GB1584303A

GB1584303A - Seismic wave detectors

Info

Publication number: GB1584303A
Application number: GB27641/77A
Authority: GB
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 1976-07-02
Filing date: 1977-07-01
Publication date: 1981-02-11
Also published as: DE2729905C2; DK143523C; GB1584304A; DK293577A; AU2655977A; DK143523B; NO144863C; NL7707298A; SE7707584L; JPS614061B2; BE856235R; NO144863B; DE2729905A1; IT1114621B; NO772313L; JPS60220886A; AU513212B2; JPS5333651A

Description

(54) SEISMIC WAVE DETECTORS (71) We, INSTITUT FRANCAIS DU PETROLE, a body corporate organised and existing under the laws of France of 4 avenue de Bois-Preau, 92502 Rueil Malmaison, France, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by by which is to be performed, to be particularly described in and by the following statement: This invention relates to seismic wave detectors, and is more particularly directed to improvements in or modifications of the invention which is the subject of our Patent No. 1 505 664 (hereinafter referred to as 'the parent patent').

Claim 1 of the parent patent recites a seismic wave detector for seismic prospecting of a medium, the detector comprising an elongate sensitive assembly that comprises a deformable electret, two electrodes located one on each side of the deformable electret, and a compressible and electrically insulative flexible member located between the deformable electret and one of the electrodes, the sensitive assembly being so arranged that at least one surface thereof defines or is adjacent to a surface of the detector that, in use, contacts the medium along the length of the detector.

In embodiments of the invention of the parent patent the deformable electret is made, for example, by polarising strips of a thickness of from 8 to 28 microns of a plastics material such as polypropylene, polytetrafluoroethylene, or polyethylene terephthalate. This polarisation can be performed by electronic bombardment, by applying an electric field while heating the material to a temperature close to the softening temperature, by a corona discharge, or by ultra-violet irradiation.

The sensitive assembly of embodiments of the invention of the parent patent may be formed by stacking several elements, namely a metal strip or braid forming a first of the electrodes, a compressible element made of an electrically insulative material and comprising a strip regularly perforated or made by weaving, an electret disposed on the compressible element, and a metal layer or strip forming the second electrode.

When the sensitive assembly is subjected to mechanical stresses, the metal layer forming the second electrode and the electret are deformed and the distance between the electrodes varies. A voltage is induced between the electrodes by the electric field permanently generated by the electret. The arrangement of the electret between the compressible element and the second electrode in the embodiments of the patent in question involves a number of disadvantages due to technological limitations in the manufacture of the electret.

A first disadvantage results from the fact that the electret fulfils two separate functions: that of generating an electric field and that of transmitting mechanical deformations. Now, some materials used to manufacture electrets (polytetrafluoroethylene for example) are incapable in practice of transmitting the mechanical stressses to which they are subjected.

A second disadvantage results from the fact that the metal layer forming the second electrode may comprise a metal coating deposited under vacuum on the surface of the electret. The metallisation of the surface of certain electrets is detrimental to the electrical characteristics of the material since certain metal ions diffuse in the plastics material forming the electret. It is preferable in this case not to use a metallised electret.

According to the present invention there is provided a seismic wave detector for seismic prospecting of a medium, according to claim 1 of the parent patent, wherein a first of the electrodes of the sensitive assembly acts as a support for the deformable electret and the compressible and electrically insulative flexible member is disposed between the deformable electret and the second electrode, the second electrode being in the form of a flexible film.

The second electrode may be entirely of metal or may comprise a strip of plastics material and a metal deposit on one of the faces of the strip of plastics material.

In embodiments of the invention disclosed hereinbelow the abovementioned disadvantages are avoided or at least alleviated due to the fact that the electret is no longer used as a mechanical transmission agent but only for generating an electric field. The deformations resulting from the stresses are only transmittted through the compressible element and the second electrode. Moreover, the second electrode is in contact with the compressible member and not with the electret.

The invention will now be further described, by way of illustrative and nonlimiting example, with reference to the accompanying drawing, wherein: Figure 1 shows a seismic wave detector constituting an embodiment of the invention; Figure 2 shows a first variant of the embodiment of Figure 1; and Figure 3 shows a second variant of the embodiment of Figure 1.

The seismic wave detector of Figure 1 comprises a flat metal strip 2, relatively thick and of sufficient flexibility for being wound, but which may be considered as locally rigid. The strip 2 forms a first electrode of the seismic wave detector. On the strip 2 is laid, adhered or welded an electret film 4 on which is laid an electrically insulative and compressible flexible element 3 formed by a strip made of solid material that has been regularly perforated. A second electrode is arranged on the insulative element 3. The second electrode comprises a metallic membrane 6 whose thickness is about a few microns. The metal of which the membrane 6 is made is selected in accordance with the desired sensitivity.

The second electrode may alternatively be constituted by a thin film of plastics material metallised on one of its faces. The mechanical properties of the plastics material are such that it acts as a membrane accurately transmitting the stresses exerted thereon. For example, the selected plastics material may be polypropylene, a polycarbonate or polyethylene terephthalate. The thickness of the film is preferably selected to be from 8 to 25 microns. The metallised surface is so disposed as to face the electret film 4.

A first variation of the embodiment of Figure 1 consists of replacing the perforated strip 3 with a strip in the form of lattice stucture (Figure 2) made of plastics material or of woven material.

A second variant of the embodiment of Figure 1 comprises replacing (Figure 3) the perforated strip 3 with a strip made of compressible elastic material (cellular material for example).

In the two above variants, the second electrode may once more comprise either a metallic membrane or a thin plastics film metallised on one of its faces.

The above described embodiments can be used in the same combinations as those illustrated in Figures 3 to 7 of the parent patent: connection in series of two sensitive assemblies by direct contact between their thicker electrodes, connection in series or in parallel of several sensitive assemblies, or stacking of sensitive assemblies with the interposition of flexible insulating strips, to substantially nullify parasitic signals due to bending.

The sensitive assembly may also be helically wound around a central core.

WHAT WE CLAIM IS: - 1. A seismic wave detector for siesmic prospecting of a medium, according to claim 1 of Patent No. 1 505 664, wherein a first of the electrodes of the sensitive assembly acts as a support for the deformable electret and the compressible and electrically insulative flexible member is disposed between the deformable electret and the second electrode, the second electrode being in the form of a flexible film.

2. A detector according to claim 1, wherein the second electrode is entirely of metal.

3. A detector according to claim 1, wherein the second electrode comprises a strip of plastics material and a metal deposit on one of the faces of the strip of plastics material.

4. A detector according to claim 1, claim 2 or claim 3, wherein the compressible member is constituted by a regularly perforated strip.

5. A detector according to claim 1, claim 2, or claim 3, wherein the compressible member is in the form of a strip of lattice structure.

6. A detector according to any one of the preceding claims, comprising a plurality of sensitive assemblies which are electrically interconnected.

7. A detector according to any one of claims 1 to 5, wherein at least one sensitive assembly is helically wound around a central core.

8. A seismic wave detector for siesmic prospecting of a medium, the detector being substantially as herein described with reference to any one of Figures 1 to 3 of the accompanying drawing.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the faces of the strip of plastics material. In embodiments of the invention disclosed hereinbelow the abovementioned disadvantages are avoided or at least alleviated due to the fact that the electret is no longer used as a mechanical transmission agent but only for generating an electric field. The deformations resulting from the stresses are only transmittted through the compressible element and the second electrode. Moreover, the second electrode is in contact with the compressible member and not with the electret. The invention will now be further described, by way of illustrative and nonlimiting example, with reference to the accompanying drawing, wherein: Figure 1 shows a seismic wave detector constituting an embodiment of the invention; Figure 2 shows a first variant of the embodiment of Figure 1; and Figure 3 shows a second variant of the embodiment of Figure 1. The seismic wave detector of Figure 1 comprises a flat metal strip 2, relatively thick and of sufficient flexibility for being wound, but which may be considered as locally rigid. The strip 2 forms a first electrode of the seismic wave detector. On the strip 2 is laid, adhered or welded an electret film 4 on which is laid an electrically insulative and compressible flexible element 3 formed by a strip made of solid material that has been regularly perforated. A second electrode is arranged on the insulative element 3. The second electrode comprises a metallic membrane 6 whose thickness is about a few microns. The metal of which the membrane 6 is made is selected in accordance with the desired sensitivity. The second electrode may alternatively be constituted by a thin film of plastics material metallised on one of its faces. The mechanical properties of the plastics material are such that it acts as a membrane accurately transmitting the stresses exerted thereon. For example, the selected plastics material may be polypropylene, a polycarbonate or polyethylene terephthalate. The thickness of the film is preferably selected to be from 8 to 25 microns. The metallised surface is so disposed as to face the electret film 4. A first variation of the embodiment of Figure 1 consists of replacing the perforated strip 3 with a strip in the form of lattice stucture (Figure 2) made of plastics material or of woven material. A second variant of the embodiment of Figure 1 comprises replacing (Figure 3) the perforated strip 3 with a strip made of compressible elastic material (cellular material for example). In the two above variants, the second electrode may once more comprise either a metallic membrane or a thin plastics film metallised on one of its faces. The above described embodiments can be used in the same combinations as those illustrated in Figures 3 to 7 of the parent patent: connection in series of two sensitive assemblies by direct contact between their thicker electrodes, connection in series or in parallel of several sensitive assemblies, or stacking of sensitive assemblies with the interposition of flexible insulating strips, to substantially nullify parasitic signals due to bending. The sensitive assembly may also be helically wound around a central core. WHAT WE CLAIM IS: -

1. A seismic wave detector for siesmic prospecting of a medium, according to claim 1 of Patent No. 1 505 664, wherein a first of the electrodes of the sensitive assembly acts as a support for the deformable electret and the compressible and electrically insulative flexible member is disposed between the deformable electret and the second electrode, the second electrode being in the form of a flexible film.