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WO1992000584A1 - Procede et appareil d'imagerie holographique et acoustique utilisës dans des equipements de telemesure acoustique marins et autres - Google Patents

Procede et appareil d'imagerie holographique et acoustique utilisës dans des equipements de telemesure acoustique marins et autres Download PDF

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Publication number
WO1992000584A1
WO1992000584A1 PCT/GB1991/001058 GB9101058W WO9200584A1 WO 1992000584 A1 WO1992000584 A1 WO 1992000584A1 GB 9101058 W GB9101058 W GB 9101058W WO 9200584 A1 WO9200584 A1 WO 9200584A1
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WIPO (PCT)
Prior art keywords
array
image
coefficients
transducer
pixel
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PCT/GB1991/001058
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English (en)
Inventor
Geoffrey Shippey
Ronald Mchugh
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Heriot-Watt University
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Publication of WO1992000584A1 publication Critical patent/WO1992000584A1/fr

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    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/18—Methods or devices for transmitting, conducting or directing sound
    • G10K11/26—Sound-focusing or directing, e.g. scanning
    • G10K11/34—Sound-focusing or directing, e.g. scanning using electrical steering of transducer arrays, e.g. beam steering
    • G10K11/341—Circuits therefor
    • G10K11/346—Circuits therefor using phase variation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S15/00—Systems using the reflection or reradiation of acoustic waves, e.g. sonar systems
    • G01S15/88—Sonar systems specially adapted for specific applications
    • G01S15/89—Sonar systems specially adapted for specific applications for mapping or imaging

Definitions

  • the present invention relates to acoustic imaging methods and apparatus.
  • the methods and apparatus disclosed are primarily intended for marine'sonar survey applications, but are also applicable to other applications of acoustic imaging.
  • MUSIC Multiple Signal Classification
  • MUSIC and other related algorithms aims at resolving far-field directional sources beyond the Rayleigh limit.
  • the methodology is closely related to that of adaptive pattern nulling, where the characteristic feature of an adaptive array is the automatic rejection of directional interference. If these algorithms are applied to the near-field situation, null-filling and directional mismatch are observed. For example adaptive nulling results in much shallower nulls, which appear in the wrong positions. It was the realization that the planar wavefront approximation is frequently inadequate that spurred research into imaging near-field objects, initially for medical imaging and non ⁇ destructive testing of materials.
  • SAFT Synthetic Aperture Focussing Technique
  • the main object of the invention is to improve the performance, and eventually to reduce the complexity and cost, of a wide range of acoustic remote sensing equipment, used to survey water volumes (Eg. fish shoal detention) , the sea-floor, and the sub-bottom region below the sea-floor.
  • the invention is applicable to standard acoustic sensing modes such as sector-scan sonar, side-scan sonar, and sub- bottom profiling.
  • the invention allows a whole insonified scene to be reconstructed from a set of digitised samples without the need for any form of special scanning equipment.
  • Swath bathymetry is an acoustic survey mode which allows the precise depth of the sea-floor or other water bottom to be measured in a strip on either side of the survey vessels track. Such survey is normally carried out with special forms of "multi-beam" sonar, which require complex electronics to produce.
  • the invention allows swath- bathymetry to be carried out using either a towed sonar array, or an array mounted on the hull of the survey vessel without the need for special electronics.
  • the Invention replaces conventional "front-end" electronics with a single Digital Sampling Unit. Subsequent processing is carried out digitally. This form of equipment can take advantage of the reducing cost and increasing performance of digital processing elements as technology develops.
  • processing modules can be used in the different acoustic sensing modes, such as sector-scan and side-scan. This brings considerable advantages to any manufacturer for product rationalization. In addition it allows processing modules to be shared between different acoustic sensors in ROV (remotely operated vehicle) applications, and other situations where space and electrical power are limited.
  • ROV remotely operated vehicle
  • Equipment embodying the Invention can offer superior performance to existing equipment, both in terms of image quality, and in the ability to select chosen regions of the whole image for larger-scale display and subsequent computer image-analysis.
  • the principles embodied in the Invention can also be applied to other acoustic imaging equipment, for example to medical ultrasonic scanners.
  • a typical problem with existing sonar equipment is to convert the image from polar to cartesian coordinates for operator interpretation, or subsequent computer processing. This conversion may require additional electronics, but in any case the image information is degraded as a result of the resampling implicit in the scan conversion procedure.
  • the coordinate system required for the image is allowed for in the Look-up Table values, and no conversion is necessary.
  • Dynamic Focus is achieved by correct computation of the Look-Up Tables.
  • sonar arrays which are non-linear in a geometric sense, for example to bend around the hull of a vessel.
  • the changes required to the imaging procedure can be allowed for in the Look-Up Table values.
  • an acoustic imaging method for marine survey and other purposes wherein a target area is insonified by periodic acoustic pulses, reflections of said pulses are detected by an array of at least two transducers which generate output signals in response thereto, and said output signals are processed so as to produce an image of all or part of the insonified target area, said image comprising an array of pixels each corresponding to a point in the insonified area and the intensity of each pixel representing the strength of the pulse reflected from said corresponding point, comprising the steps of
  • steps (c) (ii) and (iii) above are executed by:
  • a preferred form of step (iv) above to achieve pulse compression of frequency-modulated (chirp) pulses comprises adding the cross-products of the in-phase and quadrature frequency components of the composite signal with the corresponding predetermined frequency component of the transmitted pulse.
  • An adaptation of the basic method for swath bathmetry comprises the steps of:
  • a preferred method of determining the times of flight specified in step (c) (i) of the basic method uses digital Time-Delay Look-Up Tables which may be wholly or partly predetermined, such Look-Up Tables being adapted to offer specific imaging facilities including:
  • a preferred method of generating the Time Delay Look-Up Tables is based on the time delay from a given pixel point to a selected reference element on the array plus the differential delays to other transducers of the array.
  • an acoustic imaging apparatus for marine survey and other purposes comprising transmitter means for transmitting periodic acoustic pulses, an array of at least two transducers for detecting reflections of said pulses from a target area and for generating output signals in response thereto, and means for processing said output signals so as to produce an image of at least a part of said target area, said image comprising an array of pixels each corresponding to a point in said target area and the intensity of each pixel representing the strength of the reflected pulse from said corresponding points; and further comprising:
  • said data processing and image generation means is adapted to select a set of digitised samples from each transducer for each pixel of the image, said selection being determined by the time of flight a pulse from the transmitter means to the corresponding point in the target area and back to the array, to correct each selected sample set by some form of interpolation so that it is precisely aligned with the required time of flight, to derive a composite signal from the time-shifted sample sets for each pixel, representative of the strength of the reflected pulse from the corresponding point, and to derive an intensity value for each pixel from its corresponding composite signal.
  • the data processing and image generation means may be adapted to perform any of the particular optional method steps defined above.
  • the data acquisition and storage means preferably includes means for amplifying the analogue signals received from the transducers fo the array, analogue to digital conversion means for sampling the.analogue signals, and means for multiplexing the signals from each transducer channel.
  • the analogue output signals from each transducer channel are amplified and digitised separately, and the parallel, digital signals are digitally multiplexed prior to storage.
  • the analogue output signals from the transducers are amplified separately in a first amplification stage, the parallel, amplified analogue signals are analogue multiplexed, and the multiplexed, analogue signal is further amplified prior to digitisation and storage.
  • the further amplification stage is a digital switched gain stage wherein the gain is adjusted to compensate for attenuation of the reflected signals.
  • the serial implementation preferably also includes similar pre- amplification and digital switched gain stages for each transducer channel prior to digitisation.
  • the date acquisition and storage means preferably further includes timing and control means to control the multiplexing means, digitally switched gain and digitisation.
  • Fig. 1 is a block diagram of an acoustic imaging system embodying the invention
  • Figs. 2(a) and 2(b) are, respectively, schematic illustrations of alternative parallel and serial implementations of the data acquisition means of the system of Fig. 1;
  • Fig. 3 is a more detailed block diagram of a serial implementation of the serial data acquisition means fo Fig. 2(b) ;
  • Fig. 4 is a further schematic illustration of a serial data acquisition arrangement similar to that of Fig. 3;
  • Fig. 5 is a flow chart illustrating the steps of a data processing and image generation method in accordance with the invention.
  • Fig. 6 is a schematic illustration of two prior art approaches to beamforming in sonar systems
  • Fig. 7 is an illustration of a typical source signal as used in the described embodiments of the invention.
  • Fig. 8 is a simplified illustration of an insonified scene with a superimposed grid
  • Fig. 9 is similar to Fig. 8, and illustrates stabilisation against movement of the transducer array
  • Fig. 10 is similar to Figs. 8 and 9, and illustrates differential delays to different transducers of the array with the array yawed through an angle ⁇ .
  • the overall system consists of two main modules:
  • Fig. 1 is a block diagram illustrating the system, comprising a sonar head 10 consisting of an array of ultrasonic transducers and a pulse transmitter or projection.
  • Data Acquisition and Storage means 12 and Data Processing and Image Generation means 14.
  • the sonar head 10 itself is of conventional type and will not be described in further detail herein.
  • the number of transducers in the array can vary, and useful basic imaging can be accomplished with relatively few transducers down to a minimum of two.
  • the Data Acquisition and Storage means 12 is adapted to amplify, digitize, sample and store the signals received from the sonar head 10, and the Data Processing and Image Generation means 14 processes the stored samples to generate the required images. These are described in greater detail below. 5.2 Data Acquisition and Storage
  • the Data Acquisition and Storage means 12 comprises:
  • Fig. 2(a) shows a parallel implementation wherein each transducer of the sonar head 10 is connected to a digital multiplexer 16 via separate pre-amplification stages 18, digitally-switched-gain amplification stages 20 and analogue-to-digital conversion means (A/D) 22.
  • the output from the multiplexer 16 is transmitted to digital memory for subsequent data processing.
  • Fig. 2(b) shows a serial implementation wherein each transducer of the sonar head 10 is connected to an analogue multiplexer 24 via separate pre-amplification stages 26. The output from the multiplexer 24 is then transmitted via a single digitally-switched-gain amplification stage 28 to A/D 30, and thence to digital memory 32 for subsequent processing in the data processing and image generation means 14.
  • the parallel implementation represents the best option in terms of A/D performance as the conversion speed need only be above the single channel Nyquist rate.
  • the ability to match amplitude and phase characteristics in the amplifiers was at first thought to be a problem and for this reason the serial implementation was chosen instead. Additional tests on a prototype digital switched gain board however confirms that the problem of matching channels may not be as difficult as expected.
  • the DMA operation is currently performed at a rate of lOMbytes/s into a modified graphics processor board, Scanbeam. It is technically possible to increase this rate to well over 20Mbytes/s using off the shelf technology.
  • the 10 MHz DMA restriction has fundamental implications on the overall system design.
  • a 360kHz sonar array is used.
  • the Nyquist Low- pass theorem dictates a rate of over twice 360kHz. This limits the number of channels to the order of 12.
  • the number of channels in turn sets the aperture size and hence the angular resolution. If the size of the DMA RAM storage is 1Mbyte, then in combination with the digitisation rate this sets the total range to the order of 20 metres.
  • the sonar transmitter can however be thought of as a narrow-band source so the sampling rate can be equated to the Nyqulist Band-pass theorem. Given a bandwidth of typically 5% of the carrier we can see that a twenty fold increase in the aperture size is quite feasible. For the purposes of the present example this was not implemented as it was adequate for experimentation to restrict the number of channels to the order of nine..
  • the total sonar is split into 3 parts, the array itself, the amplifiers and digitisation module, and the digital storage and processing engine. Physically these three outlets are interconnected by cables.
  • the output from each element of the array feeds up individual 60 ohm twisted pairs using a differential cable driver- and into the input connecter of the amplifier and digitisation module.
  • the output of the A/D converter on the digitization unit is 8- bit ECL parallel data and is cabled into the input connector of the VME host processor.
  • Fig. 3 illustrates the architecture of a serial system embodying the invention, in this case having an array of fifteen transducer elements, comprising a sonar head 40, data acquisition means 42 and data storage/processing means 44.
  • the head 40 comprises an array of transducers and pre-amps 46, each connected to a separate output channel, and projectors and power-amps 48.
  • the data acquisition means 42 which also controls the projectors 48, comprises separate input amplifiers connected to each transducer channel and disposed on three identical cards 50, 52 and 54.
  • the outputs from the input amplifiers are connected to the inputs of analogue multiplexer 56, and the output from the multiplexer 56 to an analogue to digital converter 58 via digital switched gain card 60.
  • the output from A/D 58 is connected to the memory of the data storage/processing means 44.
  • the data acquisition means 42 further includes a power supply card 62, which provides power supplied for the analogue and digital data acquisition circuitry, a low pass filter and line driver card 64 connected to the projectors 48 of the head 40, and a main control board 66 which controls the timing of the various system components and generates pulses for driving the projectors 48 via the filter card 64,
  • Fig. 4 is a schematic block diagram of a similar, n- channel serial system wherein the outputs from the transducers 70 are fed to an analogue multiplexer 72 via input amplification stages 74(a), 74(b) and 74(c).
  • the twisted pair cables from the array are connected to input amplifiers.
  • the input amplifiers have a total of 56 dB of matched gain for each channel prior to multiplexer input. This gain is split into three blocks; a first stage 74(a) of fixed 20 dB, a second stage of gain 74(b) selectable as either plus or minus 18 dB, and finally a buffer gain 74(c) of 18 dB driving 50 ohms.
  • the selectable plus and minus 18 dB of gain is used to ensure that the power levels into the multiplexer 72 remains high for the full echo return time. This high level signal ensures optimum performance from the analogue switches in the multiplexer.
  • acoustic echo returns are such that minus 18 dB of gain maintains a high input level into the multiplexer.
  • the incoming power level falls off and to keep the signal levels high the plus 18 dB gain is then selected.
  • These input gains may require a degree of adjustment in practice.
  • the range at which the gain change occurs is set by a timing and control card 76.
  • the analogue multiplexer 72 consists of a nine-to-one channel multiplexer ( mux) . Multiplexing is achieved using Siliconix quad analogue switches(SD5002) .
  • the 5D5002 is a.. DMOS FET switch with switching times of ns.
  • the specification for the mux. is that it must switch the nine channels with a lOMhz clock rate.
  • the mux. was designed to have a 12ns switching time and the timing of the A/D 78 set to correspond to the middle of the switching period.
  • the gate drive must shift from plus 15 volts to minus 15 volts in less than 10ns, which is achieved by means of a discrete transistor circuit.
  • the gate control signals are ECL timing signals from the timing and control card 76. After multiplexing the single channel is passed through a 20 dB attenuator 80 before entering the Digital Switched Gain card(DSG) 82. This card is designed to compensate for the normal attenuation characteristics of acoustic waves in water. An analogue switched gain circuit based on log amplifier technology would have a dubious phase characteristic over the full swept gain range, so a digitally switched gain board is preferred.
  • This board uses a number of high performance operational amplifiers (0PA6 75 swop amps) and analogue switches in different gain stages at the appropriate time in the attenuation curve. This board keeps the actual gain within 6dB of the theoretical gain required. As the output of this card feeds the A/D 78 converter the DSG circuit ensures that the A/D maintains an acceptable dynamic range for the full target range. The timing for this card is derived from the control card 76. The gain selection times are pre-programmed in a ROM.
  • the Analogue to Digital converter 78 is an 8-bit Datel 8303E, a development card. This card has an input analogue bandwidth of 40 MHz, a system requirement due to the slew rate specification at the output of the multiplexer. The output data is converted into ECL signals for output on the cable connecting the software processor.
  • the control and timing card 76 performs much of the system clocks and time references.
  • This card also includes the transmitter EPROM which is pre-programmed with the windowed sine wave transmitter shape values.
  • An 8-bit D/A circuit reads the output from the EPROM and this signal is sent to the projector power amplifier 80.
  • the digital storage is performed using a Scanbeam graphics processor board modified by a separate main controller board to act as a 10 MHz DMA.
  • Fig 5 gives a general block diagram for the image generation computation.
  • the processing in the individual blocks varies in detail depending on the specific application - sidescan, sector-scan, seismic processing, chirp source etc. but the framework remains the same.
  • This schematic diagram does not exclude other possible processing steps which may assist in specific instances, for example swath bathymetry.
  • the array is steered to an angle, a, with respect to the array normal in order to point at the reflector, P, in the far-field. This point is sufficiently distant from the array for the curvature of the echo wave-front to be negligible across the array, so that the wave-front can be assumed to be plane.
  • the signal received from the transducer, Tj ⁇ is delayed by a time (K-k)*t before adding the sum amplifier.
  • This specific set of time delays aligns the signals received from P, so that they constructively add together.
  • ⁇ any far- field point subtending the same angle, ⁇ , with the array normal.
  • signals received from a point subtending a different angle destructively interfere with each other.
  • This form of processing creates a sonar receiver beam pointing in the direction, ⁇ .
  • the width of the beam depends on the carrier frequency of the sonar signal which is echoed from P.
  • the nominal value is usually given as 1/L radians, where 1 is the wavelength of sound in the medium, and L is the length of the array.
  • phased-array sonar The standard engineering for the phased-array sonar is to implement the delay-sum architecture using tapped delay lines or other analog, or hybrid digital/analog, circuitry. Processing is carried out in real-time as the echo is received, and is only carried out for one specific beam angle at any time.
  • the sum amplifier is followed by some form of detector which is used to estimate the power in the echo, averaged over a time-interval.
  • the averaging time is roughly equal to the length of the transmitted pulse-packet, although the signal may later be smoothed over an interval equivalent to the resolution of the chart recorder.
  • detection techniques are employed, ranging from simple half-wave rectifiers to correlation detectors.
  • the simple system has the disadvantages that the digitisation rate is so high that the required sampling circuitry is very expensive, and that a large number of sample values must be stored to enable the computation to be carried out.
  • estimation of the rms power in a given pixel is obtained by forming a sum of squares of the composite samples within the time window.
  • Time-domain interpolation is only accurate when the sampling rate is high in comparison with the carrier frequency, so that the number of samples within a typical pulse envelope is also large. Hence the computation required to sum the squares of such samples is also expensive.
  • the signal, S ' (t) is a copy of S (t) delayed by a time, h. Then:
  • b 1 a sin(u) + b cos (u) .
  • a « a n cos(u n ) + b n sin(u n ) n
  • the first stage of processing is to carry out a local spectral estimate for each transducer signal for each sample point in turn.
  • the processing consists of estimating the in-phase and quadrature frequency components of different subsets of the stored transducer samples for a discrete set of frequency values. These components are calculated by convolving the sample sequence for each transducer with a set of FIR filters, one pair for each frequency required. One convolution kernel in each pair gives the in-phase component a n for that frequency, f n ' and the other kernel gives the quadrature component, b n .
  • each pixel in the image corresponds to a point (X,Y) in the insonified field.
  • pixel points For a given image size with a given pixel point spacing, it is possible to precompute the time delay from transmission of the outgoing pulse for its echo to reach each transducer from each pixel point. For example, if there are 33 transducers, and 512 x 512 pixels in the image, then there are approximately 8 x 10 6 delay values to be computed. These precomputed delay values can be stored in a Look-Up Table. In practice the size of this look-up table can be reduced considerably by the use of difference techniques.
  • this pre-computed look-up table can take account of the true geometry of the insonified scan, and allow for curvature of the echo wavefront. ie. dynamic focussing can be achieved by correctly computing the look-up table.
  • tj ⁇ is the stored value of the time delay for the wave-front to arrive at the transducer, Tj from a pixel point, P. Then this value tk can be used to find the nearest sample to this time delay in the sample set, and hence the nearest set of local frequency components ⁇ a ⁇ , ⁇ b j ⁇ n ⁇ of that transducer in the Frequency Component Store. In general the nearest sample will not have precisely the required time delay.
  • the first stage of imaging is to estimate the spectral composition of the delay-sum signal. This can be carried out in the frequency domain by adding the in-phase and quadrature components over the set of transducers:
  • a n ⁇ (a'k,n ) .
  • ⁇ n ⁇ ( b'k,n ) (5.2) k k
  • the intensity of the pixel in the image corresponding to P is then made suitably dependent on p (eg. proportional to W, sqrt(W) , log(W) etc depending on the requirement) .
  • p eg. proportional to W, sqrt(W) , log(W) etc depending on the requirement
  • the processing can be subdivided into stages in many different ways. For example it is possible to carry out all processing for each frequency component in turn for all image points.
  • a 2 + B 2 to the power, W, at each n n image point (Eq. 5.3) is then added into the image register. This processing continues until all frequency components have been processed. Alternatively the processing can be carried out for each pixel in turn for all frequencies. The processing can also be split between different processing units operating in parallel.
  • the length of the kernel ⁇ k m ,j ⁇ is 2R+1.
  • the entire set of component values for a given transducer can be generated by passing the sequence ⁇ s n ⁇ through a transverse filter loaded with the particular kernel values.
  • the optimum kernel set to be used depends on many parameters:
  • Carrier Frequency(ies) Nature of imaging task (eg. survey, isolated target identification, statistical classification of seafloor) Sampling rate
  • the local echo power estimate is normally required averaged over the length on the ground corresponding to the pixel separation in the range direction.
  • the time window in each transducer signal corresponding to the pixel spacing in the image is the space distance between pixel points divided by the 2-way speed of sound, e.g. 750 m/sec in water. In the above example, the time window is 266 microseconds.
  • the number of samples in the time window depends on the sampling rate. Thus if the sampling rate is 20 kHz, we have 13 samples in each time window.
  • the echo from the insonified scene will have a frequency composition which is related to the frequency composition of the source, so that a narrow ⁇ band source will produce narrow-band echoes, and a wide-band source will produce wide-band echoes.
  • the four source signatures are:
  • the first two categories are normally used in sector- scan and side-scan sonar.
  • the seismic wavelet would be employed for sub-bottom profiling and could be employed for swath bathymetry.
  • Chirp (swept frequency) sources may be used in each of the above applications.
  • Minimum Time-Bandwidth Sources A typical source signal has a single carrier frequency with an approximately gaussian envelope (Fig 7) . Other pulse shapes with well rounded envelopes are also near minimum time-bandwidth. We assume that the length of the pulse is approximately the same as the pixel spacing in time (which is the usual situation) . Then the time-bandwidth law asserts that the uncertainty in frequency of any spectral estimate made during that time window is equal to the bandwidth of the signal itself. Hence there is no point in attempting to measure more than one frequency component.
  • Spectral estimation now reduces to the problem of determining the amplitude and phase of the carrier in the local echo.
  • the standard solution to this problem is to use a quadrature matched filter. A pair of -kernels are created whose values are just the sampled source signature, and the sampled source signature with carrier phase-shifted by 90 degrees.
  • This pair of kernels is now used to generate a single pair of Frequency Components in the Frequency Component Store.
  • the standard technique for local spectral estimation used in seismic processing is to carry out a local DFT within the time-window.
  • the sampled signal is first padded with zeros so that the length is 2**n, e.g 512 or 1024.
  • the signal is tapered to zero at each end so that it becomes approximately periodic, and the standard DFT algorithm is applied.
  • the combined operations of taper followed by DFT are equivalent to a set of convolution operations.
  • the signal component of the echo is known to consist of a limited range of frequencies. Hence all frequencies outside this range are ignored. Careful choice of sampling frequency avoids aliasing problems.
  • there are mathematical modifications to the Fourier kernels which correct for aliasing to some extent (Corrected DFT Technique) .
  • An alternative approach is to use a set of quadrature bandpass filters to estimate the local spectrum.
  • the number of such filters is equal to the time- bandwidth product of the signal to be estimated. This approach minimises the number of frequency components which are computed.
  • the DFT solution may have some merit if special hardware (DFT chips) can be 'employed in the computation.
  • a typical wavelet consists of a pair of sharp positive and negative peaks followed by a longer period ripple.
  • the spectral composition may have a maximum around 3 kHz, but the spectrum will extend from around 1 kHz up to 10 kHz.
  • Such a signal can be handled in precisely the same way as the band-limited signal discussed earlier, though here the minimum sampling frequency given by Nyquist is just twice the maximum frequency in the signal, and there is no special aliasing problem.
  • the chirp source can be treated as a band limited source with a rather wide bandwidth, and either a DFT kernel set, or the Corrected DFT technique of the Quadrature Bandpass Filter technique can be used to generate the kernel set.
  • the length of the time- window used in the kernel set should be at least the length of the chirped pulse, even if the pixel spacing is closer, if the pulse compression technique described below is to work successfully.
  • the final step in image generation should be modified to carry out pulse compression of the chirped echo.
  • Pulse compression of a chirped echo is normally carried out by matched filtering of the echo with a copy of the chirped source signature. Using digital signal processing, both the echo and the source signature are sampled.
  • Matched filtering in the time-domain is equivalent to multiplication of the complex spectrum of the echo by the complex spectrum of the chirp source in the frequency domain. This is easily achieved with the given processing scheme, since the set ⁇ A n ⁇ , ⁇ B n defined in Sec. 5.3.8 is just the complex' spectrum of the echo. If the complex spectrum of the chirp source, ⁇ P n ⁇ , ⁇ Q n ⁇ i s precomputed by the same technique as that used to estimate the local echo spectrum, then:
  • Equation 5.4 gives the local power of the pulse compressed signal.
  • Any change to the image size for example generating a zoomed display of part of the insonified scene, can be done be substituting a new look-up table.
  • image stabilization against movement of the sonar platform can be done by updating the look-up tables as the platform moves.
  • the table stores time delays. In fact it is more convenient to store values as (delay/sample period) but this is a detail.
  • the following sections show how the look-up tables can be structured to reduce storage capacity and regeneration time at the expense of a slightly greater access time. The method of updating the tables to achieve image stabilization is also described.
  • the look-up table In a basic survey mode of operation, the look-up table is fixed for a given location of the imaged scene with respect to the transducer array. Hence the Delay Table could be precomputed for each standard mode of survey and stored in ROM. However, when a 'zoom' facility is used, the Delay Table for the zoomed window must be generated rapidly if the facility is to be of operational value. An even more serious problem arises if the imaged scene requires to be stabilized against movement of the transducer array, because the look ⁇ up requires to be refreshed each ping.
  • the present invention proposes a solution based on first and second order differences from a 'Base Table' .
  • the grid spacing for the array may be a suitable grid for surveying the whole scene, or a much smaller grid suitable for zoomed, stabilised display.
  • the basic grid may be allowed to cover more of the scene, and hence be bigger than the displayed window.
  • the (R, ⁇ ) coordinates of each grid point with respect to S only are computed and stored in a pair of basic tables.
  • This R, ⁇ table will be referred to as the 'Polar Base Table• .
  • the R co-ordinates can be stored in whatever are most convenient. In this document, distance in the water medium and the corresponding time delay at the speed of sound are used interchangeably.
  • R 1 R - d cos( - ⁇ ) (la)
  • ⁇ ' ⁇ - d sin( ⁇ - ⁇ ) /R (lb)
  • Figure 10 shows the original image of Figure 8, with the array superimposed.
  • the array pointed along the Y-axis of the grid. Starting from this reference time, it is required to stabilize the image against rotation of the array in the (X,Y) plane.
  • the array makes an angle, a, with the Y axis after several pings in a stabilised mode of operation.
  • r(l) 2 R 2 + l 2 + 2R1 cos ( ⁇ /2- ⁇ - ⁇ ) ,
  • r(l) 2 R 2 + l 2 + 2R1 sin ( ⁇ + a) (3)
  • R is the range to the array centre, given by the Polar Base Table. The solution to this equation is required for each value of 1 corresponding to a transducer location on the array. If the transducer spacing is a constant equal to d 0 , and there are 2m + 1 transducers mounted on the array, then 1 takes on the values:
  • Equation 6 can be used to generate all the rj in the +ve direction. Changing the sign of d 0 gives the r in the -ve direction.
  • Equations (9) are much faster to compute than Equation (7) .
  • Equations (9) are easier to implement in fixed-point arithmetic.
  • ⁇ n+l arccos (x n + ⁇ /r n+ ⁇ )
  • ⁇ n+ l arcsin (y n + ⁇ /r n + ⁇ )
  • the absolute error in the delay values determines the precision with which the image is stabilized.
  • the tolerable error is some fraction of the acoustic travel time corresponding to the image grid spacing, which is usually of the same magnitude as the pulse length. Error in computing the delay differences between transducers leads to phase errors which degrade the imaging process itself.
  • the allowable difference error between transducers is therefore some fraction of the source signal carrier period.
  • delay differences require to be at least one order of magnitude more precise than the absolute delay values.
  • the delay differences can be converted directly into phase-differences between transducers when the spectral in-phase and quadrature components are calculated.
  • Equation (la) requires 2 add operations, 1 multiply, and a cosine table look-up for each pixel point. Assuming 50 nanoseconds per operation, the slowest execution time which can offer any kind of real-time capability, the computation time to stabilize for array translation is 50 ms.
  • the next task is to generate the differential delays for each of the transducers in the array. Computation of initial conditions for the difference equations, (8) or (9) above, will also take 50-100 ms, assuming the necessary trigonometric function tables are available. Computation of delay difference costs one or two additions per transducer per pixel point (test and branch instruction may be avoided using in-line code) . The total time or 256 x 256 pixel points and 40 transducers is around 1 sec. This time is not likely to dominate the holographic computation.
  • the computing cost is 2 additions, one division operation and 2 table look-ups per move. Assuming 50 ns per operation, the cost is 250 ns per grid point, and 62.5 ms for a 512 x 512 window. If the division operation is too expensive, the time can be reduced using a similar approximation to the one used to find the individual transducer delays, but It is probably not worth the trouble, particularly as zoom windows are likely to be smaller than 512 X 512.
  • a particular application of the above general imaging scheme is swath bathmetry.
  • the required sonar array configuration is similar to sector-scan, except that the array points vertically downwards, with the projected fan- shaped beam normal to the direction of motion of the sonar platform. As the platform moves, a section of images are obtained of the sea floor along a line below the array, approximately at right angles to the ships track.
  • the array itself may be a conventional linear array, or bent in an arc around the hull of the ship or towed fish. As no penetration is required , high frequencies can be employed as for short- range sector-scan sonar.
  • the imaging procedure is essentially the same as described above, except that the image is only required in the neighbourhood of the expected location of the sea-floor, hence the following procedure is adopted. After each ping, the image is reconstructed, and a suitable image processing algorithm is used to locate the sea-bottom horizon in the image. This horizon should be corrected to true depth and horizontal offset allowing for the attitude of the sonar (yaw, pitch and roll) .
  • the imaging area for the next ping is controlled by the horizon found in the previous ping or pings, using any prediction techniques which may be appropriate.
  • the aim is to reduce the number of pixels which are imaged, in order to speed up image reconstruction time, and hence increase the possible speed of the survey vehicle.
  • the embodiments of the invention described herein relate generally to systems having linear transducer arrays, from which images of two-dimensional sections of the target area can be generated, however the invention is equally and directly applicable to planar (n x m element) arrays, T- shaped arrays and the like which may be used to produce three-dimensional images.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Acoustics & Sound (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • General Physics & Mathematics (AREA)
  • Multimedia (AREA)
  • Measurement Of Velocity Or Position Using Acoustic Or Ultrasonic Waves (AREA)

Abstract

L'invention se rapporte à des procédés et à un appareil d'imagerie acoustique destinés à améliorer les performances et à réduire le coût et la complexité de tels procédés et de tels appareils. Ainsi, le procédé consiste à: (a) échantillonner des signaux de sortie provenant d'une multiplicité de transducteurs à une vitesse prédéterminée et entre des impulsions acoustiques périodiques; (b) numériser et mémoriser les échantillons; (c) pour chaque pixel d'une image (i) sélectionner un ensemble correspondant d'échantillons provenant de chaque transducteur; (ii) corriger chaque ensemble d'échantillons sélectionné par interpolation de sorte qu'il est précisément aligné avec le temps de trajet requis du point pixel au transducteur; (iii) dériver un signal composite représentant la puissance de l'impulsion réfléchie par le point; et (iv) déduire l'intensité du pixel à partir du signal composite. L'appareil comprend une tête de sonar (10) composée de transducteurs et d'un émetteur d'impulsions, de moyens d'acquisition et de mémorisation de données (12) et de moyens de traitement de données et de production d'images (14).
PCT/GB1991/001058 1990-06-29 1991-06-28 Procede et appareil d'imagerie holographique et acoustique utilisës dans des equipements de telemesure acoustique marins et autres WO1992000584A1 (fr)

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Cited By (6)

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US5793703A (en) * 1994-03-07 1998-08-11 Bofors Underwater Systems Ab Digital time-delay acoustic imaging
DE10334902B3 (de) * 2003-07-29 2004-12-09 Nutronik Gmbh Verfahren und Schaltungsanordnung zur Verarbeitung von Signalen, die bei der zerstörungsfreien Prüfung von Gegenständen durch Reflexion von Ultraschallwellen erzeugt werden
US8609630B2 (en) 2005-09-07 2013-12-17 Bebaas, Inc. Vitamin B12 compositions
WO2014117767A1 (fr) * 2013-01-29 2014-08-07 Atlas Elektronik Gmbh Signal sonore sous-marin, émetteur ou récepteur sous-marin, sonar sous-marin, véhicule sous-marin et complément d'équipement
CN107796473A (zh) * 2017-11-09 2018-03-13 广东美的环境电器制造有限公司 一种监测排泄数据的方法、装置以及坐便器
WO2019014771A1 (fr) * 2017-07-20 2019-01-24 UNIVERSITé LAVAL Procédé et système de détection de deuxième ordre pour applications de télémétrie

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WO1985000889A1 (fr) * 1983-08-05 1985-02-28 Luthra Ajay K Systeme de prise d'images d'un corps utilisant l'addition vectorielle de reflexions acoustiques pour obtenir l'effet d'un faisceau de balayage focalise en continu suivant un alignement geometrique predetermine
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EP0109869A1 (fr) * 1982-10-22 1984-05-30 Thomson-Csf Dispositif numérique de formation de voies sonar
WO1985000889A1 (fr) * 1983-08-05 1985-02-28 Luthra Ajay K Systeme de prise d'images d'un corps utilisant l'addition vectorielle de reflexions acoustiques pour obtenir l'effet d'un faisceau de balayage focalise en continu suivant un alignement geometrique predetermine
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Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5793703A (en) * 1994-03-07 1998-08-11 Bofors Underwater Systems Ab Digital time-delay acoustic imaging
DE10334902B3 (de) * 2003-07-29 2004-12-09 Nutronik Gmbh Verfahren und Schaltungsanordnung zur Verarbeitung von Signalen, die bei der zerstörungsfreien Prüfung von Gegenständen durch Reflexion von Ultraschallwellen erzeugt werden
US7581444B2 (en) 2003-07-29 2009-09-01 Ge Inspection Technologies Gmbh Method and circuit arrangement for disturbance-free examination of objects by means of ultrasonic waves
US8609630B2 (en) 2005-09-07 2013-12-17 Bebaas, Inc. Vitamin B12 compositions
WO2014117767A1 (fr) * 2013-01-29 2014-08-07 Atlas Elektronik Gmbh Signal sonore sous-marin, émetteur ou récepteur sous-marin, sonar sous-marin, véhicule sous-marin et complément d'équipement
WO2019014771A1 (fr) * 2017-07-20 2019-01-24 UNIVERSITé LAVAL Procédé et système de détection de deuxième ordre pour applications de télémétrie
US11460558B2 (en) 2017-07-20 2022-10-04 UNIVERSITé LAVAL Second-order detection method and system for optical ranging applications
CN107796473A (zh) * 2017-11-09 2018-03-13 广东美的环境电器制造有限公司 一种监测排泄数据的方法、装置以及坐便器
CN107796473B (zh) * 2017-11-09 2024-03-15 广东美的环境电器制造有限公司 一种监测排泄数据的方法、装置以及坐便器

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