marine surveys comprising at least two arrays of seismic sources disposed to be towed in a spaced parallel arrangement. Each array includes an elongated float having discrete seismic sources mounted thereon and provided with a steering means to maintain the float in the desired lateral position.

Abstract: A combination of fixed geometry of vibrating masses on a baseplate coupled to the ground, the masses being at a fixed angle to each other and of relatively variable phase, can be controlled to produce both compressional and shear waves simultaneously in a seismic exploration system.

Abstract: A releasable heavy mass is provided which is capable of producing by impact with the ground a shock which can be used in seismic prospecting.This mass is characterized by the fact that it comprises very many individual heavy elements connected together in a way loose enough to allow deformation of the mass without loss of cohesion on impact, and in that its base is of considerable area relative to the surface of the whole mass and is maintained substantially plane at least after release and up to impact. The mass is, in seismic prospecting, released from a helicopter.

Abstract: A seismic energy generating assembly includes a seismic energy generator positioned within a cage-like frame and adapted for vertical movement within the frame in reaction to the powerful seismic energy impulses that are transmitted downwardly into the earth. The assembly further includes hydraulic catch cylinders for easing the seismic energy generator back down to the ground within the frame after each impulse is generated. Horizontal extensions from this frame ride in guides formed by tubular channels for guiding the entire seismic energy generating assembly when lifted up from the ground by hydraulic lifting cylinders so that the assembly can be readily moved from place to place during seismic exploration. The tubular channels are pivotally mounted to an outer supporting frame on a land vehicle.

Abstract: In accordance with the present invention, an impulse generator of high portability is provided using a pair of guillotinized, separately activated hammer/anvil sub-assemblies. Since each sub-assembly is horizontal positioned, say in contact with the earth from a position below the wheel of an exploration truck, the equivalent of the "flying blade" of a guillotine, i.e., a weighted hammer element, must be provided with horizontal movement. This is done by tensioning a plurality of springs attached between the hammer element and a rectangular main frame.During cocking and release of the hammer, the explorationist is provided with a series of visual signals in the form of deactivated and activated panel lights. These signals are automatically provided by controller means of the present invention. Release of the cocked hammer, however, is operator controlled for safety reasons say by activation of a latching means attached to the main frame of the assembly.

Abstract: The present invention relates to means for coupling the horizontally-directed force of a seismic transducer into the earth as a downward going shear wave. The coupling means consists of a plurality of downwardly convergent wedge-shaped earth-engaging cleats mounted on the transducer support means. The cleats differ from each other as to depth and sharpness, the sharper cleats being the deeper ones, so that they contact the earth's surface prior to the blunter ones and penetrate hard surfaces more readily. The blunter cleats, on the other hand, slow the rate of penetration in softer materials and thus prevent bottoming out. The cleats are configured so that the total edge length of the sharper cleats is less than that of the blunter ones, further insuring a high vertical loading on initial contact with hard surfaces. This construction permits efficient coupling into the earth of the horizontal force of a vibrator in soils and other earth materials varying widely in competency while minimizing bottoming out.

Abstract: A mobile device is used which includes a target-member adapted for coupling to the earth and a means for striking the target- member so as to generate acoustic shear waves.The target-member is coupled to the earth through a deformable coupling member secured to the rigid frame of the target- member, the coupling being enhanced by the combination of a suction means and a network of channels arranged on the face of the coupling member which is in contact with the earth, whereby a partial vacuum is created between the coupling member and the earth.

Abstract: A mobile generator for producing seismic waves in the ground, which includes a mobile four-sided frame, a boom having its lower end pivotally connected to the frame, and a worm gear mounted on the frame such that by rotating the worm gear about its longitudinal axis, the boom is pivoted toward a selected impact position. A hammer, mounted on the boom, is raised and then dropped toward an anvil carried beneath the frame on the ground. The anvil has several impact-receiving surfaces such that either a P wave or a shear wave can be generated by pivoting the boom and without relocating the anvil.The anvil structure is positioned on the ground by applying a major portion of the weight of the apparatus on the anvil plate.

Abstract: A mobile device for generating acoustic shear waves comprises a structure provided with at least one axle with wheels mounted thereon, said axle being displaceable with respect to at least a part of the structure.The mobile device has mounted thereon a target-member displaceable from a position suitable for transportation to a working position where it is in contact with the earth, and means for striking the target member, when in working position, by means of a mass, to produce an impact thereon.The target member is provided at its upper part with tracks for the wheels of the axle so that the wheels can be driven, by displacing the respective axles, from a position of contact with the earth to a position where they bear on the target member with the weight of at least a part of the structure.

Abstract: A mobile generator for producing waves for seismic exploration. An elongated boom guides a wave-generating hammer along a linear path of motion to strike an anvil embedded in the ground, the boom being movable between several adjusted positions including a vertical position for generating a P wave, an inclined position for generating a shear wave, and a horizontal position for transport. An improved anvil for generating either a P wave or a shear wave is also disclosed.

Abstract: A lightweight, mobile, low-energy seismic source comprises an industrial shotgun, having a solid projectile combined with a base structure to support the gun in a substantially vertical position, slightly above the surface of the earth. The muzzle velocity of the projectile is sufficiently high that it can provide up to 9,000 foot pounds of energy impacting on the earth, which is comparable to that of some of the larger weight drop machines. Wheels or carrying bars can be provided for manual transport of the source.

Abstract: A device for generating acoustic waves in the earth by striking a mass, housed in a guiding tube, against a target member secured to the lower end of this tube and coupled to the earth in operating conditions, including means, such as an electromagnet, for holding said mass at the upper end of said tube and releasing it at will, and means to turn the tube upside down after each operation so as to reset the mass in position for a new operation.

Abstract: The seismic source consists of a tank for containing a volume of water. The tank is held in firm contact with the ground by suitable weighting means. The tank wall that contacts the ground is thinner than the other walls. The volume of water in the tank is pressurized to one to three atmospheres. An imploder is mounted in the tank, submerged in the water. When the imploder is fired, reducing the tank pressure abruptly, a negative pressure pulse is generated in the water. The negative pressure pulse is transmitted from the water, through the thin ground-contacting wall, to the ground, producing an acoustic pulse whose first motion is one of rarefaction.

Abstract: A hydraulic synchronizing system which comprises a double piston power source and a drain valve in fluid communication with the hydraulic lift cylinders of a vehicle mounted vibratory seismic energy source. The double piston power source supplies the hydraulic lift cylinders with substantially equal volumes of hydraulic fluid, and the drain valve adjusts the flow of hydraulic fluid out of the hydraulic lift cylinders. The system synchronizes the operation of the hydraulic lift cylinders and thus coordinates movement of the vibrator guide rods relative to the vehicle upon which the vibrator is mounted.

Abstract: Methods and apparatus for generating and transmitting seismic signals for the purpose of seismic exploration are disclosed. The signals are force pulses shaped to have a spectrum constrained to the range of frequencies which are necessary for penetration to desired depths within the earth and for resolution of the geological reflection surfaces therein. These pulses are provided in a non-repetitive or aperiodic train, constructed to produce a transmitted energy spectrum whose mean energy extends smoothly at a substantially constant level over the spectrum frequency range, notwithstanding that the repetition frequency of the pulses may be swept over a frequency band much narrower than the spectrum range. The transmitted spectrum can exhibit an auto-correlation function having a major lobe which is predominant over any side lobes, corresponding to a desired level of resolution of the geological reflection surfaces.

Abstract: Seismic exploration method in arctic regions involving the generation of a seismic disturbance in the water beneath the ice in areas where conventional marine and land exploration methods are functionally inadequate. Seismic disturbances are generated by an air gun assembly which automatically executes lowering air guns through apertures in the ice and retrieving them while carrying out preventive measures against freeze-ups. Seismic sensing and recording equipment are positioned within an appropriate range to detect seismic data in the form of reflective or diffractive signals generated in response to the seismic disturbance after actuating the air gun array, wherein the seismic data is indicative of sub-surface structural formations existing below the body of water.

Abstract: Apparatus for truck-mounting of vibrational seismic energy transducers, particularly shear wave transducers, that provides improved vibrational damping and energy coupling into the earth. The apparatus consists of a four column, hydraulically actuatable suspension system operative from the truck or carrier frame as opposite side pairs to support a box frame under which the vibrator and earth coupling assembly is supported by a system of lateral and vertical springs. The vibrator assembly is entirely supported by means of left and right transverse springs to transmit reactive forces resulting from isolation through the box weldment and four columns to the vehicle body. Thus, the vibrator and its associated base plate structure is entirely supported from the vehicle and positioning assembly by resilient means having the requisite interactive constants.

Abstract: A vibratory seismic energy source capable of generating significant energy over a broad frequency band. The vibrating baseplate and associated structure are designed for minimum size and weight, while retaining the structural strength to permit high actuator forces. The weight of the transport vehicle is applied to a substantially horizontal hold-down plate that is interposed between the piston-cylinder assembly and the baseplate. This horizontal plate is coupled to the baseplate through resilient means having compliance such that dynamic motion of the baseplate is decoupled from the hold-down plate and transport vehicle.

Abstract: A seismic wave generator constructed of fungible high density elements contained within a flexible envelope is dropped to impact upon the ground and generate seismic waves for seismic investigations. The non-rigid construction minimizes or eliminates bounce or reverberation after impact. Because of the lack of bounce and reverberation, recordings of the seismic waves produced are relatively free of superimposed wave forms otherwise resulting from multiple impacts.

Abstract: An improved base plate for a vibratory seismic signal generator consists of a plurality of parallel T-bars welded together. The flange of every adjacent T-bar is inverted with respect to its neighbors.

Abstract: A system for controlling the tilt of the vehicle and vibrator when the vibrator is being operated on irregular surfaces. Tilt switches detect excessive vehicle tilt. A bleeder network activated by the tilt switches reduces the pressure in the air bag on the side of the vibrator opposite from the direction of the vehicle tilt. A supply network repressurizes the air bag on the side of the vibrator opposite from the direction of the tilt.

Abstract: Apparatus for supplying a large amount of air periodically at a controlled rate to the air cushion beneath an air-cushion vehicle as particularly employed for coupling of controlled frequency seismic energy into the earth. The apparatus consists of a vehicle body which includes a compressed air supply or input to a self-contained plenum chamber space for applying air pressure through a multi-port rotary valve thereafter to be directed into an air cushion formed between the vehicle and the earth surface with periodic exhaust to the atmosphere. The rotary air modulator or valve is comprised of a cylindrical stator, a rotor, and a shutter, each of which includes a plurality of equispaced ports, and air flow from the plenum chamber to the air cushion is modulated by passage through the stator and rotor in accordance with an intermediate shutter position; pressure exhaust is similarly controlled.

Abstract: A vibratory seismic energy source capable of generating significant energy over a broad frequency band. The vibrating baseplate and associated structure are designed to have minimum weight while still retaining sufficient structural integrity to permit the use of high actuator forces. This, coupled with a large reaction mass results in the generation of significant energy levels in the earth at high frequencies.

Abstract: A hydraulic stabilization system which comprises pairs of hydraulic cylinders in fluid communication connected between the vehicle support structure of the vibrator and the vibrator ground coupling plate. This configuration maintains the ground coupling plate in vertical aligment with the vehicle support structure and isolates vibratory motions of the ground coupling plate from the vehicle support structure.

Abstract: A seismic wave generator assembly with self-contained guidance and shock-absorbing means is confined within an elongated, upstanding cylindrical housing which may be lowered into position at the end of a lateral arm or a cable supported from a transport vehicle.

Abstract: Within a stationary upstanding cylindrical housing, a piston and a base member resting on the ground are separated by an expansible detonation chamber. Means are provided for feeding successive charges of a combustible gas mixture into the chamber at a rapid rate and initiating a corresponding series of gas explosions therein. Recoil of the piston following each explosion compresses a first air spring which then forcibly thrusts the piston downwardly. Piston recovery for the next cyclic operation is thereafter slowed by means of a second air spring-dashpot combination.

Abstract: The gas detonator is especially adapted for shallow, seismographic, land prospecting. It comprises a circular bottom plate having an inwardly-directed sloping side wall. An annular top plate is mounted over the bottom plate. Means, including a plurality of springs, interconnect and resiliently maintain the plates in touching relation with each other. A housing is mounted over the top plate and forms therewith a detonation chamber. The bore of the top plate serves as the vent port for the detonation chamber. The vent port is normally closed by the bottom plate. Following the detonation of a combustible gas mixture in the chamber, the plates momentarily separate and form therebetween a cylindrical cavity which opens the vent port. A container upwardly extends from and forms with the bottom plate a reservoir which, in use, is filled with water. The spent gases of combustion escape into this water, thereby muffling the sound of the detonator.

Abstract: A seismic signal generating apparatus comprising a vehicle including a rearwardly extending bed, a ram-supporting framework pivotally mounted on the bed for pivotation about a horizontal axis, a ram slidably and reciprocably mounted in the framework, and a plurality of catapult springs connected between the framework, and one end of the ram for forceably reciprocating the ram from an elevated status in which the catapult springs are tensioned, to a lowered, impacting position in which the springs are tensioned to a lesser degree. A pair of superimposed metal plates are positioned below the ram on the ground for receiving the impact of the ram.

Abstract: Improvement in electro-fluid vibrator means used for generating seismic waves in the earth in which the flow of pressurized fluid to the vibrator is controlled in accordance with the resultant holddown pressure on the baseplate in contact with the earth, so that there is always a positive pressure on the baseplate, preventing it from being lifted off the surface of the earth. Force measuring means are provided in the holddown means. So long as this force is large and positive (in the direction to hold the baseplate to the earth) no control is required. When this positive force becomes less than a preselected value, the pressure fluid system is controlled to reduce the maximum pressure of, or rate of flow of, pressure fluid to the vibrator. The force measuring system can conveniently utilize the compliances in the holddown system as the force measuring elements.