GB1575301A - Acoustic imaging techniques - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ACOUSTIC IMAGING TECHNIQUES.

(71) We EMI LIMITED, a British com- pany of Blyth Road, Hayes, Middlesex do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to acoustic imaging techniques, and it relates especially, though not exclusively, to methods of imaging objects buried in the silt or sand of the sea bed.

It is an object of the invention to provide a technique for imaging a region in an opaque material.

According to the invention there is provided a method of forming by means of acoustic radiation an image of a region within an optically opaque material including: providing a source of an acoustic energy of a frequency between 1 kHz and 10 kHz, providing a transmitter and receiver responsive to said chosen frequency, moving said transmitter and said receiver in a known path or in respective paths adjacent to said region, transmitting acoustic energy bursts of said frequency at intervals from points along the known transmitter path, into the region of material, receiving reflected acoustic energy and assessing the range at which each reflection occurred, and determining from a series of such assessments the form of an image of said region in terms of acoustically reflective elements therein.

The energy may be directed in a beam of some 100" included angle.

The material may be the ground which may be under water.

The known path or paths may be a straight or curved line or a scanning pattern over an area. The path or paths may be determined by reference to fixed points. When the material is under water the acoustic energy may be transmitted via the water to and from the material. There may be an interface occurring at the junction of materials of different density or with a gradual change in density.

The image form may be determined by generating lines representing for each burst the range at which energy is reflected and selecting continuous line parts as being the image.

According to the invention there is also provided apparatus for forming by means of acoustic radiation an image of a region within an optically opaque material by any of the methods described above including a scannable transmitter/receiver arrangement operable to radiate a beam of acoustic energy at between 1KHz and 10KHz over an included angle at a value between 40 and 1500 and to receive rflected acoustic energy, means to assess the range of reflection of the received energy and determine therefrom the image form for display.

Embodiments of the invention will now be described with reference to the drawings accompanying the Provisional Specification in which: Figure 1(a) shows a section through a region beneath a surface and including an object, and Figure 1(b) represents a simplified reconstruction of an image of the region and object of Figure 1(a).

The embodiments to be described relate to the formation of an image of a region below the surface of the sea or other body of water. Towards the bottom of the sea, silt, and/or sand or like solid matter may be present in suspension so that there is not a clearly defined transition from liquid to solid or substantially solid material. However there is an interface beyond which visual observation is difficult, which may or may not coincide with a change in the nature of the material including the region.

Owing to the characteristic of attenuation of acoustic waves in silt, sand etc. it is necessary to use sonic rather than ultrasonic frequencies. The frequency may be, typically, in the band between 1kHz and 10kHz, corresponding to wavelengths in sea water of 1.5 metres to 0.15 metres. One purpose of such an imaging method is to distinguish man-made objects from rocks and to recognise characteristic differences between certain made made objects. For example pipelines laid on a soft sea bed may be detected, particularlv if the position in which they have beery laid is not known exactly.

At wavelengths corresponding to the frequencies used the surface of the objects tend to be smooth and to act as good specular reflectors. Energy is, therefore, only received from those parts of the surface of an object which are perpendicular to a line from a transmitter/receiver transducer. or to a line which bisects the angle between the transmitting and receiving transducers where separate transducers are used. In the case of a spherical surface only a small area thereof will satisfy the required condition from any single viewing point, and in the case of a plane surface the viewing direction must be perpendicular to that surface.To obtain information about the surface of the object in order to construct an image of the object it is therefore necessary to move the transmitter/receiver system spatially across the area above the buried object, eg in a ;;scan" movement.

The surface of the object can be defined in terms of the position of the transmitter/ receiver in three axes (X, Y and Z) and the measured range from the transmitter/receiver to the surface of the object. For convenience, the axes X, Y and Z can be assumed to be mutually perpendicular with Z being the height of the transmitter/receiver system above an arbitrary datum which may be the sea bed. It is necessary, therefore, for the position of the vehicle carrying the traflsmit- ter/receiver to be determined relative to a number (say three) of fixed reference points. Conveniently these can be defined by three transponders on the sea bed. The vehicle is caused to scan an area above the buried object, the scanning pattern being, typically, a zig-zag pattern or a spiral.

The transmitter/receiver system is required to transmit short bursts of sonic energy towards the buried object and to receive reflected energy. Since the direction of the object may not be known precisely the polar diagram of the receiver response should be broad, e.g. between 40 and 1500, preferably between 90" and 110 and typically some l0() , with the axis vertical. The resolution of the system is dependent on the duration of the burst of sonic energy and is independent of the breadth of the polar diagram.

As the vehicle carrying the transmitter/ receiver scans the area a series of measurements is recorded and stored. The recorded measurement figures are then processed to give the position of the vehicle and the corresponding range of the reflecting surface. From this data. two-dimensional display images of part of the surface of the object can be reconstructed as indicated in Figure 1, wherein the two dimensional display represents a section through the volume surrounding the buried object. The points (a) to (e) represent the projections of the successive positions of the transmitter/ receiver on the plane represented: and rl to r5 represent respectively the projections of the measured ranges of the reflecting surfaces from these positions.

The points (a) to (e) etc. may be close together if the measurements are along the path of the vehicle, but perpendicular to this direction the points would be more widely spaced as they each correspond to one traverse of the vehicle in the scanning pattern. Typically the points are spaced at a distance similar to their distance from the surface e.g. about one metre.

Reconstruction of the image of part of the surface of the object may be achieved graphically by simply drawing the arcs with appropriate centres and radii provided that only one object is present in the scanned area, as is shown in simplified form in Figure lb. Computing methods may be used to improve the displayed image by removal of unnecessary parts of the scan arcs and displaying only the representations of the continuous surfaces of the object or objects.

The power efficiency and signal/noise ratio of the system may be improved by using a narrow beam width for the irradiating and scanning systems, and guiding the beam towards the object from any position of the vehicle. This may be achieved by known means once the positions of the object and the vehicle are known relative to the transponders.

Where there are a number of objects in the viewing area the simple graphical method of reconstruction of the surfaces may not give satisfactory results and it will then be necessary to resort to computing methods of line reconstruction from the data using a computer and a suitable program. Basic features of the system which are relevant to such methods are: (i) the surface to be reconstructed has a common tangent with each spherical surface at the appropriate range from the transmitter/receiver, and (ii) the surface to be recontructed is continuous.

The techniques described above permit the formation of an image of a region within an optically opaque material, which may be solid or substantially solid, e.g. the ground or sea bed, or liquid, e.g. murky water, particularly from a vehicle transversing space above or within the material, which space may be the sea or other body of water.

WHAT WE CLAIM IS: 1. A method of forming by means of acoustic radiation an image of a region within an optically opaque material including: providing a source of an acoustic energy of a frequency between 1kHz and 10kHz providing a transmitter and receiver responsive to said chosen frequency, moving said transmitter and said receiver in a known path or in respective paths adjacent to said region, transmitting acoustic energy bursts of said frequency at intervals from points along the known transmitter path into the region of material, receiving reflected acoustic energy and assessing the range at which each reflection occured, and determining from a series of such assessments the form of an image of said region in terms of acoustically reflective elements therein.

2. A method according to Claim 1 including directing said energy in a beam of between 40 and 1500 included angle.

3. A method according to Claim 2 in which the included angle is between 90" and 110 .

4. A method according to any one of claims 1 to 3 including moving along a known path substantially parallel to an interface of the material and transmitting the energy along a direction substantially perpendicular to the interface.

5. A method according to any one of Claims 1 to 4 including determining the image form by generating lines representing for each burst the assessed range at which reflection occurred and selecting contiguous line parts as being the image form.

6. A method according to Claim 5 including providing a program to operate a computer to generate and select said line parts.

7. A method of forming by means of acoustic radiation an image according to any one of claims 1 to 6 the image being of a region beneath submerged land.

8. Apparatus for forming by means of acoustic radiation an image of a region within an optically opaque material by a method according to any preceding Claim including a scannable transmitter/receiver arrangement operable to radiate a beam of acoustic energy at between 1KHz and 10KHz over an included angle at a value between 40 and 1500 and to receive reflected acoustic energy, means to assess the range of reflection of the received energy and determine therefrom the image form for display.

9. A method of forming by means of acoustic radiation an image of a region within an optically opaque material substantially as herein described with reference to the drawing accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. the formation of an image of a region within an optically opaque material, which may be solid or substantially solid, e.g. the ground or sea bed, or liquid, e.g. murky water, particularly from a vehicle transversing space above or within the material, which space may be the sea or other body of water. WHAT WE CLAIM IS:

1. A method of forming by means of acoustic radiation an image of a region within an optically opaque material including: providing a source of an acoustic energy of a frequency between 1kHz and 10kHz providing a transmitter and receiver responsive to said chosen frequency, moving said transmitter and said receiver in a known path or in respective paths adjacent to said region, transmitting acoustic energy bursts of said frequency at intervals from points along the known transmitter path into the region of material, receiving reflected acoustic energy and assessing the range at which each reflection occured, and determining from a series of such assessments the form of an image of said region in terms of acoustically reflective elements therein.

2. A method according to Claim 1 including directing said energy in a beam of between 40 and 1500 included angle.

3. A method according to Claim 2 in which the included angle is between 90" and 110 .

4. A method according to any one of claims 1 to 3 including moving along a known path substantially parallel to an interface of the material and transmitting the energy along a direction substantially perpendicular to the interface.

5. A method according to any one of Claims 1 to 4 including determining the image form by generating lines representing for each burst the assessed range at which reflection occurred and selecting contiguous line parts as being the image form.

6. A method according to Claim 5 including providing a program to operate a computer to generate and select said line parts.

7. A method of forming by means of acoustic radiation an image according to any one of claims 1 to 6 the image being of a region beneath submerged land.

8. Apparatus for forming by means of acoustic radiation an image of a region within an optically opaque material by a method according to any preceding Claim including a scannable transmitter/receiver arrangement operable to radiate a beam of acoustic energy at between 1KHz and 10KHz over an included angle at a value between 40 and 1500 and to receive reflected acoustic energy, means to assess the range of reflection of the received energy and determine therefrom the image form for display.

9. A method of forming by means of acoustic radiation an image of a region within an optically opaque material substantially as herein described with reference to the drawing accompanying the Provisional Specification.