WO2003104842A1

WO2003104842A1 - Method and apparatus for detecting objects on or in the ground

Info

Publication number: WO2003104842A1
Application number: PCT/NL2003/000417
Authority: WO
Inventors: Franciscus Petrus Gijsbertus Driessen
Original assignee: Nederlandse Organisatie Voor Toegepast- Natuurwetenschappelijk Onderzoek Tno
Priority date: 2002-06-10
Filing date: 2003-06-10
Publication date: 2003-12-18
Also published as: NL1020818C2; EP1512030A1; AU2003241923A1

Abstract

A method for detecting objects on or in the ground, comprising the steps of transmitting one or more acoustic signals to the ground, so that, after reflection, at least a first and second signal are received with a detectable delay time difference, and detecting a delay time difference between first and second reflection signal. The method is characterized in that it comprises the steps of setting up the sources and/or receiving units in a series at a fixed height above the ground, calculating a delay time difference from a path length difference of the reflection signals, on the assumption that the reflection signals have been reflected by the ground surface, with the ground surface being modeled according to a predetermined geometry, and deriving the presence of an object on or in the ground by comparing the detected delay time difference with the calculated time difference.

Description

Title: Method and apparatus for detecting objects on or in the ground

The invention relates to a method for detecting objects on or in the ground, comprising the steps of: transmitting one or more acoustic signals to the ground, so that, after reflection, at least a first and second reflection signal from the ground surface are received by the receiving units, and detecting a delay time difference between first and second reflection signals. Such a method is known from the US patent specification No. 5,563,848. This describes an acoustic detection apparatus, in which signals are compared of two microphones set up parallel to the ground. If one of the microphones is held above an object with an acoμstic resistance and the other microphone above a reference area, there is a detectable difference between the received signals, from which the prβsence of the object can be derived. The apparatus can be used as a detector for detecting objects in the ground by moving the apparatus parallel to the ground. However, the apparatus is unsuitable for uses where the signals travel a larger distance before they are detected by the microphones, because the resolution decreases fast in that the signals impinge in the same region of the ground surface and no longer differ from each other. Further, the apparatus is only suitable for measurements right under the measurement setup. It is an object of the invention to provide a method in which the drawbacks of the abovementioned state of the art do not occur or occur to a reduced extent. Further, it is an object of the invention to provide an apparatus which yields reliable detection at •larger distances, enabling the detection of objects on or in the sea bottom. This object is achieved by providing a method of the abovementioned type, further comprising the steps of: setting up the sources and/or Receiving- μnits in a series in a fixed position above the ground surface; calculating a delay time difference from a path length difference of the reflection signals, on the assumption that the reflection signals have been reflected by the ground surface, with the ground surface modeled according to a predetermined geometry; deriving the presence of an object on or in the ground by comparing the detected delay time difference with the calculated delay time difference.

The method according to the invention makes it possible to correct for the reflection properties of the ground by removing from the signal the reflections coming from the ground, so that these do not affect the measuring result.

In a preferred embodiment, the method comprises the further steps of transmitting a pulse signal and receiving the reflection signal by two receiving units set up at different positions in relation to the ground surface. Such a setup allows a reception of two temporally separated pulses, which have both been reflected by the same reflection surface. From the time difference, it can be derived whether the reflection surface is the ground surface or another surface of an object located below or above it. Preferably, the method further comprises the steps of: calculating a corrected reflection signal by displacing the first reflection signal in time over a time interval corresponding to the calculated delay time difference; and determining a difference value by subtraction of the corrected first reflection signal and the second reflection signal, which difference value is indicative of the presence of an object on or in the ground.

In a first detection, the receiving units can be set up at a first position, from which detection a radial distance of an object to the receiving units can be derived, and subsequently, in a second detection, the receiving units are set up at a second position, so that the position of the object can be determined by means of triangulation.

In a further preferred embodiment, the method comprises the further steps of calculating a corrected signal by displacing the first signal in time over a time interval corresponding to the calculated delay time difference and determining a difference value by subtraction of the corrected first signal and the second signal, which difference value is indicative of the presence of an object on or in the ground. Correction and subtraction of the signals yield a difference signal which is theoretically equal to zero in the absence of reflecting objects on or in the ground. A deviation from this zero value is thus indicative of an object with a reflective surface.

A refinement of the technique is achieved by using a correction for the calculated amplitude difference as a result of a calculated path length difference of the signals. The method is preferably used for detecting objects in a sea bottom, with the chosen acoustic frequency being so low that a depth of penetration into the sea bottom varies from 0-20 meters, while the detection takes place from a height varying from 3-500 meters above the bottom surface and the array setup is such that an area to be probed is in the lateral angle range of 5 - 70°.

The delay time difference can be calculated on the assumption that the ground surface has a flat geometry. In a more accurate approach, a delay time difference is calculated after determining the ground geometry by means of bathymetry. The bathymetry can be carried out by means of an echo sounder or a high-frequency side-scan interferometry.

The invention further relates to an apparatus for detecting objects on or in the ground according to a method of one the abovementioned aspects, further comprising at least two parallel arrays of hydrophones, set up at a distance from each other, with fixing means for setting up the apparatus, in use, under a vessel, so that the arrays are mutually different in height relative to the ground, and an acoustic source which is provided at a fixed height relative to the arrays, which acoustic source is arranged for transmitting an acoustic pulse in a low frequency spectrum suitable for penetrating a few meters into the seafloor. . For obtaining a desired flexibility in tuning the apparatus, the distance between the arrays is preferably adjustable. The acoustic source can be set up between the arrays. In operation, the apparatus is preferably tiltable through an angle relative to a vertical direction of the setup. This makes it possible to vary an angle range at which an area of interest is to be probed. The working depth of the apparatus is preferably adjustable.

The invention will be explained in more detail with reference to the Figures, in which: Fig. 1 shows a preferred setup of the apparatus according to the invention;

Fig. 2 shows a side view of the apparatus according to the invention in operation;

Fig. 3 shows a front view of the apparatus according to the invention in operation;

Fig. 4 shows a diagrammatic setup of a calculation example according to the method of the invention; and

Figs. 5a-5c show the results of a 2D model analysis using the method of the invention. In the Figures, the same or corresponding parts are designated by tKe same reference numerals.

Fig. 1 diagrammatically shows a hydrophone setup 1 for detecting objects on or in the ground. The hydrophone setup 1 comprises two exactly or virtually parallel arrays 2 of hydrophones 3, which are suitable for receiving acoustic signals transmitted by an acoustic source 4. The arrays 2 are preferably kept in the right position relative to each other by a steel frame 5 and are preferably acoustically decoupled from the frame. The source is, for instance, an air gun transmitting a pulse in the range from 10 Hz - 3 kHz. The source can be set up at a distance, but can preferably also be fixed in or to the apparatus 1. The distance between hydrophones 3 should be adapted to a frequency range of the acoustic signals and can measure some tens of centimeters. A preferred distance for a geometry whereby a depth of penetration of a few meters into the seabed is provided, is a hydrophone setup of two rows, each with twenty hydrophones, set up at a mutual distance of one meter. The rows are set up at a distance of 2-20 meters from each other, preferably about 5 meters. This makes it possible to use the method for detecting objects in a seabed, with the chosen acoustic frequency being so low that a depth of penetration into the seabed varies from 0-10 meters, while detection takes place from a height varying from 10-500 meters above the seabed surface and the array setup is such that that an area to be probed is in the lateral angle range of 20-70°. The intervals mentioned serve as an example; also outside these intervals, good results can be achieved in practice. Figs. 2 and 3 show how the setup 1 is active in operation. The hydrophone arrays 2 are suspended from a ship 6 which has been sailed to an area of interest. The ship is kept stationary relative to the area, and signals are transmitted by the source 4. Due to the specific array setup, difference signals from outside the area of interest 7 are suppressed in a manner known per se. Thus, the probed area 7 is a small strip of the seafloor directed transverse to the longitudinal direction of the apparatus 1. Here, the width of the strip depends on the length of the arrays 2. Known techniques such as the synthetic aperture can be used for further increasing the resolution of the hydrophone setup. Fig. 3 further shows that the frame 1 can be varied slightly through an angle α relative to a vertical direction of the setup. This makes it possible to set an optimal sensitivity.

Fig. 4 shows a diagrammatical setup in which the method of the invention can be used with a simplified calculation example. In the Figure, at a first height, a source si and a hydrophone hi are located, at coordinate (0,3). At a second height, a source s2 and a hydrophone h2 are located, at coordinate (0,2). A signal transmitted by si or s2 reflects from via a position d on the seabed at coordinate (4,0). An object b is located under the seabed at coordinate (3,-1). According to the following calculation example, the delay time difference between a reflection via b or d is indicative of the presence of the object b in the seabed. According to a first measurement, a signal travels a path 11 from si via d to hi measuring 10 m

(i.e. 2x V4² -t- 3² ). In a second measurement, a signal travels a path from s2 to d to h2 of a length 12 = 8.94 m (i.e. 2 x /4² +2² ). A time displacement ("compression") for this point by a factor 0.894 will cause the signal at hi to coincide with the signal at h2.

Next, the same compression factor is used with a reflection via an object b buried in the sea bottom. The signal path length 11' from si via b to hi is again 10m (thus, 8.94 after compression). The signal path length 12', however, has changed now: this goes from s2 via b to h2 and thus measures 2 x 3² + 3² = 8.49. This means that, using a compression factor 0.894, a difference of 0.45 m (viz.: 8.94-8.49) occurs, resulting in a difference signal in which the reflection from b becomes visible. This delay time difference is observable and is indicative of the presence of an object b in the ground. Fig. 5a shows a diagrammatic setup in 2D, in which two hydrophones 8, 9 are set up at 10 and 10.5 meters above a modeled surface 10 with 400 diffractors which have an equal distance of 5 cm relative to each other but have a random height between 0 and 20 cm. Use was made of a pulse form in a frequency range of 1 to 3 kHz. The source 11 was set up at a height of 10 meters. Fig. ϋb^ shows a signal such as it has been received by one of the hydrophones 8 or 9. A reflecting object 12 was modeled at 10 m distance from the vertical, 1 meter under the ground surface.

Fig. 5c shows a remaining signal 13 and envelope 14 after time correction and subtraction of the reference signal. The response resulting from the presence of the object 12 is clearly visible. The displacement in the x-direction (the peak 15 is beyond 10 m) is caused by the object 12 located under the surface being projected to the surface 10 along a circular path with the receiver as its center; in other words, the peak represents the radial distance from receiver to object. By repeating the measurement from a second position, the actual location of the object, and the depth at which the object is located under the ground, can be determined.

Although a flat ground geometry has been modeled in the example of Figs. 5a-c, it is evident that the method can also be used with other ground geometries, which have been determined by means of depth measurement. Such measurements can be carried out using conventional bathymetry, such as, for instance, a high-frequency interferometric side scan.

The invention is not limited to the examples shown in the description, but also comprises all kinds of variations and modifications thereof. Such variations are considered to be within the scope of the invention as defined in the following claims.

Claims

1. .A method for detecting objects on or in the ground, comprising the steps of: transmitting one or more acoustic signals to the ground, so that, after reflection, at least a first and second reflection signal are received from the ground surface by receiving units, and detecting a delay time difference between first and second reflection signals; characterized in that the method comprises the further steps of: setting up the sources and/or receiving units in a series at a fixed position above the ground surface; calculating a delay time difference from a path length difference of the reflection signals, on the assumption that the reflection signals have been reflected by the ground surface, with the ground surface being modeled according to a predetermined geometry; - deriving the presence of an object on or in the ground by comparing the detected delay time difference with the calculated delay time difference.

2. A method according to claim 1, characterized in that the method further comprises the steps of: - transmitting a pulse signal; and receiving a reflection signal by two receiving units set up at different positions relative to the ground surface.

3. A method according to claim 1 or 2, characterized in that the method further comprises the steps of: calculating a corrected reflection signal by displacing the first reflection signal in time over a time interval corresponding to the calculated delay time difference; determining a difference value by subtraction of the corrected first reflection signal and the second reflection signal, which difference value is indicative of the presence of an object on or in the ground.

4. A method according to at least one of the preceding claims, wherein, in a first detection, the receiving units are set up at a first position, from which detection a radial distance of an object to the receiving units can be derived, and wherein next, in a second detection, the receiving units are set up at a second position, so that the position of the object can be determined by means of triangulation.

5. A method according to at least one of the preceding claims, wherein a correction is applied to the calculated amplitude difference as a result of a calculated path length difference of the reflection signals.

6. A method according to at least one of the preceding claims, characterized in that the method is used for detecting objects in a seabed, wherein the chosen acoustic frequency is so low that a depth of penetration into the seabed varies from 0-20 meters, wherein the detection takes place from a height varying from 3-500 meters above the seabed surface and wherein the array setup is such that an area to be probed is in the lateral angle range of 5-70°.

7. A method according to at least one of the preceding claims, characterized in that a delay time difference is calculated on the assumption that the ground surface has a flat geometry.

8. A method according to at least one of the preceding claims, characterized in that a delay time difference is calculated after determining the ground geometry by means of bathymetry.

9. A method according to claim 8, characterized in that the bathymetry is carried out by means of an echo sounder or high-frequency side scan interferometry.

10. An apparatus for detecting objects on or in the ground according to the method of at least one of claims 1-9, comprising at least two parallel arrays of hydrophones set up at a distance from each other, with fixing means for setting up the apparatus, in operation, under a vessel, such that the arrays mutually differ in height relative to the ground, and an acoustic source provided at a fixed height in relation to the arrays, which acoustic source is arranged for transmitting an acoustic pulse in a low frequency spectrum suitable for penetrating a few meters into the seabed.

11. An apparatus according to claim 10, characterized in that the distance between the arrays is adjustable.

12. An apparatus according to claim 10 or 11, characterized in that the acoustic source is set up between the arrays.

13. An apparatus according to at least one of claims 10-12, characterized in that, in operation, the apparatus is tiltable through an angle relative to a vertical direction of the setup.

14. An apparatus according to at least one of claims 10-13, characterized in that the working depth of the apparatus is adjustable.