WO2010046685A1

WO2010046685A1 - Acoustic bore inspection device

Info

Publication number: WO2010046685A1
Application number: PCT/GB2009/051380
Authority: WO
Inventors: Peter Farthing; Helge Nareid
Original assignee: Offshore Marine Technology Limited
Priority date: 2008-10-24
Filing date: 2009-10-15
Publication date: 2010-04-29
Also published as: GB0819532D0

Abstract

An inspection device is disclosed, particularly useful for inspecting a bore of a pipe, the inspection device having a long body adapted to move through the bore, and having an acoustic transducer with a number of transducer elements, able to transmit acoustic signals from the device toward an inspected object and to receive returned signals from the inspected object in response to the transmitted signals. The transducer elements are arranged to inspect along the longitudinal axis of the inspection device, typically on the end face of the device, in an orientation to inspect along the axis of the bore. This allows the gathering of information relating to objects in the bore ahead of the device.

Description

ACOUSTIC BORE INSPECTION DEVICE

The present invention relates to an inspection device, particularly useful for inspecting pipes.

It is known to inspect pipework by means of acoustic inspection devices which pass through the bore of the pipe and radially transmit acoustic signals from the device to the pipe wall, and determine the condition of the pipe wall (e.g. depth) by means of the signals returned from the wall and gathered by a receiver on the device.

The present invention provides an inspection device for inspecting a bore, the inspection device having a body having a longitudinal axis and being adapted to move through the bore, the inspection device comprising:

- an acoustic transducer comprising a plurality of transducer elements, the plurality of transducer elements incorporating

- a transmitter mechanism adapted to transmit acoustic signals from the device toward an inspected object to generate returned signals from the inspected object in response to the transmitted signals, and

- a receiver mechanism for receiving acoustic signals returned from the inspected object,

and wherein the plurality of transducer elements are arranged in an orientation to inspect along the longitudinal axis of the inspection device. Typically the inspection device has at least one end portion and the plurality of transducer elements are optionally arranged on the at least one end portion in an orientation to inspect along the axis of the bore, which is typically co-axial to the longitudinal axis of the device.

In some embodiments of the invention, the array of transducer elements is adapted to transmit and receive acoustic signals, typically at substantially the same time, and typically the array is adapted to gather information about the shape or direction of the bore, obstructions or deviations of the bore, or debris located in the bore, or other information about the bore.

In a typical embodiment, the device has the transducer elements arranged in a sparse array typically on the leading end of the device, each transducer element being spaced apart from at least one other transducer element in the array so that each element "points" at a particular area of the bore being inspected. This sparse array can typically improve the resolution of the device.

Typically the array of elements is planar.

In certain embodiments of the device, the array can be circular in cross section, but the transducer need not be circular, and the circumference of the array can have a square or some other cross-sectional shape. In preferred embodiments, the cross sectional shape and size of the array is typically chosen to match that of the bore being inspected.

The transducer elements can be arranged at the leading end of the device, pointing in the direction of forward travel through the bore. Alternatively, the transducer elements can be arranged on the trailing end of the device. The transducer elements of the array permit the device to measure and compare the return time and/or distance from the device to different features of the bore being inspected, and this allows a qualitative or optionally a quantitative assessment of debris, blockages, narrowing or diversion in the bore, which frequently occurs in oilfield tubulars.

Embodiments of the invention allow the generation of target data using a single transmitted pulse (or a plurality of single pulses, but a single pulse is sufficient in principle), although it is frequently advantageous to use multiple transmitted pulses to maximise the information content. The returned signal is typically received simultaneously by multiple receivers. The image is typically constructed from the information carried by the time, frequency and phase responses of the combined responses from the receivers.

Embodiments of the invention can obviate the need for scanning.

Certain embodiments of the present invention permit the recording of different characteristics of the returned signal. For example in some embodiments, the intensity and optionally the phase of the signals (e.g. waves) reflected from the target object can be recorded, together with the pulse return time. This can optionally be done simultaneously across all the elements of the detector array.

The data from the reflected wavefront, typically the amplitude and phase distribution across the detector, can optionally be numerically backpropagated to the location of the object, which can for example, be determined by the return time of the ultrasonic pulse. In the case of the object or objects in the image (or parts of the object) being at different distances from the detector, this backpropagation can be performed to a sequence of distances, thus allowing generation of a true 3-dimensional image of the object.

The timing of the return pulse can be used to provide accurate range information. This can be used to determine the appropriate focus for the image.

Typically the acoustic transducer transmits (and optionally receives) at a particular frequency, e.g. between 10OkHz- 5MHz. The frequency of the transmitted signal can be selected in accordance with the desired range of the device. Lower frequencies can yield longer ranges of reliable inspection, whereas higher frequencies can provide higher resolution but at lower ranges. Optionally the device can transmit more than one acoustic frequency. Typically a single acoustic transducer element is adapted to transmit and receive at a single preset frequency, and the device can optionally have a number of different transducer elements adapted to transmit and receive at different frequencies.

For example, the transducer can comprise a plurality of first transducer elements operating at 20OkHz, and a plurality of second transducer elements operating at the same or a different frequency, for example 50OkHz, and scanning at the same or a different range or resolution from the first transducer elements. The second elements can be spaced from the first elements. A third and fourth group of elements operating at other frequencies again can also be included, dependent on the range of the device. In some embodiments the device can transmit and/or receive more than one type of acoustic signal. For example, in some cases, each transducer can emit different frequencies of acoustic signal, optionally at the same time. In particular embodiments of the invention, the device can have a simple planar array of low frequency transducer elements arranged in the head of the device to penetrate through scattering media such as debris that is suspended in the fluid in front of the device, to detect obstacles and obstructions in the bore. A basic pulse ping method can be used to detect the distance of obstructions ahead of the device. Different transducer elements can optionally be incorporated within the same array, or optionally can be disposed in discrete arrays that are separate from one another. Transducer elements operating at the same frequency can be arranged together on the device, but this is not essential.

Typically a single transducer contains both a receiver mechanism and a transmitter mechanism within the same transducer element. In certain embodiments it would be possible to provide transmitter mechanisms and receiver mechanisms within separate transducers within the array. The number of transducer elements able to act as receivers influences the achievable field over which the resolution can be achieved, and it may be desirable to include the appropriate number of sensing elements to support a reasonable field of view. Generally, it is useful to have approximately the same number of receiver elements along a dimension as the number of points of resolution required along the same dimension in the image plane.

Some transducer elements can typically comprise a coil and a core. Passing electrical current through the coil can energise the core and cause it to transmit the acoustic signal. Reception of an acoustic signal by the element can energise the coil, which can result in a detectable electrical impulse in the coil and a detectable signal at a controller. In some embodiments the transducer elements can include a piezoelectric material, adapted to deform when an electrical voltage is applied to it, or to generate an electrical voltage when mechanical force is applied to it. The transducer element can be tuned to a particular resonant frequency, and when a signal at or close to the resonant frequency is applied to the transducer element, an acoustic pressure wave is typically generated by the transducer. In a similar manner, an electrical signal can be generated by the transducer element when it encounters an acoustical pressure wave at or near the resonant frequency of the transducer element. Typically the elements deform optimally when the wave is incident on the transducer element. Piezoelectric transducer elements can be used as transmitters, or receivers or both. The array is typically arranged with the elements parallel to the leading end of the device.

Typical embodiments have a high resolution forward looking transducer array for high resolution imaging. An exemplary operating frequency is in the range 5 to 8 MHz, and transducers are typically arranged in a sparse array filling the front surface of the probe. This array is designed to work at short range at maximum resolution.

One or two transducer arrays can optionally operate at lower frequencies, typically 200 kHz and 1 MHz. The lower frequencies penetrate further through the various media, but at correspondingly lower resolutions. Because of the lower resolutions achievable with these frequencies, correspondingly fewer array transducer elements are required. These lower frequency transducer arrays will be used for two purposes - to penetrate further into blockages than the highest frequency during short range operation or to operate at longer ranges. The different frequencies of transducer can be incorporated in discrete arrays if desired, or can be incorporated into the same planar array in certain embodiments. The recorded data can optionally be transmitted to topside during deployment using wireline, and can also be stored on-board e.g. in flash memory in case of communications failure.

The device can have two main operational modes - a ranging mode where the instrument will be lowered towards an obstruction while using the lower frequency transducers to find the remaining distance to an obstruction, and a static or near static mode with the instrument held at a distance of e.g. 200 to 500 mm from the obstruction using a high frequency transducer array to obtain a high-resolution image, while optionally locating the back surface of the obstruction using the lower frequency arrays to assess the depth of the obstruction, which is useful information to assess the method of removal of the obstruction.

According to a second aspect, the present invention also provides a method for inspecting a bore, the method comprising the steps of:

inserting an inspection device into the bore, the inspection device having an acoustic transducer and a longitudinal axis;

generating acoustic signals from the acoustic transducer within the bore, so that acoustic signals from the transducer are returned from the object and are received at the transducer;

wherein the acoustic transducer has a plurality of transducer elements and; wherein the plurality of transducer elements are oriented to inspect the bore along the longitudinal axis of the inspection device.

Embodiments of the invention can thereby detect the presence and optionally the shape of an object in front of the imaging tool by collecting an acoustic image of the object, which can provide information about the geometry of the object. The object to be observed is typically insonified (i.e. irradiated by sound waves, such as ultrasound waves) at one or more frequencies, and a fraction of the sound waves reflected from the object are optionally captured by a detector on the device. The captured wavefront is optionally stored in memory and can be backpropagated numerically to reconstruct an acoustic image of the shape of the object, which can be stored or displayed.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic side view of an inspection device, located in a bore, and detecting narrowing of the bore;

Fig. 2 is a schematic view of an array of transducer elements detecting debris in the path of an inspection device; Figs. 3-6 show schematic end views of inspection devices, with different arrays of transducer elements in each figure. Fig. 7 is a schematic side view of an inspection device, located in a bore, and detecting an obstruction of the bore; Fig 8 is a schematic side view of the Fig. 7 device showing the wavefront being reflected back to the device from the obstruction; and Figs. 9 and 10 show alternative designs of transducer array with more than one frequency of transducer element, suitable for use in devices according to the previous figures.

Referring now to the drawings, Fig 1 shows a first inspection device 1 in a bore comprising a casing C1. The inspection device 1 is arranged to detect an acoustical image of an object S in front of the leading end 11 of the imaging tool in downhole conditions. In this example, the object S is scale build up on the inner surface of the casing C1 , causing a narrowing of the bore of the casing C1 below the device 1 , and presenting an impedance to the passage of the device 1 through the bore.

The device 1 has a body with a long axis 4 and is typically deployed on the leading end of a tubing string (not shown). Also, the device 1 could be deployed on wireline or the like, without using the tubing. The device 1 has a single transducer 2, comprising a planar array 3 of transducer elements 5a, 5b and 5c (only three are shown in this example, but the number of elements can be varied) which are arranged in a planar forward looking array 3 at the leading end 11 of the device. The transducer elements 5 in the array 3 are typically identical. Each of the elements 5 can transmit and receive acoustic signals in a forward looking direction, downwardly from the leading end 11 of the device 1. The transducer elements 5 are typically piezoelectric elements that incorporate a piezoelectric material which is excited by an electric signal. This electric signal induces size variation in the piezoelectric material, which is transmitted as pressure variations into the surrounding medium. These pressure variations propagate as sound waves in the fluid or solid surrounding the device 1. The piezoelectric transducers 5 can either be arranged as dedicated transmitters or can function as both a transmitter and receiver. In this example the transducers 5 are set up to transmit and receive. In this embodiment, each of the transducers typically fires while the others receive. Each transducer fires by receiving an electrical signal from a controller (not shown) which induces a configurational change in the piezoelectric material to produce the sound wave. In listening or receiving mode, the piezoelectric material in each transducer 5 is flexed by the sound wave hitting it, and this causes the piezoelectric material to generate an electrical signal which is sent to a processor P.

As shown schematically in Fig. 2, in a simple embodiment, an electrical signal is sent from the processor P to a single transducer that is set to transmit, while the other transducers in the array are set to receive. Sound waves generated by the flexing of the piezoelectric material are propagated from the transmitting transducer and the wavefronts from the transmitting transducer strike objects in front of the array. The objects return the wavefronts, which are picked up by the listening transducers in the array. This causes a discrete signal to be sent from each listening transducer corresponding to the particular time each listening transducer picks up a return signal from the objects, providing the processor P with information that e.g. with respect to Fig. 2, the closest transducer to the object 1 is 5x, and the closest to object 2 is 5y. From the time delay in the signals, the processor P can also derive the distance between the objects 1 and 2 and each individual transducer element that reports a return signal, and therefore identify how far ahead the objects are, and optionally, by comparing the date from each listening transducer and triangulating the positions, the 3 dimensional locations of the objects in the bore C1.

The data can optionally be stored in on-board memory M. Optionally the processor P and memory M can be remote from the device 1. The transducer 2 can optionally have a protective housing to prevent abrasion, and a cable (not shown) or wireless transmitter devices for reporting data. The transducer elements can comprise active composite elements mounted on a backing material, which holds the active elements in the correct position, and which dampens acoustic waves propagating back into the processor housing, in order to reduce multiple reflections and crosstalk between the transducer elements. In addition, a matching layer can be used to match the acoustic properties of the piezoelectric material and the surrounding liquid so as to minimise losses and reflections at interfaces.

The embodiment shown in Fig 1 can adequately scan the bore of the casing C1 , and as well as detecting the presence (and optionally the position) of debris in the path of the device, it will detect narrowing at S, or differences in the calculated distances or measured time returns between the device 1 and the walls of C1 , which would tend to indicate oval distortion of the casing string C1. Return signal times can simply be measured in simple embodiments, and measured time discrepancies can be interpreted as discontinuities in the casing wall, for example. The Fig. 1 embodiment can also detect the presence of debris in front of the device 1 , as shown in Fig. 2. In some embodiments with quantitative assessment capabilities, the measured return times can be converted to calculated distance, based on a comparison of the time for a transmitted signal to travel from the transducer to the object and travel back again as a return signal, and deriving from this time measurement the corresponding distance using standard values of speed of sound in e.g. air, water, and most metals. Speed of sound values can be readily found in published literature, for example, in "Handbook of Chemistry and Physics" Chemical Rubber Company, Cleveland OH, table E-28; J. David N. Cheeke: "Fundamentals and Applications of Ultrasonic Waves", CRC press 2002, ISBN 0-8493-0130-0, appendix B; Kaye & Laby: "Tables of Physical & Chemical Constants", National Physical Laboratory (NPL) , http://kavelaby.npl.co.uk/, section 2.4.1 - "The speed and attenuation of sound." For example, given that speed = distance/time and knowing that the speed of sound in water is 1400m/s and knowing the time taken for the signal to bounce back from the casing to the transducer through the water, it is possible to calculate the distance from the transducer to the casing. The materials of the casing are generally known beforehand, and the speed of sound in the casing (e.g. speed of sound in steel is 5000m/s) will also be information known or readily available to the skilled person. It is also possible to construct qualitative and not necessarily quantitative embodiments which detect discontinuities in the distances, without necessarily calculating the distances themselves.

In some cases, the speed of sound in the medium immediately surrounding the device can optionally be measured by including an optional speed-of-sound test cell in the device. A speed of sound test can typically be implemented by measuring the traverse time of an ultrasonic pulse over a known distance. A simple test cell located on the device can optionally comprise a simple annular (or other shaped) sleeve arranged around a reference transducer, and defining a reference cavity of fixed and known dimensions, which occupies a part of the annulus between the device and the bore in which it is disposed, and which has an opening to allow the test cavity to fill with the fluid medium surrounding the device, e.g. with the water or oil surrounding the device in the bore of the casing. Thus the distance between the reference transducer and the reference cavity is known, and the frequency and wavelength of the reference transducer is also known, and so upon measurement of the time taken for the return signal from the reference sleeve to reach the reference transducer can be used to confirm the speed of sound in the fluid medium surrounding the device. The data can be logged on optional memory storage M as the device is run into the hole on the tubing T, and the discontinuities such as oval distortions, blockages and scale are optionally logged with respect to depth, either by the onboard circuitry or by remote processors and/or data storage devices, and optionally mapped.

Referring now to Figs 7 and 8, a bore inspection device 1 can detect an obstruction O occluding the bore of the casing C1. The transducer elements in the transducer 2 emit a wavefront 6 that travels from the end of the transducer 2 toward the obstruction O. The transducer elements in this example can again be piezoelectric elements which are excited by an electric signal to induce a size variation in the piezoelectric material, which is transmitted as pressure variations into the surrounding medium. These pressure variations propagate as sound waves in the fluid surrounding the device 1. The piezoelectric transducers used as a source in this case can function both as transmitters and receivers. The receiving transducers can pick up the signal returned from the obstruction O.

The waves generated by the transducer 2 can optionally be varied in accordance with different requirements. For example, in a simple case, like that shown in Fig. 2, the wavefront can be in the form of a very short pulse excitation of a single transmitting transducer element, normally consisting of a small number of oscillations. Pulse excitations are acceptable for basic embodiments of the invention, but better results can be obtained by using other types of wavefront. For example, if a pulse excitation has a very short duration (e.g. 2 or 3 oscillations) the margins of error can be increased because of limitations to the wavelength of the pulsed wavefront. In such cases, it can be a better option to use a continuous wave form comprising a long sequence of waves with little or no variation over a large number of oscillations. An alternative solution is to use a chirped wave comprising a sequence of oscillations where the frequency varies over the sequence, e.g. from a lower to a higher frequency, which can control the shape of the waveform. Chirped waveforms are often better than pulsed waveforms as they allow more energy to be emitted from the transducer elements and so a larger return signal can be expected. One benefit of varying the frequency in chirped forms is that a chirped signal can be used to create a "synthetic pulse", which will be mathematically equivalent to an actual pulse. Since a chirped signal lasts significantly longer than a pulse, more energy can optionally be transmitted in a chirped signal without increasing the signal intensity.

As well as varying the intensity and time of the wave emitted from the transducer elements, it is possible to vary the direction and behaviour of the waveform.

In some cases, the waveform (e.g. pulsed, continuous or chirped) can be transmitted from a single (or a few) transducer element(s) thereby providing a point source to the wave. A point source is a source where the emitting area is small relative to the effective wavelength of the ultrasonic sound. The corresponding wave will be emitted as an expanding wavefront, with the angular divergence of the wavefront inversely proportional to the size of the emitter source.

In some other cases, the wave form can be generated as a plane wave. A plane wave is a wave where the sound pressure maxima of the ultrasonic wave form substantially parallel planes that are typically normal to the direction of propagation (in an isotropic medium). A plane wave can be formed by exciting a large number of adjacent elements in an area transducer at substantially the same time. In other cases, the waveform can be transmitted as a shaped wave. One type of shaped wave useful in certain embodiments is a focused wave, where the waveform forms a focal spot or line at a distance from the transmitting transducer. This can be done either by means of an acoustic lens or by exciting the elements an array transducer with appropriate calculated delays. Both plane and shaped wavefronts may be scanned by electronic or mechanical means to cover specific regions of the object under observation.

In order to generate plane waves, focused waves or more specialised shaped wavefronts, a number of transducer elements are excited simultaneously or at pre-calculated time intervals. By controlling the timing of the excitation of the individual transducer elements, the beam can also be steered in a desired direction. This technique is normally referred to as a phased transducer array.

The ultrasonic field is detected over a known area. This normally requires a number of measurements to be made at different locations over the detection area. In the general case, the detector area is a 2-dimensional surface, but in some applications a 1 -dimensional detector can suffice. The measurement of the acoustic field over the detector area may be done by mechanical scanning using a single detector or a simplified multielement transducer over the detection area, but our preferred solution is to use an area detector with multiple transducer elements located in a pattern over the detection area.

Several transducer geometries are possible. Simple embodiments can have linear detector geometry, comprising a number of transducer elements arranged along a line. This type of detector has adequate spatial selectivity in the direction along the line of elements and can be used in some application areas where the alignment of the object under observation is known. The main advantage of this arrangement is that relatively high resolution can be achieved with a small number of elements.

Alternative embodiments can have Cartesian geometry where the transducer elements are located on a regular or semi-regular grid pattern. This arrangement is conceptually and computationally simple and can yield high resolution, but can result in the requirement for a large number of active elements in order to achieve the desired resolution and field of view.

In other case, the array can have polar geometry, with the transducer elements organised on a radius/azimuth basis. This does not necessarily involve computational difficulties in that most of the required equations can be reformulated in a polar form. There is little discernible advantage to a Cartesian pattern, however, and will require a similar number of active elements.

In particularly advantageous embodiments the array is a sparse array of irregularly spaced transducer elements. A sparse array is one where not all possible active element positions are filled. It is possible to do this in a manner which does not significantly impact the resolution and field of view of the array. Normally this involves a quasi-random distribution of active transducer elements over the detector area. This method optimises resolution with a minimum number of active transducer elements.

The detection of the incident acoustic field can be sampled simultaneously over the entire detector surface. It is also possible to detect sequentially over part of the detector array. The detected acoustic field is stored electronically for further processing and analysis.

One useful method of reconstruction of the image of the object is a numerical backpropagation algorithm. The reflected wavefront will be recorded simultaneously at all the transducers across the front of the probe. The resulting picture will then contain an image of the wavefront of the reflected wave as received at the transducer array. The image is then typically reconstructed with numerical calculations based on this data in the processor, by a process known as backpropagation.

The process of wave propagation through a medium is theoretically well understood, and suitable calculation techniques for converting the data gathered from the listening transducers into spatial information is within the skill of the normal person. It is possible to use the acoustical field recorded at one surface to calculate the field at other locations along its propagation path. In particular, by reversing the sign of the time variable in the appropriate equations, it is possible to calculate the field properties before it was detected. This is called time reversal, and forms the basis for the process called backpropagation, where the wave equations are used to computationally reconstruct the field at a distance before it was recorded. It can be shown that this is mathematically equivalent to the propagation in the forward direction, using time reversal. Thus having an understanding of the theory of wave propagation and a sufficiently detailed record of the wavefront over a reference surface, it is possible to calculate the wavefront at another location.

Backpropagation can be performed on the gathered data using a variety of methods, including but not limited to the methods described below: The Fresnel transform allows accurate calculations of the field except in the very near vicinity of the observation plane. It is based on a Fourier transform of the field in the observation plane with compensation for phase variations over the observation plane and the calculated plane. The Fresnel transform can be written as:

U{x,y, z) = - e^^(χ2+y2)T{U(x₀,y_0l 0)e^^x'^{>2 +yo2)}}

JAZ

In the above equation, U(x,y,z) is the sonic field at a distance zfrom the array, with the value U(xo,yo,O) is the field at z=0 at the surface of the array, i.e. the wavefront recorded by the transducer array, the symbol F{...} indicates a 2-dimensional Fourier transform, λ is the wavelength and j is the imaginary square root of -1. Using this equation, the field can be calculated at the distance zfrom the transducer array as long as certain conditions are met, which mainly involves that the object is at sufficient distance from the observation field. The Fresnel transform is best suited to fields with a continuous wave.

Angular spectrum calculations are like the Fresnel transform based on a Fourier transform of the field at the detector plane. The Fourier transform can be interpreted as a representation of an infinite continuum of elemental plane wave propagating in all forwards directions from the observation plane. These plane waves are propagated to the object plane, and the field at the object plane is found by an inverse Fourier transform at the object plane. This method involves an extra 2-D Fourier transform compared to the Fresnel transform, but is more accurate than the Fresnel transform, particularly in locations close to the observation plane. Like the Fresnel transform, the angular spectrum calculation is best carried out with a continuous wave. Time of flight calculations is suitable for short pulses, where the geometric path length is calculated from the time of flight only. In other words, this method operates in the time domain, unlike the two previously mentioned methods which operate in the frequency domain. Time of flight can also be estimated from a chirped signal.

A number of hybrid methods combining the merits of operating in the time and frequency domains are possible. One possibility is combining the Fresnel transform with the extra information provided by the time of flight data. Other possibilities include using a wavelet transform which has the property of combining time and frequency data.

The embodiment in Figs 7 and 8 uses a plane wave method where all of the transducer elements in the array 3 are fired substantially simultaneously to emit a planar wavefront 10 toward the obstruction O. The signal wavefront returned from the obstruction O is non-planar, because of the irregular shape of the obstruction O, and so the wavefronts R returning to the array 3 energise the transducer elements at the centre of the array first. The geometry of the array 3 in the Fig. 7 embodiment is typically that shown in Fig. 6, i.e., a sparse array. Thus the returning wavefront R energises the transducer element in sequentially expanding rings 5i, 5j, 5k, thereby allowing the processor P to calculate the shape of the obstruction O, as well as its distance, and the extent to which it is blocking the whole of the bore of the tubular C1.

The embodiment shown in Figs 7 and 8 can optionally be provided with a different array of transducer elements. For example the arrays shown in Figs 9 and 10 show different transducer elements within the same plane in the planar array. In the Figs. 9 and 10 examples, the squares indicate transducer elements with one frequency, e.g. 50OkHz. The circles indicate different transducer elements with a different frequency e.g. 1.5MHz. The Fig. 9 example has two different types of transducer element, whereas the Fig. 10 example has the same 500 kHz and 1 MHz elements but also has 2MHz transducer elements represented by the triangles. All of the transducer elements in the arrays shown in Figs 9 and 10 are typically similar to the arrays 3, in that all of the transducer elements are normally arranged in a planar plate on the leading end of the device, and each particular frequency is arranged in a sparse array as shown in Fig. 6.

However, in some embodiments, the arrays can be stacked or otherwise spaced axially from one another, so that one array composed entirely of e.g. 50OkHz elements is stacked underneath a second array of 1 MHz elements. Of course it can be seen from this that other arrangements are possible, for example 3, 4, 5 or more types of transducer elements in the same or different layers or planes

The different elements typically transmit and receive at different frequencies. The frequencies are exemplary and other frequencies can be used without departing from the scope of the invention. In each case, the acoustic signal is transmitted along the axis of the device to look ahead of (or behind) the device. Any discontinuities in the medium through which the signals pass causes a reflection of the signal back to the device which is picked up by the relevant transducer operating at the required frequency. The time taken between the transmitted signal leaving the device, and the return signal reaching the device is measured by the processor P, which can be local to the device 1 or can be located elsewhere e.g. topsides on a rig, and this measurement is converted into distance as a function of the speed of sound in the particular medium concerned, which is generally known beforehand. The three different frequencies of the three types of transducer element give the respective signals transmitted by the transducer elements different ranges. The lower the frequency the longer the range of the signal. Since some loss of signal can occur on longer ranges, the lower frequency transducers operating at 50OkHz (squares) can optionally have more rows of transducer elements than the higher frequency transducers represented by circles and triangles. The different transducers are therefore focused to inspect particular areas of the wellbore. For example, the high frequency transducers are arranged to focus respectively on the areas of the bore close to the end of the device. The lower frequency transducers are arranged to focus on areas of the wellbore that are further away from the device. Thus the device can pick up discontinuities in the casing C1 or obstructions inside the wellbore or in the annulus, in real time, without moving parts, and can give data allowing the assessment of ovality and concentricity, as well as suspended debris immediately ahead of the device, all within a single trip into the wellbore. This data can be correlated with the measured depth of the tubing string on which the device is inserted, and a map of the casing string C1 can thereby be constructed non-invasively. Clearly it will be recognised by a skilled man that although not shown in the drawings for reasons of clarity, additional transducers can easily be added to focus on deeper parts of the bore.

In some alternative embodiments, the wavefront emitted from the transducer can be shaped. Figs 11 , 12 and 13 show some alternative shapes of wavefront, which can be generated by the embodiments shown in Figs 7 and 8, by firing individual transducers 5 sequentially in different patterns to emit different shapes of wavefront. For example, in the Fig. 11 embodiment, the wavefront is steered in the direction of the arrow Z, by firing the elements 5 on one side of the transducer before the elements on the opposite side. The Fig. 12 wavefront is focussed on a focal point F, by firing the elements on the outside of the transducer before the elements in the centre (or adjacent to the chosen focal point F, if that is not central). In the example shown in Fig 13, the wavefront is both steered in the direction of the arrow Z' and focussed on the (non-central) focal point F', by a combination of the aforementioned techniques. The effect of this steering is to inspect a particular section of the bore and to increase the returned signal from that chosen section, and to reduce noise from other parts of the bore that are of less interest.

Modifications and improvements can be incorporated without departing from the scope of the invention.

Claims

Claims:

1 An inspection device for inspecting a bore, the inspection device having a body having a longitudinal axis and being adapted to move through the bore, the inspection device comprising:

and wherein the plurality of transducer elements are arranged in an orientation to inspect along the longitudinal axis of the inspection device.

2 An inspection device as claimed in claim 1 , having at least one end portion and wherein the plurality of transducer elements are arranged on the at least one end portion to transmit and receive signals in an direction that is co-axial to the longitudinal axis of the device.

3 An inspection device as claimed in claim 1 or claim 2, wherein the array of transducer elements is adapted to transmit and receive acoustic signals at substantially the same time. 4 An inspection device as claimed in any preceding claim, wherein the transducer elements are arranged in a sparse array on the leading end of the device, each transducer element being spaced apart from at least one other transducer element in the array so that each element transmits signals to and receives signals from a particular area of the bore being inspected.

5 An inspection device as claimed in any preceding claim, wherein the array is circular in cross section.

6 An inspection device as claimed in any preceding claim, wherein the array is arranged in a single plane that is perpendicular to the long axis of the device.

7 An inspection device as claimed in any preceding claim, wherein the transducer elements are arranged at the leading end of the device, pointing in the direction of forward travel through the bore.

8 An inspection device as claimed in any one of claims 1 to 6, wherein the transducer elements are arranged on the trailing end of the device, pointing away from the direction of forward travel through the bore.

9 An inspection device as claimed in any preceding claim, wherein the transducer comprises a plurality of first transducer elements operating at first frequency, and a plurality of second transducer elements operating at a second frequency that is different to the first frequency.

10 An inspection device as claimed in any preceding claim, incorporating a data recorder to store data concerning returned signals. 11 An inspection device as claimed in any preceding claim, incorporating a calibration mechanism within the device in the form of a speed of sound test cell.

12 An inspection device as claimed in any preceding claim, incorporating a processor adapted to construct an image of an object in the bore by manipulation of data selected from time, frequency and phase data from the signals received by the receiver mechanism.

13 A method for inspecting a bore, the method comprising the steps of:

- generating acoustic signals from the acoustic transducer within the bore, so that acoustic signals from the transducer are returned from the object and are received at the transducer;

- wherein the acoustic transducer has a plurality of transducer elements and;

wherein the plurality of transducer elements are oriented to inspect the bore along the longitudinal axis of the inspection device, whereby acoustic signals are directed along the bore.

14 A method as claimed in claim 12, wherein the returned signals are processed in a processor to detect characteristics of an object in the bore. 15 A method as claimed in claim 13 or claim 14, wherein the bore is irradiated by sound waves at more than one frequency.

16 A method as claimed in claim 13, 14 or 15, wherein the acoustic signal generated by the device is in the form of a wave, and wherein a selected number of transducer elements on the device are excited at pre- calculated times to influence the characteristics of the wave

17 A method as claimed in any one of claims 13-16, wherein acoustic signals returned to the device from different parts of the bore are processed to compare characteristics of the different inspected parts of the bore.

18 A method as claimed in any one of claims 13-17, wherein the device is moved through the bore when generating and receiving acoustic signals.

19 A method as claimed in any one of claims 13-18, wherein the acoustic signals generated by the device are in the form of chirped waves comprising a sequence of oscillations where the frequency varies over the sequence

20 A method as claimed in any one of claims 13-19, in which the acoustic signals generated by the device are transmitted from a point source in the form of fewer than 6 transducer elements and wherein the point source is small relative to the effective wavelength of the acoustic signal, and wherein the signal is emitted as an expanding wavefront, with the angular divergence of the wavefront inversely proportional to the size of the emitter source. 21 A method as claimed in any one of claims 13-19, wherein the acoustic signals generated by the device are emitted from the device as a plane wave where the sound pressure maxima of the wave are substantially parallel and normal to the direction of propagation.

22 A method as claimed in any one of claims 13-19, wherein the acoustic signals generated by the device are transmitted from the device as a shaped wave.

23 A method as claimed in any one of claims 13-22, wherein an image of an object in the bore is generated by recording data relating to the reflected wavefront of acoustic signals received by the device, using simultaneous recording at multiple receiving transducers across the front of the device, and using the recorded data to construct an image of the object using backpropogation.

24 A method as claimed in claim 23, wherein the backpropogation is calculated using the Fresnel transform

where U(x,y,z) is the sonic field at a distance z from the array, the value U(xo,yo,O) is the field at z=0 at the surface of the array, λ is the wavelength and j is the imaginary square root of -1.