| Claims: 1. Apparatus for sensing motion of a surface, comprising: a stabilised platform comprising a support structure and a moveable mass resiliently suspended with respect to the support structure: a transmitter transducer and a receiver transducer mounted on the moveable mass, the transmitter transducer arranged to transmit an acoustic wave towards the surface; and the receiver transducer arranged to receive a reflected wave from the surface; and detection means for measuring motion of the surface based on a Doppler shift in the reflected wave. 2. Apparatus according to claim I5 wherein the detection means comprise an amplifier arranged to receive a Doppler modulated signal from the receiver transducer, and a phase detector arranged to receive an amplified signal from the amplifier and to provide a demodulated output signal indicative of the motion of the surface. 3. Apparatus according to claim 2, wherein the amplifier is arranged to equalise the amplitude of the Doppler modulated signal. 4. Apparatus according to claim 2 or 3. wherein the amplifier lias a dynamic range of at least 14OdB, 5. Apparatus according to any of claims 2 to 4, wherein the phase detector comprises a phase locked loop. 6. Apparatus according to an}' of claims 2 to 5, wherein the phase detector is arranged to detect a phase shift corresponding to a vibration velocity of the surface down to lμm/s. over a bandwidth from at least 5 to 150 Hz. 7 Apparatus accoidmg to am preceding claim, comprising an arra) of transmittei transducers 8 Apparatus according to any preceding claim comprising an array of receivei transducers 9 Apparatus according to any preceding claim, wherein the support structure comprises a casing which houses the moveable mass 10. Apparatus according to an}' preceding claim, wherein the stabilised platform includes an electrically conductive coil and a magnet, whereby relative movement of the moveable mass and support structure provides an induced electrical output signal across the coil dependent on the relative movement. 1 1 Apparatus according to claim 10. wherein the moveable mass includes the electrically conductive coil and the support structure includes the magnet 12 Apparatus according to claim 10 or 11. wherein an electrical resistor is connected across the coil to provide electric damping of the relative movement 13 Apparatus accoidmg to any preceding claim, furthei comprising an accelerometer attached to the mass whereby movement of the mass provides an electrical output signal dependent on the movement 14 Apparatus according to claim 13, wherein the accclerometer is a microelectromechamcal system (MEMS ) acceierometei 15 Apparatus according to any preceding claim, wherein the moveable mass is resiliency suspended with respecl to the support structure by one oi more springs oo 16 Apparatus according to an}' of claims 2 to 15 furthei comprising lelative motion compensation means arranged to lemove from the demodulated output signal phase noise caused by motion of the transducers whereb) to provide a surface motion output signal 17 Apparatus accoidmg to claim 16 wherein the relative motion compensation means comprise a low-cut filtei, arranged to filtei the demodulated output signal 18 Apparatus according to claim 17 wherein the low-cut filtei has a cut-off frequency at least one decade above a natural resonant frequency of the stabilised platform 19 Apparatus according to any of claims 16 to 18, wherein the relative motion compensation means comprise means foi estimating the phase noise, the relative motion compensation means being furthei arranged to subtract the estimated phase noise from the demodulated output signal 20 Apparatus according to claim 19 wherein the phase noise estimating means are arranged to determine the estimated phase noise on the basis of an estimate of the velocity of the mass 21 Apparatus according to claim 20 when dependent on claim 13 oi 14 wherein the phase noise estimating means are arranged to receive the output signal from the acoeleiometei. and furthei comprise means foi integrating the received accelerometei signal to estimate the mass velocity 22 Apparatus according to claim 19 oi 20 wherein the phase noise estimating means are arranged to determine the estimated phase noise using an electrical analogical model of the stabilised platform 23 Apparatus according to claim 21 wherein the phase noise estimating means are arranged to receive a signal dependent on relative movement between the moveable mass and the support structuie. and to estimate the mass velocitj on the basis of the relative movement signal and stored parameters relating to mechanical impedances of the stabilised platform the model topology and respective mechanical constants 24 Apparatus according to claim 19. wherein the phase noise estimating means are arranged to receive a voltage signal q(t) dependent on relative movement between the moveable mass and the support structure, and to estimate the phase noise on the basis of the signal q(t), a modelled transfei function t(t) of the platform relating the relative movement to the mass velocity, and a stored calibration factor (G/α) 25. Apparatus according to claim 19, wherein the phase noise estimating means are arranged to receive a signal dependent on relative movement between the moveable mass and the support structure, and to estimate the phase noise on the basis of the relative movement signal q(t) and a stored total transfer function (G/α) (t(t)} relating the Telative movement to the phase noise 26 Apparatus according to claim 19, wherein the phase noise estimating means are arranged to receive a signal dependent on relative movement between the moveable mass and the support structure, and to appty a least squares method to estimate the phase noise on the basis of the relative movement signal q(t) and a statistical knowledge of the surface motion output signal s(t) 27 Apparatus accoidmg to any of claims 23 to 26 when dependent on claim 10, wherein the relative movement signal is the induced electrical output signal q(t) from the coil 28 Apparatus according to claim 21 wherein the phase noise estimating means are arranged to estimate the phase noise on the basis oF the estimated mass velocity and a stored calibration factor (G/β) 0£ 29 Apparatus according to claims 2" and 2S wherein the phase noise estimating means aie aπanged to estimate the phase noise on the basis of both a first phase noise estimate derived fiom the induced electrical output signal from the coil and a second phase noise estimate derived from the acceleiometei output signal 30 Apparatus according to claim 29 wherein the phase noise estimating means are arranged to average the first and second phase noise estimates 31 Apparatus accoidmg to an)' preceding claim wherein the receiver transducei is positioned relative to the transmittei transducei so as to detect motion of the surface in a first direction substantially normal to the surface the apparatus furthei comprising at least second and third receivei transducers mounted on the moveable mass and orthogonal!}' offset from the transmittei transducei so as to detect components of the motion of the surface in respective directions orthogonal to the first direction 32 Apparatus according to claim 31 comprising an array of second receive: transducers and an arraj' of third receivei transducers 33 Device foi sensing motion of a surface, comprising a support structure and a plurality of sensors, wherein the support structure is a stress membei . and each of the plurality of sensors comprises an apparatus according to any piecedmg claim, mounted on the stress membei 34 Device according to claim 33, wherein the stress membei and the plurality of sensors are encased ID an outei housing 35 Device according to claim 34, furthei- comprising means foi piopulsion o/ (he housing into position on the surface 36. Device according to an)' of claims 33 to 35. further comprising means for orienting the sensors with respect to the surface, so as to enable measurement of the surface motion. 37. Method for calibrating a sensing apparatus according to claim 27. comprising: attaching the support structure of the sensing apparatus to a rigid structure such that the support structure is prevented from moving and the transmitter transducer is arranged to direct an acoustic wave towards a fixed, acoustically reflecting surface of the rigid structure, operating the apparatus to transmit the acoustic wave and receive and demodulate the reflected wave, applying a known signal f(t) to the coil to induce forced motion of the mass, measuring the demodulated output signal r(t) from the detector while the known signal f(f) is applied, using the. relationship r{ϊ)~(G/ά) {f(t}}, where G/α is a calibration factor of the apparatus, to calculate the calibration factor, and storing the calculated calibration factor. 38. Method for calibrating a sensing apparatus according to claim 27. comprising: attaching the support structure of the sensing apparatus to a rigid structure such that the support structure is prevented from moving and the transmitter transducer is arranged to direct an acoustic wave towards a fixed, acoustically reflecting surface of the rigid structure, operating the apparatus to transmit the acoustic wave and receive and demodulate the reflected wave. applying a sharp impulse δ(t0) to the coil using an impulse generator, to induce forced motion of the mass, electronically disconnecting the coil from the impulse generator at the time when the impulse terminates, so that the coil is in an open circuit, measuring the demodulated output signal O(f) from the detector at least immediately following the impulse, using the relationship O(f)=(G/α){T(f)}, where G/α{T(f}} is the lotaJ transfer function of the apparatus, to determine the total transfer function, and storing the determined total transfer function. 39. Method for calibrating a sensing apparatus according to claim 28. comprising: attaching the support structure of the sensing apparatus to a rigid structure such that the support structure is prevented from moving and the transmitter transducer is arranged to direct an acoustic wave towards a fixed, acoustically reflecting surface of the rigid structure, operating the apparatus to transmit the acoustic wave and receive and demodulate the reflected wave. inducing a forced motion of the mass, measuring the demodulated output signal R(f) from the detector, and the output signal D(f) from the accelerometer, during the induced motion. using the relationship [R(f)}/{D(f)}=(G/β); where G/β is a calibration factor of the apparatus, to calculate the calibration factor, and storing the calculated calibration factor. 40. Method according to claim 39, wherein the sensing apparatus is as claimed in claim 29, and the step of inducing a forced motion of the mass comprises appfying a signal to the coil. 41. An apparatus for sensing motion of a surface, substantially as hereinbefore described, with reference to the accompanying drawings. 42. A method for calibrating a sensing apparatus, substantial])' as hereinbefore described, with reference to the accompanying drawings. |
The present invention relates to sensors for detecting motion of a surface, and in particular uncoupled sensors for detecting vibration using Doppler shifts in transmitted acoustic waves,
Background of the Invention
Contact (coupled) sensors based on inductive, piezoelectric or capacitive transducers, are used almost universally in a host of applications ranging from the detection of vibrations in nianmade structures, e.g. bridges, to the monitoring of the earth's naturally occurring, or artificially generated, tremors. For example, in seismic exploration for hydrocarbons and minerals, tremors produced by a source of elastic energy are picked up by detectors placed at or near the earth's surface, in water covered areas, on the ocean bottom and in boreholes.
There are two main drawbacks inherent in coupled sensors: a. The response of a vibrating structure can be altered by the coupled mass of the sensors; b. The deployment and retrieval of large numbers of sensors is very inefficient and costly. In seismic exploration over land, man)' hundreds to several thousand sensors, i.e. geophones or accelerorneters, are placed on the earth's surface. Each sensor has attached to its body a metal spike which must be driven individually into the ground by hand, to provide good earth-to-transducer coupling. Vast manpower and logistical support are required to place and move such a large number of sensors over exploration areas measuring from hundreds to thousands square kilometres.
Non-contact (uncoupled) sensors are more suited to applications where transducer coupling effects can alter the response of the vibrating structure to be measured. Vibration measurements made with uncoupled sensors are mostly based on optical waves, electromagnetic microwaves and acoustic ultrasonic waves. The embodiment of these sensors consists of a transducer which emits and directs waves toward the vibrating surface under investigation. The vibrating surface modulates the incident and reflected waves in frequency and phase (Doppler shift). The reflected waves are detected by a receiver sensor and subsequently demodulated to extract the Doppler shift, which is proportional to the velocity of the surface vibrations. Because of the t3φe of measurement performed, these devices are also called velocimeters.
Generally, velocimeters have man}' inherent drawbacks: a. Starting with optical sensors, known as Laser Doppler Velocimeters (LDV) 5 they are bulk}', expensive, very sensitive to surface roughness and the frequency range of the measured vibrations does not extend below IkHz. However, their resolution (the smallest detectable vibration amplitude) can be as high as a few nanometers. b. Microwave Doppler Velocimeters (MDV) are also bulky and their resolution is several orders of magnitude lower than that obtainable by LDVs. In addition, the accuracy of the Doppler shift measurement is affected by interferences from spurious reflections and other events originating at or just below the surface. c. Ultrasonic acoustic sensors have found only limited applications in acoustic Doppler vibrometry (ADV) mainly because, compared to LDVs, their resolution is much lower and they lack response to vibrations above approximately HcHz. d. Finally, it is a further very important consideration that all types of uncoupled sensors need to be mounted on an ultra-stable platform, in order to minimize the movement induced in the sensor by ambient noise. Since the surface vibrations are measured relative to the static position of the sensor, sensor movement is one of the major sources of errors in Doppler measurements of surface vibrations. General]}', for experimental set-ups and outdoor applications the platform is provided by a sturdy support . , like a rigid purpose built tripod, whereas in more permanent installations, the uncoupled sensor is mounted on a damped large mass or in an an echoic chamber. For very precise measurements inertial platforms are sometimes employed. For use outside of the laboratory and in the field, to obtain for example seismic data measurement, all the above platforms are either too cumbersome or very expensive to implement, or both. In order to overcome the drawbacks of coupled sensors, it is therefore desirable to provide an uncoupled sensor which is relatively inexpensive, and which is small and durable enough to be deployed easily in large numbers, In order to be useful in applications where coupled sensors are usually employed, it is desirable for the uncoupled sensor to have sufficient resolution to measure earth surface vibrations, but without being too sensitive to surface roughness. Furthermore, it is desirable to provide an uncoupled sensor which is mounted on a platform which is sufficiently stable to allow the sensor to provide accurate measurements, but is durable and inexpensive enough to be used in large numbers in rough terrain and can withstand rough handling.
Summary of the Invention
Despite the drawbacks of ultrasonic acoustic sensors in terms of resolution, when compared with LDVs, it has been found that for low frequency applications, e.g. sensing of seismic waves, ranging from a few Hertz up to several hundred Hertz, acoustic Doppler vibrometers (ADVs) can outperform LDVs in resolution. In fact, with an electronic demodulation system having the appropriate wide d3'namic range, it is possible to achieve resolutions in the order of nanometers. Other advantages of ADVs are the small size, low cost and low sensitivity to surface roughness. The latter property is extremely important for sensing seismic waves on the naturally rough earth surface. In addition, their low cost and small size make the ADVs ideally suited for the rapid and economical deplo3Tiient, and retrieval, of very large multi-sensor arrays.
The present invention provides an apparatus for sensing motion of a surface, comprising: a stabilised platform comprising a support structure and a moveable mass resiliently suspended with respect to the support structure; a transmitter transducer and a receiver transducer mounted on the moveable mass, the transmitter transducer arranged to transmit an acoustic wave towards the surface, and the receiver transducer arranged to receive a reflected wave from the surface, and detection means for measuring motion of the surface based on a Doppler shift in the reflected wave.
Preferably, the detection means comprise an amplifier arranged to receive a Doppler modulated signal from the receiver transducer, and a phase detector arranged to receive an amplified signal from the amplifier and to provide a demodulated output signal indicative of the motion of the surface. Preferably, the apparatus further comprises relative motion compensation means arranged to remove from the demodulated output signal phase noise caused by motion of the transducers, and to provide a surface motion output signal.
The support structure may comprise a casing which houses the moveable mass. In one embodiment of the invention the platform for the ADV transmitting-receiving transducers is provided by a damped mass-spring system. This platform is relatively inexpensive, can be made small, and is simple to implement.
The invention further provides a device for sensing motion of a surface, comprising a support structure and a plurality of sensors, wherein the support structure is a stress member, and each of the plurality of sensors comprises an apparatus according to any preceding claim, mounted on the stress member.
In another aspect, the invention provides methods for calibrating a sensing apparatus, as defined in claims 37, 38 and 39.
Brief Description of the Drawings
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Figure 1 is a schematic diagram of an uncoupled acoustic Doppler sensor according to the invention; Figure 2 is a diagram showing the frequency domain response of the stabilised platform of the sensor;
Figure 3 is a schematic representation of the mechanical components of the stabilised platform:
Figure 4 is a schematic cross- sectional view showing the components of the sensor;
Figure 5 shows schematically the geometry of acoustic transducers in a further embodiment of the sensor, for sensing vertical and horizontal components of the surface vibration velocity;
Figure 6 is a schematic diagram of a cable assembly housing a plurality of sensors; and
Figure 7 is a cross- sectional diagram of a sensor mounted on a cable.
Description of the invention
The invention outlined herein, consists of an acoustic transmitting-receiving sensor for accurate measurements of micro-vibrations on a surface by means of Doppler shift. The sensor requires no coupling to the measured surface and can be placed at an arbitrary distance from the surface. The maximum sensor-to-surface separation is dictated by the strength of the received signal above the noise; it can typically range from a few centimeters to more than a meter.
The following description is for the implementation of a continuous acoustic wave Doppler sensor. However, the invention equally applies to vibration measurements made with pulsed acoustic sensors, laser sensors, and electromagnetic wave sensors. Also the invention equally applies to the use of chirps or other forms of coded signals for measuring Doppler shift. An embodiment of a sensor according to the invention is shown diagrammatical])' in Figure 1 ,
The uncoupled acoustic Doppler sensor 10 is arranged to measure movement of a rough vibrating surface 22, the movement of which is denoted s(t). The sensor comprises a earner frequency generator 12 for providing a carrier frequency to an ultrasonic transmitter transducer 24 mounted on a stabilised platform 14 togethei with a receive! transducei 26. the transmittei arranged to direct an acoustic signal towards the surface 22 such that the signal is Dopplei -shifted by the movement of the surface and reflected back to the receivei AD amplifiei 16 is arranged to receive the Dopplei modulated signal from the receiver, and the output of the amplifiei is then supplied to a detectoi 18 which provides a demodulated output p(t) In tins embodiment there is furthei provided a relative motion compensatoi 20, which receives the demodulated signal ρ(t) as well as a transducei relative motion raw signal q(t) from the stabilised platform 14, and provides an output Dopplei signal proportional to s(t), eitbei m digitally coded or analog form foi example to a single oi multichannel recordei
A carriei frequency signal is generated m the carπei frequency generatoi 12, and is used to drive the piezoelectric transmitter transducei 24 m the stabilised platform 14 The generated signal is a highly stable sine wave generated by a 4 MHz quartz oscillator The quartz frequency is divided down 100 times to obtain 40 kHz carriei frequency fo Aftei powei amplification the carrier is fed to the 40 kHz transmitter piezoelectric transducer In this particulai example, the choice of a 40 kHz transducer provides a suitable balance between resolution and sensitivity to surface roughness as discussed m more detail latei, but transducers resonating at othei frequencies may also be used More than one transmittei transducei, oi an array thereof, can be advantageously employed in the stabilised platform to obtain the desired directivity and bandwidth iesponses
The ensuing acoustic signal is projected from the transmittei oi transmitters 24 onto the vibrating surface As a result of the surface time variant displacement, both incident and reflected waves are phase modulated (Dopplei shifted) by an amount fd given by fd = ±2fo(vd/c), where vd is the vibration velocity and c is the velocity of sound in air Because of the iouglmess of the surface, the reflected acoustic beam ma}' depart considerably from speculai ieflection angle and may also undergo scattering Foi this reason the receivei transducei 26 must have a wide aperture angle Also its bandwidth must be sufficiently broad to prevent undue attenuation of the modulation sidebands In some implementations, electrostatic transducers ma)' be advantageously used ID oi dei to meet these requπ ements more effectively than piezoelectric transducers although piezoelectric transducers aie more readily available in sizes small enough foi use in arrays of large numbers of sensors
More than one leceivei transducei 01 an array theieof can be advantageous^ employed to obtain the desired directivity, sensitivity and bandwidth responses
An important aspect of the invention is the method used foi isolating both the transmittei and receivei transducers from motion that could be imparted to them by ambient noise and indeed by the vibrating surface being sensed as the surface may provide directly or indirectly, physical support foi the transducers Motion imparted to the transducers' bodies will produce a signal at the output of the detectoi which is indistinguishable from that generated by the vibrating surface Ideally the objective would be to impart zero motion to the transducers' bodies In practice, as shown schematically in Figure 1 , the transducers' isolation from motion is provided, partially within a finite bandwidth, by a platform 14 consisting of a mass spring, dashpot system The transmitter and receivei transducers 24 26 are mounted on the mass of the system Provided the bandwidth of interest starts above the system's natural resonant frequency (fn). the mass displacement has been found to decrease at a rate of -40 dB pel decade of frequency This attenuation can be seen in Figure 2. which shows the frequenc)' domain response of the stabilised platform More specifically Figuie 2 shows the mass displacement magnitude plotted against frequency foi different values of the damping coefficient, ζ (ζ=0.01. ζ=0 71 and ζ=1.00) The natural resonant frequency of the platform is indicated as fn Naturally the damping of the system must be close to critical, to avoid a sharp peak at fn (as shown in Figure 2) Furthei details of the platform will be given below
To ensure high detection sensitivity of the surface vibration velocity a very low noise wide dynamic range amphfiei 1 6 is used, followed by an accurate phase detectoi 18 (Figuie I J The total system noise, referred to the input of the amplrfiei 16, must be in the ordei of IUV/Λ'HZ and the dynamic range not less than 140 dB To achieve this wide dynamic range a noise floor of O.l μY rms over a minimum signal bandwidth of 150Hz (—5 Hz to 150 Hz) is required. Moreover, the amplifier must equalize the amplitude of the phase modulated signal, to compensate for changes in the signal strength caused by the vibrating surface roughness. Generalfy. phase and fin detectors tend to become unstable with large input signal amplitude variations.
A highly accurate low noise detector IS consisting of a digital or analog phase locked loop (PLL); or some other form of interferometric demodulation scheme, is preferably used to meet the required accuracy and sensitivity of phase demodulation. The PLL must provide a reliable output for surface vibration velocity down to 1 μm/s over a bandwidth from a few Hertz to at least 150 Hz.
Relative Motion Compensator
The transducer relative motion compensator (EMC) 20, shown in Figure 1, will now be described in more detail. The purpose of the PJs-IC is to further reduce phase noise n(t) produced by residual mass motion that the platform cannot prevent. This motion will be detected as motion of the vibrating surface if it is not compensated for.
As shown in Figure 2, the platform 14 does not prevent mass motion from taking place between zero frequency and a frequency, fc, which is above the natural resonant frequency, fn, of the platform. For this reason, an- appropriate low-cut filter h(t) is applied, to adequatel3' attenuate frequencies below fc in the phase demodulated detector output p(t). Unfortunately within the frequency range 0 < f < fc, not only the undesired mass motion components n(t) are attenuated by the filter h(t), but also any signal component of the surface vibrations s(t) that falls within the filter frequency attenuation range. Therefore, both fn and fc should preferably be chosen to make the lowest frequency of interest in s(t) fall at least -4OdB from the maximum of the platform response, i.e. fc = fsmin, fc > lOfn; where fsmin is the minimum frequency of interest in s(t). Obviously fn must be as low as is practical to minimize loss of the signal low frequency bandwidth. In spite of the mtimsic attenuation affoided b) the stabilized platform (-40 dB/ decade] togethei with the low-cut filtei ven low magnitude vibrations in s(t) (of the ordei of nanometers) can be overwhelmed by much strongei noise n(tj (of the ordei of millimetres) Furthei attenuation of phase noise is possible by estimating the noise n(t) and then subtracting it from the demodulated output p(t) since p(t) = s(t) + n(t) Of course, the effectiveness of this method of noise attenuation depends laigely on the accuracy with which ri(t) can be estimated The procedure consists of first estimating the mass velocity u(t) mathematically by means of an electrical analogical model foi the platform Then the model can be implemented eithei by electronic hardware (analog) or by software (digital) The inevitable differences between the theoretical model and the real platform together -with the inherent inaccuracies in the analog circuitry are likely to make the analog realization unreliable if not outright unstable The software only solution is therefore the more desirable This is because all the fine tuning to take account of differences between the theoretical model and the real platform can be done by making use of the flexibility available with software The downside of the software approach is that the computations are done in "computei time" rathei than real time Alternative!}', a hybrid model implementation based on microprocessor technology ma)' provide both hardware computational speed and the flexibility of software This means that the computations can be advantageously carried out m pseudo real time
Without going into the model's algebraic details the expression giving the mass velocity, iefened to the measured surface is lelatively simple to interpret XJ (f) = W(f)x[ZM/ZP], 01 more compactly U(f) = W(f)T(f) wheie T(f) = ZM/ZP ZM is the impedance of the mass, and ZP is the impedance of the parallel combination of the sprmg-dashpot while XJ(Fj and W(f) are respectively the mass velocity and the velocity acioss the sprmg-dashpot, both expressed as a function of frequency Figure 3 is a schematic representation of the mechanical components of the stabilised platform, showing the mass M to which the tiansmittei and jeceivei transducers 32 34 are mounted, a spring K and dashpot D The mass velocity, as a function of tune, is denoted u(t) and the velocity of the support structuie (or casing) to which the mass is attached is denoted v(t). The relative velocity of the mass and casing, which corresponds to a velocity which can be measured directly within the stabilised platform, is denoted w(t). and corresponds to v(t) - uft).
Simply .stated, with above relation it is possible to calculate U(f) in terms of the velocity across the spring- dashpot and the respective mechanical impedances of the mass-spring- dashpot. The impedances can be easily derived from the physical constants characterizing these three mechanical components, i.e. weight, stiffness and damping coefficient. With reference to Figure 1. the parameter W(f) can be conveniently derived from the platform relative motion transducer output q(t). A detailed description of this and other features of the platform will be given below, It suffices now to point out that W(f) = (l/α)Q(f). where: α = volt/velocity = the transducer's electrical -to-mechanical transfer function (transduction factor) and Q(f) is the frequency domain expression of q(t). Once U(f) has been calculated, as indicated above, it can be transformed into a voltage proportional to TST(f), which is the frequency domain expression of n(t), i.e. the mass motion component of p(t). Thus. N(f) = prop. {U(f)} = G (U(f)} = (G/α) (Q(f)T(f)) . The measurement of the proportionality/transduction factors G/α. will be discussed in some detail below.
Stabilised Platform
The physical implementation of the stabilized platform in a preferred embodiment is shown in Figure 4. This arrangement is based on a construction very similar to that of a velocity- sensitive geophone transducer. A t)φical geophone comprises a casing which houses a spring-mounted coil moving in a magnetic field, arranged such that vibration of the casing causes relative movement between the casing and the coil, which generates an electrical signal. This signal can be used to measure the vibration to which the casing is subjected. In particular, when the casing is subjected to vibration within a particular frequency range, the spring-mounted coil remains substantially stationary due to its inertia, such that the relative movement corresponds mainly to the vibration of the casing. In this way. ground particle velocity is converted into a voltage signal, The present invention preferably uses a similar principle to provide a stabilised platform onto which the transmitter and receiver transducers are mounted. However, other types of mechanical . , electromechanical or electronic devices, including micro electromechanical systems (MEMS), capable of providing a stable platform fully integrated with the ultrasonic transmitter and receiver transducers, and providing a mass moveable with respect to a support structure, can be used. The reasons for choosing a geophone-based construction for the platform are that it is well proven technology, with over 60 years of use by the oil industry as the only detector type for land seismic prospecting, and over the years, large numbers have been manufactured and used in severe environments such as polar, desert, rain forest, etc. In accordance with the invention, this technology can be adapted to the requirements of the stabilized platform for the uncoupled acoustic sensor.
Figure 4 shows the essential components of the geophone-based platform 40. The support structure 42 serves as an anchor point upon which the platform casing 44 is fitted. For seismic field recording applications, the support structure is a stress member, measuring up to several kilometers in length and capable of carrying many hundreds to several thousands of individual acoustic sensors spaced along its length. The entire assembly of the stress member and acoustic sensors may typically be encased in a structure suitable for self- propelling or towing along the ground surface. Figure 6 shows a cross section of a stress member-sensor assembly 70. comprising a flexible stress member 72 encased in a flexible cylindrical sheath 74. The assembly 70 is shown in side view (a), and a front view (b). A plurality of individual sensor assemblies 76 are suspended from the stress member, and are each housed in an enclosure 77, having acoustical]}' absorbent interna] enclosure walls 78 and an acoustically transparent window 19 fitted around the cable sheath. A variety of towing or self propelled mechanisms can be fitted external])' onto the cylindrical sheath.
Needless to say, a suitable mechanism must be provided, e.g. gimbals, to keep the sensors levelled. Figure 7 shows in more detail a front view of an individual sensor assembly 76, mounted on the stress member 72 (shown in cross section). Orthogonal gimbals 82 are used to suspend the sensor 84 from the stress member, the pivota] connections being made by means of damped shafts 86. The assembly 76 is pivotally mounted on the stress member ■using an arrangement of ball bearings 87 in a damping fluid 88. It should be noted that it is important that all of the pivotal joints are suitably damped, in order that the sensors can be used to obtain reliable measurements of the surface vibrations.
Alternatively, the acoustic sensors according to the invention could be deplo3'ed on the ground and retrieved either singly, or in patterns, from a moving craft.
Returning now to the stabilized platform details, as shown in Figure 4, the mass consists of a moving double coil 46 suspended within the air-gap 47 between the poles of a permanent magnet 48, by means of flat spider-web springs 49. A single coil may alternatively be used. Electric damping is provided by a resistor KD connected across one of the coils. The required amount of damping applied to the mass-spring motion (see Figure 2). is obtained by adjusting the value of RI). Both transmitter and receiver ultrasonic transducers 52, 54 are mounted on the lower part of the moving coil former 50. In the present embodiment, the transducers 52, 54 are 40 kHz miniature piezoelectric transducers, or arrays thereof, each weighing only a few grams. Other types of transducers can be employed, such as capacitance, polymer, MEMS, etc. The choice for the transducers' frequency is a compromise between keeping the sensitivity to surface roughness as low as possible, while achieving an acceptable level of maximum velocity resolution of the sensed vibrations. As mentioned above, an electrostatic receiver transducer may be particularly suited to the requirements of broad directivity and bandwidth, so long as it can be provided in a suitably small size. Acoustic insulation 51 placed between the two transducers prevents the high acoustic power transmitted by the transmitter transducer 52 being picked-up by the receiver transducer 54. A low acoustic, attenuation window 53 is fitted on the bottom of the casing 44, opposite the ultrasonic transducers 52, 54, to allow two way passage of acoustic waves.
Further inspection of Figure 4 reveals that any motion on the support structure 42 is also imparted to the platform casing 44. This motion is expressed as a velocity v(t) in Figure 3. The behavicmi of mass displacement resulting from the casing motion can bε seen from the curves m Figure 2 plotted as a function of frequency with the damping coefficient as a parameter As discussed above m the description of the relative motion compensator, the low frequency components, below fc ma)' be attenuated by a low-cut filtei h(t), as shown in Figure 2 This is a way of preventing undue interference on the signal s(t) resulting from the mass motion velocity component n(t) of the detectoi output p(t). The platform attenuation of frequencies above fc is -40dB/decade Highei attenuations can be achieved by cascaded mass-sprmg-dashpot systems, although the additional attenuation is obtained at an increased cost in mechanical complexity and reduced reliability
As already described above, the output of the signal from the upper moving coil, q(t), is used m the relative motion compensator (EJvIC) to estimate the noise component n(t) in the demodulated output p(t) Furthermore, it was also pointed out that the ratio of a proportionality factor "G" and transduction factor o (G/α) was needed for the estimation of n(t). i.e. N(f) = G/α{Q(f)T(f)} . One way of obtaining G/σ is to perform a '"factor}'" calibration of the entire uncoupled acoustic sensoi system First, the platform casing 44 is clamped to a rigid baffler, and an appropriate reflecting surface is placed undei the ultrasonic transducers 52, 54 The reflecting surface may be a part of the rigid structure to winch the casing is clamped A known signal f(t). e.g a sine wave, is then applied to the terminals of the uppei moving coil 46 As a result, only the coil assembly and hence the ultrasonic transducers 52, 54 will be set into uniaxial "forced" motion m sympathy with the applied signal. The movement of the transducers will Dopplei modulate the transmitted carrier frequenc)' fo which m turn will produce a detectoi output. r(t) = (G/α) (f(t)}. Since r(t) and f(t) are both known, the calculation of G/σ is a simple matter. The assumption made in the equation for r(tj is that the moving coil resistance and inductance are negligible compared to the impedance of mass-sprmg-dashpot components
An alternative approach would be to feed a very sharp impulse δ(t 0 ) to the platform moving coil At the instant I 1 (t o <ti), when the impulse terminates, the coil is disconnected electronically from the impulse generator such that the coil looks into an open circuit With these initial conditions, the mass velocity in the frequency domain is given b} ? : Uffj = K(I /α) (TCf)] , where K is the '-white' ' spectrum of the impulse. Hence, the output O(f) of the detector can be found by: O(f) = (G/α) [T(f)j . The constant K has not been included in the last expression since its value is known and can be compensated for. The chief advantage of this approach is that no analogous mods] for T(f) is required since this parameter is obtained from the impulse measurement. Thus, the noise component n(t) in p(t) can be calculated either in the frequency domain as: N(f) = O(f)Q(f), or in the time domain as: n(t) = o(t)Qq(t). where, o(t) is the time domain expression (Fourier transform) of O(f) and α stands for "convolved with".
As shown above in the description of the relative motion compensator, the surface vibration velocity s(t) can be computed by: s(t) = [p(t) - n(t)]. Upon expanding ti(t) ; this expression can be written as: s(t) = [p(t) - (G/α) {t(t)}D (q(t)}]. where (G/α) is a gain factor defined above as the ratio of proportionality and transduction factors (see the description of the relative motion compensator above), t(t) is the time domain expression of the velocity ratio of mass/(spring-dasbpot) in the platform, while q(t) is the output signal from the platform moving coil. The noise term total transfer function (G/α){t(t)} can be determined using the impulse measurement method described above.
The uniaxial motion of the platform mass can also be advantageously monitored by means of a single accelerometer fitted on the media ) mass. A MEMS accelerometer is ideally suited for this purpose because of its small size and excellent low frequency sensitivity. IvIEMS accelerometers are available with a variety of outputs: frequency, digital and analog. For generality and compatibility with the mathematical notation used above, the symbols used in this description will represent physical quantities such as acceleration velocity and voltages.
Since the unit used for the detector output p(t) is velocity (see Figure I), the. mass acceleration a(t), picked up by the accelerometer, must also be converted into velocity b(t) before it can be applied to estimate the noise n(t). Conversion of a(t) into b(t) is accomplished by convolution of a(t) with an integrator operator ift). such that: b(t) = | a(t))s fi(tj} . For simplicity, it is assumed that the velocity signal b(t) is also the mass motion velocity u(t). i.e. b(t) = u(t). The output voltage c(t) produced b) ; the accelerometer. is related to u(t) thiOUgh the accelerometer transduction factor β = voltage/velocity. Thus: c(t) = β !u(t)J .
Employing the same factor}' calibration procedure, described above, for appfying "forced" motion to the platform mass, the signal appearing at the output of the detector is identical to that already defined above and it is repeated here for convenience: r(t) = (G/α) ff(t)} .
However, the accelerometer output converted into velocity, resulting from the application of f(t) to the platform coil, is defined as: d(t) = (β/α){f(t)} . Solving both expressions in the frequency domain for the ratio (G/β). the following relationship is obtained: (G/β) = (R(f)}/ (D(f)} . Since R(f) and D(f) are respectively the frequency domain expressions of the detector and accelerometer outputs, measured with the platform casing rigidly clamped to a baffler, the ratio (G/β) can be obtained quantitatively.
The expression for the noise n(t) can now be derived by combining both the accelerometer output (see above), defined in the frequency domain as C(f) = β fU(f)j , and the ratio (G/β) as defined above. Recalling from the description of the RMC above that the noise N(f) found at the detector output is given by N(f) = G fU(f)}, then N(f) = [R(f)|/ [D(f) } C(f). It should be noted that, when using an accelerometer to estimate the phase noise, the expression for (G/β) does not depend on a knowledge of the induced mass movement, such that any movement of the mass can be used to calibrate the sensor. By contrast, the derivation of (G/α). described above, requires full knowledge of the induced mass movement, and hence the use of a known input signal
Finally, all the operations on the MEMS accelerometer output leading to the estimation of the noise term n(l) are preferably earned out in the RMC unit. As already stated above, an additional purpose of the PJvIC unit is to provide noise cancellation from the detector output p(t).
The main advantages of using a MEMS accelerometer to monitor the mass motion velocity u(t) are: a. The noise n(t) (accelerometer derived noise ADN) can be readily calculated without the need for an electromechanical model of the accelerator transducer. b. It is possible to continuously compare ADN to the noise derived from the platform moving coil output (coil derived noise CDN). where both an accelerometer and a moving coil are provided. This comparison can considerably increase the confidence in the estimated noise. c. Improved accuracy of noise estimation can be achieved by using both the ADN and the CDN, and in particular by averaging these two different noise estimates.
Because of their small size. MEMS accelerometers are particularly suited to being fitted on the platform mass.
Lastly, under certain conditions, the transfer function of the noise term n(t) can be estimated with only a statistical knowledge of s(t) and the measured signal q(t), by means of well known least squares algorithms, e.g. Wiener-Levinson. This method will yield useful results provided there is negligible correlation between s(t) and q(t).
The closely spaced transmitter and receiver ultrasonic transducers layout, mounted on the mass of the platform in Figure 4, can only sense the vibration velocity component normal to the surface, To sense velocity components parallel to the vibrating surface, the value of the grazing angle of the reflected Doppler modulated carrier must depart considerably from the vertical. This condition is satisfied at relatively large source-receiver offsets of the transducers assembled on the platform mass, and an embodiment of the invention using this arrangement is shown in Figure 5. in side view (a), bottom plan view Cb). and front elevation (c). Figure 5 shows schematically (not to scale) the transducer substrate 6O 5 and the transducers mounted thereon. The main feature of the layout in Figure 5. is the two long offset transducers (long offset y receiver 64 and long offset x receiver 66). placed orthogonally at a distance from transmitter transducer 62. to sense the components of the x.y velocity field. On the same assembly there is a short offset transducer 68 for the (near) vertical, z, velocity field. The three velocity field components (3C) can be sensed with a minimum of four transducers, i.e. one transmitter and three receivers, A greater number of transducers would obviously improve the directivity and gain performance, but such an improvement has to be weighed against added complexity.
The dimensions of the orthogonal array depend on the value of the grazing angle and the separation of the substrate from the vibrating surface. Assuming a grazing angle of 60° and a substrate-to-surface separation of 0.02 m, the transmitter-to-receiver offset is 0.01m. Preferably, the offset should be kept well below the critical value at which the first received arrival is the energy refracted along the earth's surface, or otherwise processing means may be provided for distinguishing between desired and undesired signals. With this scheme for acquiring 3C vibration velocity data and appropriate computer processing software, it is possible to obtain reliable vibration velocities along x ; y,z coordinates. This feature is particularly useful for deriving seismic elastic wave parameters for characterization of the earth subsurface.
An alternative to the arrangement shown in Figure 5 is an array consisting of many transducers filling-up a space similar to that of the substrate shown in figure 5. With such an arrangement and adequate software, it is possible to simulate optimum arrays (including phased arrays) capable of discriminating the desired from the undesired signals in the vibrating surface.
A further modification of .the invention would be to use IvEBMS technology, as discussed above, to implement a fully integrated 3 C Doppler sensor inclusive of noise cancellation electromechanical and electronic components. I b
In conclusion the uncoupled acoustic sensoi CUASj of a preferred embodiment of the present invention foi measuring surface vibrations consists two novel components a An acoustic Dopplei velocimetei (ADV) capable of measuring components of the x,y.z vibrations velocity field (3C) without requiring physical coupling to the measured surface b An electromechanical stable platform which together with electronic Dopplei noise cancellation, prevents the signal generated by the surface vibrations being corrupted by noise resulting from the ADV transducers ' relative motion
In addition to the capability of the ADV to acquire 3C data, it has a sensitivity m the order of Iμm/s. a d3'namic range of typically 14OdB, broad bandwidth extending several hundred Hertz and tolerance to surface roughness The lattei property is extremely important for sensing seismic waves on the naturally rough earth surface
The ADVs low cost and small size makes it ideally suited foi the rapid deployment and retrieval of very large multi-sensoi arrays In a preferred arrangement, the deployment and movement of very large numbers of uncoupled sensors over vast areas can be carried out very rapidly and efficiently by means of highly mechanized means A typical mechanized system consists of a stress membei, a few kilometers long, to provide the support structure foi man)' hundreds to several thousands of sensors, spaced along its length. The entire stress membei -sensoi assembly may be encased m a structure suitable foi self propelling, or towmg. close the ground
Since there is no need Io "plant" the uncoupled sensors, the manpower requirements foi seismic surveys will be substantially reduced As a result, land and transition zone seismic surveys are made more cost effective and can be executed withm a much shorter time compared to conventional surveys employing coupled sensors To make the UAS more compact and lightweight MEMS teclinolog)' can be employed to implement an integrated construction of the platform and ADV In partzculai. by using appropriate technologies, it is possible to implement the ennie UAS including the tiansmittei and receive? transduceis and the phase noise cancellation system ( PJvIC) on a single chip
Furthermore b) positioning the sensoi 's leceivei tiansduceis orthogonally at short and long offsets with iespect to the transmittei it is possible to acquire 3C vibiation velocity data This featuie is particularly useful foi multi-component seismic data acquisition With the appropriate computei piocessmg software 3 C seismic data can be tiansfoπned into elastic wave parameters that provide extremely important information foi locating subsurface featuies where there is a high piobabihty of oil and gas accumulations
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