Microwave reflection survey technique for determining depth and orientation of buried objects

• US 4,728,897 A
• Filed: 07/13/1987
• Issued: 03/01/1988
• Est. Priority Date: 10/17/1984
• Status: Expired due to Fees

• Current Assignee: British Gas Trading Limited (Centrica PLC)
• Original Assignee: British Gas Trading Limited (Centrica PLC)
• Inventors: Gunton, David J.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US07/073,328
• Time in Patent Office: 232 Days
• Field of Search: 324/329, 324/337, 342/22, 342/459, 343/797
• US Class Current: 324/329
• CPC Class Codes: G01S 13/04 Systems determining presence of a target based on relative movement of target G01S13/56
• CPC Class Codes: G01S 13/885 for ground probing prospecting or detecting using electromagnetic waves G01V3/12
• CPC Class Codes: G01S 7/026 involving the transmission of elliptically or circularly polarised waves
• CPC Class Codes: G01V 3/12 operating with electromagnetic waves operating with millimetre waves G01V8/005

Abstract

A method and antenna assembly for transmitting into the ground polarizeable radiation whereby buried objects, and particularly long thin objects such as pipes can be located, the position determined, and the pipe direction ascertained, by taking measurements from a signle point, without mechanical movement of the antenna, and thereby allowing for good suppression of spurious signals and a reduction in false indications. The method includes the steps of transmitting into the ground polarizeable radiation, receiving reflected signals, indicative of the objects, incident upon at least two co-located angularly disposed transducers and comparing the signals thus received, wherein the radiation is transmitted from a transducer source which is co-located but angularly disposed with respect to each receiver transducer. An antenna assembly suitable for use in the method of the invention includes a substantially planar arrangement of at least three, co-located, two-terminal antennas wherein the antennas are angularly disposed with respect to each other around a common point and each antenna is adapted to be capable of selectively transmitting polarizeable radiation into the ground and receiving reflected radiation from out of the ground.

Claims

1. A method of producing information indicative of the depth and orientation of an elongated object buried beneath the ground using electromagnetic radiation transmitted from and received by an antenna apparatus comprising mutually electrically orthogonal transmit and receive elements, said radiation being elliptically or circularly polarized or nearly so, said method comprising the following steps:

(a) positioning an elongated reference object at known angular orientation with respect to a reference direction on the ground in a position to intercept radiation emitted from said antenna apparatus with the transmit and receive elements thereof in positions of known angular orientations with respect to the lengthwise direction of said reference object;

(b) energizing the transmit elements of said apparatus with a first electric signal to cause radiation to be transmitted therefrom;

(c) receiving radiation backscattered due to step (a) by means of said receive elements and producing first data corresponding to the received backscattered radiation;

(d) storing said first data;

(e) repositioning said transmit and receive elements at least once to new positions angularly separated from the immediately previous positions by a known angle other than an integer multiple of 90°

about an axis transverse to the lengthwise direction of said reference object;

(f) repeating steps (b), (c) and (d) after each repositioning effected by step (e) to obtain at least second data;

(g) algebraically combining at least said stored first data and said second data to reduce clutter and errors due to departure from exact orthogonality of said transmit and receive elements upon each performance of step (f) and based on said algebraic combining producing reference data defining a known orientation of said reference object in relation to a known reference direction;

(h) storing said reference data;

(i) positioning said antenna apparatus over the ground with said transmit and receive elements in position at known angular orientations with respect to a reference direction on the ground; and

(j) energizing said transmit elements with a signal identical to said first electrical signal to transmit radiation towards the ground;

(k) receiving radiation back-scattered due to step (j) by means of said receive elements and producing third data corresponding to the received back-scattered radiation;

(l) storing said third data;

(m) repositioning of said transmit and receive elements at least once to new positions angularly separated from the previous positions by a known angle other than integer multiple of 90°

about an axis directed into the ground;

(n) repeating steps (j), (k), and (l) after each repositioning effected by step (m) to obtain fourth data;

(o) algebraically combining at least said stored third data and fourth data to reduce clutter and errors due at least to departure from exact orthogonality of said transmit and receive elements and producing search data based on the algebraic combining of said third and fourth data;

(p) storing said search data; and

(q) detecting correlation between said stored reference data and said stored search data to yield said information and further to reduce clutter in said stored fourth data, said correlation being detected using a predetermined filtering technique;

wherein steps (a)-(h) are optionally performed either before or after steps (i)-(p).

2. A method according to claim 1, wherein said transmit and receive elements are repositioned at least twice in step (e) and in step (m).

3. A method according to claim 1, wherein steps (c) and (k) comprise:

receiving backscattered radiation on at least two receive elements which are so related to said transmit elements as to be equivalent to a single receive element electrically orthogonal to said transmit elements.

4. A method according to claim 3, wherein step (c) and (k) comprise:

using pairs of receive elements arranged as dipoles with respect to said transmit elements.

5. A method according to claim 1, comprising:

using spirally shaped transmit and receive elements.

6. A method according to claim 1, wherein said steps (b) and (j) comprise:

supplying to said transmit elements said first electrical signal having a time waveform such that said back-scattered radiation gives rise at said receive elements to a time waveform of chirp type in which the frequency varies rapidly with time.