GB2395280A

GB2395280A - Processing data from an inductive sensor to detect an electrically conductive target

Info

Publication number: GB2395280A
Application number: GB0322415A
Authority: GB
Inventors: Simon Jordan
Original assignee: VT TSS Ltd
Current assignee: VT TSS Ltd
Priority date: 2002-09-25
Filing date: 2003-09-25
Publication date: 2004-05-19
Anticipated expiration: 2023-09-25
Also published as: GB2419956A; GB0222186D0; GB2395280B; GB0322415D0; GB2419956B; GB0526127D0

Abstract

A method of processing a signal from an inductive sensor is disclosed for detecting an electrically conductive target such as a pipe or cable below the seabed. The method comprises obtaining information about the sea water response from the measured signal and determining the component of the signal arising from the electrically conductive target. The value of a parameter of the signal such as the magnitude of the signal voltage waveform for time windows 31,32,33 centred on three different times ta,tb,tc in the signal lifetime is obtained and used in the determination of the component of the signal arising from the electrically conductive target. The information about the sea water response is further determined from the measured signal eliminating the need to model the sea water response. In another embodiment, information about the sea water response may be determined by obtaining the values of first and second parameters of the signal for at least two different heights of the sensor above the sea bed in the absence of a target, and making a linear or quadratic fit to the values of the first and second parameter.

Description

Inductive Detection of Target and Data Processing of Sensor Signal The

present invention relates to a method of and an apparatus for processing data from an inductive sensor to detect an electrically conductive target. The present invention 5 also relates to a method of and an apparatus for inductively detecting an electrically conductive target making use of such data processing techniques.

There are several known techniques for detecting the presence of, and locating the position of, electrically conductive targets such as pipes and cables below the scabcd.

10 One example of such techniques is based on elcctro-magnetic induction and involves generating a pulsed magnetic field in a search region surrounding one or more coils.

The pulsed field induces eddy currents in conductive materials within the search region,

which represents the range of sensitivity of the system. Following each pulse, the induced eddy currents in conductive materials within the search region decay and 15 induce in the or each search coil a voltage waveform, which is processed to indicate the range of any target within the search region.

In addition to the contributions from any targets in the search region, the voltage signal measured in a search coil contains contributions from other sources, generally referred 20 to as "background" sources. For example, salt water is electrically conductive so, where

such a system is operating within or in the vicinity of a salt water body, the pulsed magnetic field will induce eddy currents in the sea water and the decay of these eddy

currents will induce a voltage in the search coil. The measured voltage signal in the search coil therefore contains a substantial background contribution from the sea water.

25 Furthermore, the or each search coil is generally mounted on a remotely operated vehicle (ROV), and any metal parts of the ROV will also give rise to a background

signal. A further source of background signal is the sea-bed itself, which may have

electrical conductivity arising from, for example, metallic contamination.

30 Figure 1 illustrates some of the components of a measured waveform obtained when locating a 27cm diameter pipe buried below the seabed. The components have been displaced slightly in the vertical direction so as to make them more easily visible.

The sea water in the search region produces an approximately exponential decay signal in the search coil, and this decay signal Is shown at S(). The pipe has two decay modes caused by current crculatmg circumferentally and axially of the pipe. The saddle 5 decay curve 51 represents the decay signal from current circulating circumferentially and the axial decay curve 52 represents the decay signal from current circulating axially.

In Figure 1, the "result" 53 is equal to the sum of the decay curves 50, 51 and 52 and represents a signal measured at a search coil from which the contribution of the ROV 10 has been subtracted. (Whereas the response of the seawater can vary substantially depending on the prevailing conditions when the measurements are made' the response of the ROV on which the search coil is mounted is substantially constant irrespective of conditions. This may therefore be measured and subtracted from each measured signal so that the contribution of the ROV can be substantially eliminated. However, if 15 necessary or desirable, curve fitting to the component resulting from the ROV may be included.) The problem faced in processing data such as the measured voltage waveform 53 of Figure l is to extract the target signal from the measured voltage waveform. 20 In order to reduce the effects of such background signals, each decaying waveform, or

summed set of decaying waveforms, has its amplitude measured at two time windows, with the separation between the time windows being chosen to provide a balance between good sensitivity and good rejection (i.e., attenuation) of the background

signals. From the two amplitude measurements, it is possible to provide an indication 25 of the approximate position, such as the range or distance from the coil to the target or the depth below the coil, of any target within the search region. However, because the background signals cannot be eliminated, such measurement is of limited range and

accuracy. 30 The magnetic pulse is generated by establishing a large current in the or each coil and then abruptly switching off the current. In theory, the search range can be extended and/or the accuracy of range measurements can be improved by Increasing the current

in the coil, and hence the magnetic pulse when the current is switched off. However, this also results m increased contributions l'rom the background signals. This is

particularly severe when operating in sea water with the result that little or no improvement m performance can be achieved merely by increasing the current and 5 using the conventional processing technique to analyse the resulting data.

One approach to improving the accuracy of determining the sea water response is to use three search coils, one mounted on the centreline of an ROV, and the others mounted on the left side and right side of the ROV respectively. A geometric technique, the "three 10 coil algorithm" is used to separate the target signal and the sea water signal. However, this method involves measuring the difference between two lengths, which become very similar at large ranges to the target - as the height of the triangle formed by the coils and the target increases, the length of the base becomes very small in relation to the lengths of the other two sides. Increasing the baseline by increasing the coil separation might 15 appear to help, but in practice this generally results In the voltages from the coils becoming too small to measure. This limits the range to approximately l.Sm. It would be pret'erable to use just two search coils, since this would allow the target position to be measured directly, rather than from a difference, and this should increase the range.

20 It has been suggested that it would be possible to fit the measured signal to two exponential or quasi-exponential decay curves, one curve corresponding to the sea-

water background and another curve corresponding to the signal from a target object.

However, it has found that the sea water response is not exactly an exponential curve.

Furthermore, the time constant of the best exponential fit to the sea water response 25 depends on the depth of sea water below the search coil, and this will not be constant during a survey - as noted above, the or each search coil is typically mounted on an ROV, and the ROV will move up and down as it travels through the water, to avoid hitting obstacles protruding from on the sea-bed. It has therefore been found to be very difficult to fit the measured signal accurately to two exponential decay curves.

According to a first aspect of the invention, there is provided a method of processing a signal from an inductive sensor to detect an electrically conductive target, the method

composing: obtaining mformaton about the sea water response from the measured signal; and dote m''ning the component of the signal arising from the electrically conductive target from the information about the sea water response dctemnned from the measured signal.

s The present invention enables inl\rmation about the sea water response to be derived trom the measured voltage waveform, in contrast to the prior art approach of attempting

to model the sea water response.

10 According to a second aspect of the invention, there is provided a method of processing a signal from an inductive sensor to detect an electrically conductive target, the method comprising: obtaining the value of a parameter of the signal at three different times in the signal lifetime; and determining the component of the signal arising from the electrically conductive target from the values of the parameter of the signal at the three 15 different times.

According to a third aspect of the invention, there is provided a method of processing a signal from an inductive sensor, the method comprising: determining the value of a first parameter of the signal for at least two different heights of the sensor above the sea bed; 20 and obtaining information about the sea waler response from the values of the first parameter of the signal at the different heights.

The method may further comprise determining the value of a second parameter of the signal for at least two different heights of the sensor above the sea bed; and obtaining 25 information about the sea water response from the values of the first and second parameters of the signal at the different heights.

The parameter of the signal according to the second aspect may be the magnitude of the voltage waveform. The first and second parameters of the signal according to the third 30 aspect may be the magnitude of the voltage waveform at particular times in the signal lifetime.

According to a fourth aspect of the mvention, there is provided a method of inductively detecting an electrically conductive target in a search region of space, comprsmg: generating a magnetic treed pulse In the search region; measuring a signal induced in an inductive sensor by currents flowing In the search region after the magnetic field pulse;

5 and determining the component of the signal aris ng from the electrically conductive target using information about the sea water response obtained according to a method of the third aspect.

According to a fifth aspect of the invention, there is provided an apparatus for 10 processing a signal from an inductive sensor to detect an electrically conductive target, comprising means for determining the component of the signal arising from the electrically conductive target from the values of a parameter of the signal at three different times in the signal lifetime.

15 According to a sixth aspect of the present invention, there is provided a computer programmed to perform a method according to the hrst, second, third or fourth aspect of the invention.

According to a seventh aspect of the invention, there is provided a program, for 20 programming a computer to perform a method according to the first, second, third or fourth aspect of the invention.

According to an eighth aspect of the invention, there is provided a storage medium containing a program according to the seventh aspect of the invention.

According to a ninth aspect of the invention, there is provided a method of inductively detecting an electrically conductive target in a search region of space, comprising: generating a magnetic field pulse in the search region; measuring a signal induced in an

inductive sensor by currents flowing in the search region after the magnetic field pulse;

30 obtaining the value of a parameter of the signal at three different times In the signal lifetime; and determining the component of the signal arising from the electrically

conduclvc target from the values of the parameter of the signal at the three different times. According to a tenth aspect of the mventon, there Is provided an apparatus for 5 inductively detecting an electrically conduct ve target in a search region of space, comprising: means for generating a magnetic field pulse m the search region; a sensor

for sensing currents flowing in the search region after the magnetic pulse; and means for determining the component of the signal arising from the electrically conductive target from the values of a parameter of the signal at three different times in the signal 10 lifetime.

According to an eleventh aspect of the invention, there is provided an apparatus for processing a signal from an inductive sensor, the apparatus comprsmg: means for determining the value of a first parameter of the signal for at least two different heights 15 of the sensor above the sea bed; and means for obtaining information about the sea water response from the values of the parameter of the signal at the different heights.

According to a twelfth aspect of the invention, there is provided an apparatus for inductively detecting an electrically conductive target in a search region of space, 20 comprising: means for generating a magnetic field pulse in the search region; a sensor

for sensing currents flowing in the search region after the magnetic pulse; and means for determining the component of the signal arising from the electrically conductive target using information about the sea water response obtained by an apparatus defined in the eleventh aspect.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a waveform diagram of typical sensor signal components against time; figure 2 indicates the method of the present Invention applied to a typical voltage waveform obtained at an inductive sensor such as a search coil;

Figure 3 11ustratcs the determmaton of the parameter t2; Figure 4 illustrates the determination of the parameter s2; s Figure 5 Illustrates results obtained by a method of the invention; Figure 6 is a diagrammatic side view of a remotely operated vehicle (ROY) including at least part of an apparatus for detecting a target constituting an embodmcnt of the 10 invention; Figure 7 is a diagram illustrating the detecting apparatus on the ROV and in a surface support vessel; 15 Figure 8 is a block flow diagram illustrating a method of the present invention applied to a typical sensor signal; and Figure 9 is a schematic block diagram of an apparatus according to the present nvcntion. Figure 2 illustrates the principle of the present invention. Figure 2 shows a signal V(t) measured at a search coil following generation of a magnetic pulse. The signal is the voltage measured at the search coil, and corresponds to, for example, the voltage waveform 53 of Figure 1. Ignoring the response of the ROV on which the search coil is 25 mounted, which may be corrected for as described below, the signal V(t) in general contains a component T(t) arising from a target object within the search region of the coil, and a sea water response component S(t), that is V(t) = S(t) + T(t) (l) The object of processing data acquired by the search coil is to fit the measured signal V(t) to a target response and a sea water response. The prior art approach has been to

attempt to fit the measured signal by modellmg the sea water response S(t) . The present invcuton does not attempt to model a response but, Instead, determines the response empirically from the measured signal. The t'itting process can be though of as a "black box" which analyses the Input waveform V(t), and outputs the target and seawater strengths. In fact, this may be further smplil''ed since, as there arc only two output parameters, the black box needs only two input parameters from the waveform.

Although more parameters may help, two well-chosen parameters are sufficient If the waveform is noiseless and If the parameters can be obtained with suitable precision.

This gives two equations in two unknowns, and the black box merely multiplies its 10 inputs by the inverse of a 2x2 matrix.

Having obtained the 2x2 matrix, it needs to be ted with two parameters from the measured trace. The present invention uses the approach of determining a parameter of the signal at three different times within the signal lifetime, so as to obtain two values IS reprcsentmg the change of this parameter over time; these two values provide the two inputs. In a preferred cmbodmcnt, the chosen parameter is the value (magnitude) of the measured voltage signal withm a small sample window, since this has proven highly effective and easy to implement. All that needs to be done is to measure within three sample windows, rather than within two sample windows as in the prior art, to reduce

20 the measured voltage signal to two numbers.

Figure 2 shows the way in which the three voltages are measured, at times ta, tb, tc, where tc > tb > tat The times ta, tb, to are at the centre of respective time windows 31, 32, 33, and the average magnitude of the voltage waveform V(t) is measured in each time 25 window according to any conventional technique. The measured average magnitudes of the voltage waveform at the windows 31,32,33 are then used to define two new voltages, as follows: V(th)-V(tc) = V1 (2)

V(ta)-V(tc) = V2 (3) In these equations, V(ta) denotes the average vo] tagc measured over the time window 31 centred on ta' and so on.

The three time windows represent "early", "late" and "zero" time windows. The "early" window 31 is preferably placed as early as possible in the decay curve to make the resulting 2x2 matrix as far from singular as possblc. However, placing the "early" window too early in time results In the loss of dynamic range, since the acquisition 10 system must encode the falling seawater response. The positions of the "late" and "zero" time windows 32,33 represent a compromise between sensitivity to the decay ol the target signal while obtaining good attenuation of seawater noise. It has been found that good results are achieved with time windows 10Ols wide and centred on 120lls, 300,us and 1000,us, but the invention is not of course limited to these time windows.

Again on the assumption that the ROV response has been corrected for, each of the average voltages V(ta), V(tb) and V(tc) is the sum of a target signal and a sea water background signal, so V(ta) = S(ta) + T(ta), and so on. The voltage V1, which will be

referred to as the "late" voltage, will therefore also always be a sum of a seawater signal 20 and a target signal, and so can be written as: Vl =S+ T (4) It should be noted that there is an implied normalization here, and that from here on the 25 terms S and T relate to the seawater and target signals in terms of the "late" voltage signal Vl.

The "early" sample voltage, V2, will be a different combination of seawater and target signals, and so can be written as: V2=s2S+2T (5)

This makes the assumption that the seawater signal always has the same shape regardless of the height of the search coil above the sea-bed. As noted above this assumption has been found not to be true, but it is possblc to add a non-lncar correction, y, to equation (5) to compensate for the variation in the sea water signal with 5 the search coil height. Effectively, the correction factor describes the change In time constant of the sea water signal decay curve as the sea water amplitude (de, the depth of sea water below the search coil) increases. Including the correction factor modifies equation (5) to read: 10 V2 = (s2 + yVl)S + t2T (6) The dimensions of y are reciprocal volts.

In order to apply the invention, the parameters 92 and t2 must be determined. For an 15 experiment performed on land, S is always zero, so equations (4) and (6) reduce to: V2 = t2 T (7) Vl = T (8) The "target enhancement parameter" t2 describes how the target voltage is enhanced by moving the sample window to the early position 31. The ratio of V2 to V1 can be rapidly determined from a few measurements. Figure 3 shows two sets of results obtained using a 340 sensor system of TSS (UK) Limited, one set relates to a large 25 buried pipe and the other relating to a small buried pipe. The "early" sample voltage V2 is plotted along the y- axis, and the "late" sample voltage V1 is plotted along the x-axis.

It will be seen that both data sets show a linear relation between V2 and V1, and the two straight lines in Figure 3 are the best "least squares" linear fits to the two data sets. The data relating to the small pipe yields t2 = 3.0 and the data relating to the small pipe 30 yields t2 = 2. 4. Other tests have shown that the parameter t2 has an approximate average value of 2.6; its exact value has been found to make little difference to the reported range and no difference to the elimination of the seawater signal.

In order to determine s2 and it, consider the case when no target is present, i e. when T = 0. Then, equations (4) and ((I) reduce to: 5 V2=(s2 +yVl)S (9) V1=.S (10)

Rearranging these equations gives V2=s2Vl+yVl2 (11) Thus, to determine s2, V1 and V2 are determined in absence of a target. In one method, a search coil is initially disposed on the sea-bed, away from any electrically conductive 15 target. A magnetic pulse is emitted, and the resultant voltage waveform Is detcrmincd.

The measured voltage waveform will consist of a sea water response and a background

signal, for example from the background response of the ROV on which the search coil

is mounted. The average magnitude of the recorded voltage waveform is determined for the "early" and "late" time windows 31,32. This step is a "background compensation"

20 step, since the average voltages determined for the early and late time windows in this step enable the ROV response to be corrected for.

The search coil is then raised from the sea-bed, and voltage waveforms are obtained at various heights above the sea-bed. For each of these voltage waveforms, the average 25 magnitude of the voltage is determined for the "early", "late" and "zero" time windows 31,32,33. The average magnitudes of the voltage determined for the "early" and "late" windows for each search coil height are then corrected by subtracting the average voltage magnitudes determined for the respective time window when the search coil was disposed on the sea-bed, to eliminate the ROV response. The early and late 30 voltages Vl and V2 are then determined for each voltage waveform, from the average magnitude of the voltage determined for the "zero" time window for that waveform and

the corrected average magnitude of the voltage determined for the "early" and late" time windows for that waveform.

The pairs of values (V1,V2) for each height of the search coil will yield a near-straight 5 line whose gradient is s2. If desired, a non-linear fit can be used to dctcrminc y. Note that in this case, fitting is apphcd to filtered, averaged data resulting from measurements on the measured voltage waveform, and not trom the waveform itself.

This process of determining the parameters s2 and can be considered as a "dynamic lo compensation" process. The process of lifting the search coil provides information about the sea water response, and this may be used to compensate an recorded voltage waveform for the sea water response. It is a compromise over the height above the sca-

bed to which the search coil is taken, and Will give an average target signal of zero over the entire height range used. Preferably, the coils are lifted over a height range similar 15 to that expected from the ROV during the survey.

Although the voltage waveform used for background compensation was recorded with

the search coil disposed on the sea bed in the above-described method, this is not essential. The voltage waveform used for background compensation may In principle

20 be recorded with the search coil at any height above the sea bed.

In one trial, a search coil was dropped to the sea-bed and an initial voltage waveform was recorded. The search coil was then lifted by Sm. and then lowered back to the sea-

bed. Data was recorded as the search coil was raised and lowered via a modified 25 version of the 340 firmware from TSS. This firmware measured the magnitude of the voltage waveform in three sample windows, centred on 150, 300 and l,000ps.

The conductivity of the seawater was found to be essentially constant with depth after the top, warmer layer of water was passed.

Figure 4 Illustrates typical results obtained. The y-axis shows the value of V2, the "early" voltage, and the x-axs shows Vl, the "late" voltage. (The values of the average

voltage for the early and late time windows were background-compensated as described

above before detenninaton of VI and V2.) These data were obtained using an ROV on which three search coils were mounted, and dficrent symbols are used to denote the data points for (V1,V2) obtained using the dflerent search coils. The data were 5 obtained in water of 16m depth. The heights of the search crisis above the sea-bed at which the data points were obtained are indicated in Figure 4.

It will be noted that best lit to the results of Figure 4 is not a straight line, and this is a consequence of the term in y in equation (6). The dashed line in Figure 4 is the best 10 linear fit to the data.

The full curve in figure 4 is the best quadratic fit to the data, having the general quadratic form y =yX2 + s2 x + k. This fit gave results of s2 = 20.0 and y = -0.12 (pV).

The y-'ntercept, k, is ignored.

It will be noted that the data pomts deviate from the best quadratic fit near the origin (corresponding to the search coils being disposed only a short distance above the sea-

bed). This arises from metallic contarmnation of the sea-bed.

20 Having found the parameters s2 and t2, the problem of determining the target signal can be restated as iV1] 1 1]: T] (12) 25 Next, a new parameter c is defined as follows: T = VI.c (13) Substituting for T in equation (12) using equation (3) leads to the following expression: F(c) = V2 - (s2Vl(l - c)) - (yVl 2 (1 - C)2) _ t2VlC = 0 (14)

This equation can have several different forms depending on the input parameters, but in general, there is a root which gives the solution of interest. Equation (14) may be solved by any suitable technique, for example by bisection.

In a case where the target signal is large compared to the seawater, or if the solver does not find a root, it Is possible to use a callback. This first uses equation ( 15) below: T V2 - s2V1 - yVIZ (15) t - s - yV1 If the value returned by equation (15) is greater than a preset threshold, for example greater than 601lV, the quadratic parameter y is set to zero, and equation (15) is used again. Setting the quadratic parameter y to zero in equation (15) results in the following equation: 7 V2 - s2V1 (16) t2 'S'2 In practice, s2 is typically 25 and is approximately -0.1. Given the exponential nature 20 of the range curve, t2 has little effect on reported range.

Figure 5 shows the results of applying equation (15). A search coil was mounted on an ROV, and was initially disposed on the sea-bed at a location where no target was present. At time t 12 seconds, the search coil was lifted from the sea-bed, and was 25 raised to around 5m above the sea-bed. This height was reached at time t 50 seconds, and the search coil was maintained approximately at this height above the sea-bed until t 135 seconds. The search coil was then lowered back to the sea-bed, reaching the seabed at time t 155 seconds. The search coil remained on the sea-bed for approximately 30 seconds, and then was then lifted off the sea-bed again, reaching a 30 height of approximately 5m above the sea- hed at time t 210 seconds. The search coil

was maintained at a hcght of approximately Sm above the sca-hed for the remainder of figure S. Trace (a) in Figure 5 is the rcadng of an altimeter attached to the 1lOV on which the S search coil was mounted, and gives the altitude of the search coil (its height above the sea-bed) (right hand scale).

Trace (b) in Figure S is the late sample voltage V1 (left hand scale) determined from the voltage measured in the search coil. This voltage was again measured using a 340 10 search coil from TSS. The ROV response has been eliminated, using the background

compensation technique described above. Since there is no target in the search region, the change in the measured voltage arises from the change in sea-water response as the altitude of the search coil changes.

15 Trace (c) in Figure S is the sensor voltage (left hand scale) after elimination of the component arising from the sea water response. More fonnally, trace (c) shows the larger component of the measured voltage as determined using equation (13), with the values of s2 and y as determined from the data of figure 4.

20 It will be seen that the target response is, as expected, close to zeroat all times. The method of the invention is therefore effective at determining the target response. (It will be seen that a slight correlation between the seawater response and the determined target response remains, owing to higher order terms in the relation between Vat and V2.) 25 The apparatus for inductively detecting electrically conductive targets is illustrated in Figures 6 and 7 for use in a marine environment for detecting and locating pipes or cables buried under the seabed. Figure 8 Is a block flow diagram of a suitable processing technique.

30 Parts of the apparatus are shown fitted to a remotely operated vehicle (ROY) 1, which is capable of operating at great depths on or near the seabed. The apparatus comprises search coils, and in this embodiment two search coils 2,3 are attached to the ROV 1 by

A. . 1 6

non-metallic supports 5 and 6. The supports 5 and 6 extend forwardly of the ROV I and the front of the support 6 Is provided with a buoyancy aid 7. The coils 2,3 are arranged side by side with the wndmg axis oriented vertically in normal use and with the coils being spaced by 60 to 100cms In front ol the main structure of the ROV 1.

s The coils 2,3 are independently connected to electronics on board the ROV I in environmentally sealed containers 10 and 11. The apparatus within the containers 1() and 11 supplies current pulses to each of the coils 2,3 in turn and performs some processing on the signals received from the coils 2,3 before supplying the pre-processed 10 signals via an ROV umbilical 14 connecting the ROV l to further apparatus 15 on board a surface support vessel (not shown). The apparatus Is performs further processing to provide information about the detection and location of targets below the seabed within the search range of the apparatus. The apparatus 15 comprises, for example, a computer programmed by suitable programs to process the data from the ROV and having a 15 display for providing a graphical representation of tracking data, located target data, Images from video cameras on board the ROV l and any other desired information.

The search coils interface with transmit (Tx)/receive (Rx) electronics 20 which supply drive to the coils and return signals detected or measured by the coils. A timing 20 generator 21 supplies timing signals which synchronise the operation of the apparatus.

The timing signals are used to select each of the coils in turn and to supply thereto a current pulse having an amplitude typically in the range of 20 to 40 amps and a duration typically of 1,500 microseconds. The inductance of each coil determines the time constant with which the resulting magnetic field increases to a substantially constant

25 level and, at the end of the pulse, the current through the coil is abruptly switched off so as to generate a rapidly decaying magnetic field which induces currents in electrically

conductive material within the search range of the apparatus.

After the current through each coil has been switched off, the coils are connected to a 30 preamplifier 22 and an analogue-digital converter (ADC) 23, which samples the signal induced in each coil as a result of currents mduced m the search region. The coil signal

Is sampled every four microseconds at 12 bit resolution and the resulting data together with sync signals arc supplied to the next process 24.

The process 24 is performed by the apparatus on board the ROV. The proccssmg 5 comprises data collection and averaging. In particular, in order to improve the sgnal-

to-noisc ratio of the measured signal, each set ol'eght consccutve waveforms following eight consecutive magnetic pulses is averaged by summing the waveforms ol' each set.

A random access memory (RAM) 25 is provided to support this processing. The resulting data are supplied to the next process 26 which is also performed on board the 10 ROV. The process 26 performs data reduction so as to reduce the rate at which data are supphed to the support vessel via the umbhcal 14. For example, the process 26 may perform filtering in the frequency domain to remove signal energy outside the frequency band of interest. An example of this is low pass filtering to remove noise. Further data reduction to reduce the necessary data transmission rate may also be performed in the 15 form of polynomial decomposition. For example, the waveform following each magnetic pulse may be approximated by a polynomial curve with a limited number, such as eight, parameters. It is then merely necessary to transmit the parameters via the umbilical 14 and, from these, the apparatus 15 can reconstruct the approximation to the waveform with acceptable accuracy.

The reduced data are transmitted via the umbilical 14 to the apparatus 15, which then determines the target component of the measured voltage signal (or the approximation thereof). 25 The target signal is found according to the invention in the process block 27, for example by using equation (13).

The results of this analysis are supplied to a co-ordinate resolution algorithm 29. The algorithm 29 converts the range information from polar coordinates to rectangular 30 Cartesian coordinates so as to give "vertical range to target" and "lateral offset" measurements of the target relative to the ROV 1. The algorithm 29 also assesses the quality of the data so as to provide an error range in the vertical range and lateral offset

measurements. Smoothing (such as low pass fltcring) may be performed to reduce or remove the effects ot Judderng of the vehicle caused, for example, by mechanical vibration or movement OTI an uneven seabed.

5 The tracking display 3() displays mforTnation about any largess which have been detected. For example, the display may show a "snail trail" Image of the located target, such as a pipe, against a scrolling background image representing movement of the

ROV across the seabed. The colour of the trail may vary depending on the quality of the data assessed by the algorithm 29.

With the type of arrangemcut Illustrated In the drawings and with the centres of the coils being spaced apart by about l metre, it TS possible to detect pipes having damctcrs of the order of 30 inches at a range of 3 metros. Where the ROV I is intended to operate on the seabed itself, then the height of the co'Is above the base of the ROV reduces this 15 range slightly but, nevertheless, pipes can be detected at of the order of 3 metros below the seabed.

By using spaced coils as shown in Figure 7, it is possible to perform triangulation to give an indication not only of the vertical depth below the coils of a target, but also of 20 the lateral offset of the target with respect to the coils. Although a minimum of two coils is required to perform triangulation (when it is assumed that the target is not longitudinally offset from the coils), the presence of one or more additional coil allows the accuracy to be improved and any number of coils may, at least in theory, be used.

25 The technique may be performed in real time with a latency dependent on the actual processing power available or convenient. With the present convenient arrangements, a latency of 1 to 2 seconds can be achieved.

Figure 9 is a schematic block diagram of an apparatus 15 that is able to perform a 30 method according to the present invention.

The apparatus 15 comprises a programmable data processor 34 with a program memory 35, for Instance In the form of a read only memory (ROM), storing a program for controlling the data processor 34 to process seismic data by a method of the Invention The apparatus funkier comprises non-volat]e read/wrte memory 36 IOT stormg, for 5 example, any data which must be retained in the absence of a power supply. A "working" or "scratch pad memory for the data processor is provided by a random access memory RAM 37. An Input device 38 is provided, for instance for receiving user commands and data. One or more output devices 39 are provided, for Instance, for displaying information relating to the progress and result of the processing. The output 10 device(s) may be, for example, a printer, a visual display unit, or an output memory.

Sets of data for processing may be supplied via the input device 38 or may optionally be provided by a machine-readable data store 40.

15 The results of the processing may be output via the output device 39 or may be stored.

The program for operating the system and for performing the method described hereinbefore is stored in the program memory 35, which may be embodied as a semiconductor memory, for instance of the well known ROM type. However, the 20 program may well be stored in any other suitable storage medium, such as a magnetic data carrier 35a (such as a "floppy disk") or a CD-ROM 35b.

Although the invention has been described with particular reference to a search coil, the invention may be applied to any inductive sensor.

Claims

CLAIMS:

l. A method of processing a signal from an nductivc sensor to detect an electrically conductvc target, the method composing: obtaining the value of a 5 parameter of the signal at three different times in the signal 1'fctmc; and determining the component of the signal arising from the electrically conductive target from the values of the parameter of the signal at the three different tunics.
2. A method of processing a signal from an inductive sensor to detect an 10 electrically conductive target, the method composing: obtaining information about the sea water response from the measured signal; and determimng the component of the signal arising from the electrically conductive target from the information about the sea water response determined from the measured signal.

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3. A method as claimed In claim I wherein the parameter is the magnitude of the signal voltage waveform.
4. A method as claimed m claim 3 and comprising dctermning the voltages Vl and V2 according to V(t2) - V(t3) = V1 and V(tl)- V(t3) = V2 where V(tl), V(t2) and V(t3) are magnitudes of the signal voltage determined at times ta, 25 tb and tc respectively, and where ta < tb < le.
5. A method as claimed in claim 4 and comprising determining the component of the signal arising from the electrically conductive target according to 30 I-(c) = V2 - (s2Vl(1 - c)) - (yV1 2 (1 - C)2) - 2Vlc = 0
6. A method as claimed m claim 4 and comprising determining the component of the signal arising from the electrically conductive ta get according to V2-., Vl-yVI2 t2 -s2 -yVI s
7. A method as claimed in claim 4 and comprising determining the component of the signal ar sing from the electrically conductive target accord ng to V2 - S

lO T= t - s
8. A method as claimed in claim 6 wherein s2 and were determined from a quadratic fit to the values of Vl and V2 obtained for a plurality of signal voltage waveforms obtained at different heights of the Inductive sensor above the sea-bed in the 15 absence of an electrically conductive target.
9. A method as claimed in claim 7 wherein s2 was determined from a linear fit to the values of Vl and V2 obtained for a plurality of signal voltage waveforms obtained at different heights of the inductive sensor above the sea-bed in the absence of an 20 electrically conductive target.
10. A method as claimed in claim 5, 6, 7, 8 or 9 wherein t2 was determined from a Imear fit to the values of Vl and V2 obtained for a plurality of signal voltage waveforms obtained at different heights of the inductive sensor above an electrically conductive 25 target on dry land.
11. A method substantially as described herein with reference to Figure 8 of the accompanying drawings.
12. A method of processing a signal from an inductive sensor, the method comprising: obtammg the value of a first parameter of the signal for at least two dff'crcnt heights of the sensor above the sea bed; and obtaining information about the sea water response from the values ot' the first parameter of the signal at the dt'fcrcnt 5 hcghts.
13. A method as claimed in claim 12 and further comprising obtaining the value of a second parameter of the signal for at least two dff'crcnt heights of the sensor above the sea bed; and obtaimng mformaton about the sea water response from the values of the 10 first and second parameter of the signal at the different heights.
14. A method as claimed in claim 12 or 13 and comprising making a linear fit between the first parameter and the second parameter.
15 15. A method as claimed in claim 12 or 13 and comprising making a quadratic fit between the t'irst parameter and the second parameter.
16. A method as claimed in any of claims 12 to 15 and comprising disposing the sensor at a location at which no electrically conductive target is present within the 20 search region of the sensors.
17. A method of inductively detecting an electrically conductive target in a search region of space, comprising: generating a magnetic field pulse in the search region;

measuring a signal induced in an inductive sensor by currents flowing in the search 25 region after the magnetic field pulse; and determining the component of the signal

arising from the electrically conductive target using information about the sea water response obtained according to a method defined in any of claims 12 to 16.
18. An apparatus for processing a signal from an inductive sensor to detect an 3() electrically conductive target, comprising means for determining the component of the signal arising from the electrically conductive target from the values of a parameter of the signal at three different times m the signal lit'ctime.
19. A computer programmed to perform a method as defined in any of claims 1 to 17. 5
20. A program for programming a computer to perform a method as defined in any of claims 1 to 17.
21. A storage medium containing a program as defined In claim 20.

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22. A method of inductively detecting an electrically conductive target in a search region of space, composing: generating a magnetic field pulse in the search region;

measuring a signal induced in an Inductive sensor by currents flowing m the search region after the magnetic field pulse; and determining the component of the signal

arising from the electrically conductive target according to a method defined in any of 15 claims 1 to 11.
23. An apparatus for inductively detecting an electrically conductive target in a search region of space, comprising: means for generating a magnetic field pulse in the

search region; a sensor for sensing currents flowing in the search region after the 20 magnetic pulse; and means for determining the component of the signal arising from the electrically conductive target from the values of a parameter of the signal at three different times in the signal lifetime.
24. An apparatus for processing a signal from an inductive sensor, the apparatus 25 comprising: means for determining the value of a first parameter of the signal for at least two different heights of the sensor above the sea bed; and means for obtaining information about the sea water response from the values of the parameter of the signal at the different heights.

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25. An apparatus for inductively detecting an electrically conductive target in a search region of space, composing: means for generating a magnetic field pulse In the

search region; a sensor for sensmg currents flowing m the search region after the

magnetic pulse; and means for detcrmming the component of the signal arsmg from the electrically conductive target using information about the sea water response ohtamed by an apparatus defmed m clarion 24.

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