WO2011010933A1

WO2011010933A1 - Electrical sensor for marine csem prospecting

Info

Publication number: WO2011010933A1
Application number: PCT/NO2010/000276
Authority: WO
Inventors: Jostein Kåre KJERSTAD
Original assignee: Advanced Hydrocarbon Mapping As
Priority date: 2009-07-24
Filing date: 2010-07-09
Publication date: 2011-01-27
Also published as: NO20092754A; NO329716B1

Abstract

A sensor for marine geophysical prospecting is described, in which, in a sensor casing (1) made of an organic material, an electrode (3) is provided, formed of at least a pressed and sintered, fine-grain, homogeneous mixture of silver and silver chloride arranged on an electrode seat (2, 2a) projecting upwards from an internal end surface of the sensor casing (1), and the sensor casing (1) being filled with a particulate filling material (8) mixed with a saturated, aqueous solution of sodium chloride (NaC1) or potassium chloride (KC1), the filling material (8) being provided by a chemically inert, particulate material mixed with silver chloride particles, and the sensor casing (1) being provided with a filter (9) arranged to retain the particulate filling material (8) and convey fluid between the interior of the sensor casing (1) and its surroundings.

Description

ELECTRICAL SENSOR FOR MARINE CSEM PROSPECTING

A sensor for marine, geophysical prospecting is described; more specifically, with a sensor casing made of an organic material and in which an electrode is provided, formed of at least a pressed and sintered, fine-grain, homogeneous mixture of silver and silver chloride and arranged on an electrode seat projecting up from an internal end surface of the sensor casing, the sensor casing being filled with a particulate filling material mixed with a saturated, aqueous solution of sodium chloride (NaCl) or potassium chloride (KCl) , the filling material being provided by a chemically inert, particulate material mixed with silver chloride particles, and the sensor casing being provided with a filter arranged to retain the particulate filling material and convey fluid between the interior of the sensor casing and its

surroundings .

Instrumentation used in marine hydrocarbon controlled-source electromagnetic (CSEM) prospecting contains electrical sensors intended for the registration of an electric field. Normally, non-polarizable electrodes of silver / silver chloride (Ag-AgCl) are used in the electric field recorders. At least two spaced sensors are used for measurements of electrical potential. The total noise level produced by two sensors is always higher than the internal noise of one sensor and is determined mainly by the noisiest sensor. The electric field sensors designed for marine

electromagnetic prospecting exhibiting a silver wire

galvanically plated with silver chloride are well known. The design, technology of preparation and galvanizing methods for a silver chloride electrode are described by, for example, Webb et al . , 1985, and resulted in the American patent US 5770945. According to this article, the sensor exhibits a porous polyethylene tube containing a foil of pure silver wrapped around a plastic-coated metal rod. The conductor of an underwater connector is soldered to one end of the silver foil, and the joint is potted into a plastic cap and sealed by the use of epoxy. The other tube end is glued onto the cap. A mixture of 35 g/1 NaCl solution with diatomaceous earth and silver chloride surrounding the metal rod covered with silver foil is used as filling material.

The electrode is galvanized after assembling. Optimal contact with the sea water is achieved through the surface of the porous polyethylene casing. The role of the filling material consists in forming the maximal diffusion-path length for the silver and chlorine ions, which determines the service life of the electrode and its noise level.

Direct contact between a filling material and the surface of the electrode covered by a thin layer of silver chloride is possible even in quiet conditions during transport, impacts and moving of the electrode, and also because of gravity.

These contacts lead to rupturing of a silver chloride layer, to exposure of the silver metal electrode body, and as a result, the initial potential (drift) is changed and

additional noise occurs. A difference in temperature

coefficients of expansion of the materials silver and silver chloride in the body of an electrode also leads to the occurrence of microcracks and additional noise. In attempts to eliminate this problem, silver sensors are repeatedly "revived" . The electrodes are then cleaned and recoated with AgCl. However, in spite of this measure, satisfactory results are not always achieved. Reviving performed regularly between measurements results in additional work and prevents

5 continuous use of the electrodes.

A potential difference between two electrodes is the

continuous or very slowly variable difference in voltage which appears between electrodes of the same type placed at a short distance in a suitable substrate. Evidently, for two io identical electrodes the potential difference should be null.

Another construction of a sensor with Ag-AgCl electrodes of great and long-term stability, which is necessary for work in sea water, has been proposed by Y. Hamano et al . , 1984, and Perrier et al . , 1997. The sensor consists of a porous, cera- i5 mic tube containing silver metal in the stem and the mesh, and having a mixture of silver / silver chloride and silver oxide. The ceramic tube with the specified structure is placed in an electric oven at a temperature of 440-450 ⁰C for nearly 20 minutes. The temperature must not exceed 455 ⁰C

2o which is the melting temperature of silver chloride. The

optimal contact with the sea water is achieved through the surface of the porous ceramic casing.

Extruded and then sintered Ag-AgCl electrodes are applied as non-polarized electrodes for batteries and for electric field

25 potential measurements on a human body, for example in

electrocardiography, electroencephalography, electromyography etc. Owing to the fact that the particles of finely dispersed silver become covered with the melted silver chloride, these electrodes have a small and stable difference in their

30 potentials. At the same time, these electrodes have principle disadvantages in that their potential difference is varying with a period of 0.5-10 seconds when the electrodes are submerged in a salt solution or sea water. Sea water has a much more complicated chemical structure than a simple salt solution which can be prepared from the

chemically pure components water and sodium chloride or potassium chloride. Therefore, the chemical processes in sea water are more various and unpredictable. It is well known that the base factors that result in degradation of the parameters of the sensors are aquation, formation of complex compounds, oxidation and pH modification of the filling material. The variations in potential may reach several μV. The phenomenon which is of a complicated chemical nature, which has to do with the formation of complex compounds of silver and components in sea water, is undesired for sensors designed for marine geophysical prospecting with a controlled electrical-current source (CSEM) because the variations in potential difference create noise which makes difficulties during measurements of weak, useful signals. The spectral distribution of the noise and the useful signal are

overlapped, and their separation becomes difficult.

For the suppression of these undesirable variations, the electrode is surrounded by an additional, neutral, finely dispersed filling material with a finely porous filter. This essentially increases the diffusion-path length for the silver and chlorine ions. The resistance between electrode pairs is increased as well as the level of thermal noise, but this increase is smaller than the decrease in the value of potential-difference variation.

During manufacturing of the electrode, the uniform mixture of small silver and silver chloride particles is pressed with a thin silver wire which is used for the output of the electric signal. After pressing, the wire has numerous mechanical and electrical contacts with the silver particles which provide electrical conductivity. During melting, the silver chloride meets the silver wire and silver particles and produces a multitude of microelectrodes . However, the majority of the silver particles do not connect electrically with each other. The connection between sensors during measurements is

realized through the chain: electronic conductivity of silver - ionic conductivity of silver chloride / salt solution - ionic conductivity of silver chloride - electronic

conductivity of silver. The general electromotive force (EMF) (electrode voltage) is determined by a total amount of microelectrodes' potentials

in which E± is the potential of the i-th microelectrode, N is a number of microelectrodes. Such electrodes may therefore be termed polyelectrodes .

It is also known that decreasing the silver particle size as the total weight is fixed, decreases the capacity and

resistance of the electrode, increases the stability of the electrode over time, and decreases the degree of polarization under direct-current effect.

The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art .

The object is achieved through features which are specified in the description below and in the claims that follow.

In marine controlled-source electromagnetic (CSEM)

prospecting, the measurement response voltage can be very small, in the order of the first nV; correspondingly, the level of inherent noise should be less than 1 nV. This factor makes additional demands on the inherent noise of the sensor. These demands are substantially higher than for on-land prospecting. In the present invention, the sensor which is in the form of a vessel, is manufactured from an organic material with an internal ledge at the bottom, on which an electrode in the form of a disc, cylinder or other form made of pressed and sintered mixes of fine granules of silver and the silver chloride and connected to a target cable, is placed. The vessel is filled with a filling material consisting of a particulate material, a mixture of the solution of water saturated with NaCl or KCl, and granules of a chemically inert substance with an amount of particulate silver chloride admixed. A porous filter which has a pore diameter which is smaller than the diameter of the granulated substance is placed on top of the filling material. To the particle mixture is added a saturated solution of NaCl or KCl. The space between the filter and an upper edge of the sensor vessel is filled with a hydrophobic material, and this is impregnated with the same salt solution that is added to the particle mixture. The vessel is hermetically closed by a cover to provide a storable electrode.

One preferred embodiment of the invention is based on the realization that if an electrode made of silver / silver chloride is used, the electrode surface will also remain substantially unchanged even if the electrode wears or is scratched, because the electrode consists of the same

material throughout .

A substance which will not react with the components of the electrolyte, not form complex connections with ions of silver and chlorine is selected as the filling material. Finely ground silicon oxide (SiO₂) , aluminium oxide ((X-Al₂O₃) in granular form, used in column chromatography, or a chemically inactive, finely dispersed organic material of the polythene type can be used as such a filling material. The invention relates more specifically to a sensor for marine geophysical prospecting, in which a sensor casing is provided as a vessel with an open top, the sensor casing accommodating an electrode connected to an external sensor cable arranged to transmit signals from the sensor to associated recording means, and the sensor casing being filled with a particulate filling material mixed with an electrolyte formed of a saturated salt solution,

characterized by

- the sensor casing being manufactured from an organic material;

there being a columnar electrode seat projecting from an internal first end portion;

an electrode provided by at least a pressed and

sintered, fine-grain, homogeneous mixture of silver and silver chloride being arranged on the end portion of the electrode seat projecting upwards;

the salt solution being an aqueous solution of sodium chloride (NaCl) or potassium chloride (KCl) ;

- the filling material being provided by a chemically inert, particulate material mixed with silver chloride particles; and

the other end portion of the sensor casing being

provided with a filter arranged to retain the particulate filling material and convey fluid between the interior of the sensor casing and its surroundings.

The sensor casing may have a cylindrical shape. This enables rational production and provides optimal conditions around the electrode.

A space between the filter and the open top of the sensor casing is arranged to receive a hydrophobic material which is impregnated with an aqueous solution of sodium chloride

(NaCl) or potassium chloride (KCl) . The other end portion of the sensor casing may be arranged to be closed temporarily by a fluid-tight cover. The advantage of this is that the sensor can be stored and transported while it is filled with the salt solution.

The electrode may have the form of a conical, smoothed pin. Thereby a large contact area towards the filling material is provided, in order thereby to optimize noise, drift and lifetime .

The electrode seat may have a length determined by the grain distribution in the filling material. By varying particle size in the filling material, a sorting during filling of the sensor will thereby result in filling material of an optimal particle size surrounding the electrode.

The filling material may be provided as a granulated

aluminium oxide ((X-Al₂O₃) mixed with silver chloride

particles. Alternatively, the filling material may be

provided as a fine-grain quartz (SiO₂) mixed with silver chloride particles. Both aluminium and quartz are easily available materials with stable, desired properties.

The connection of the sensor cable to the electrode may extend through a closable cut-out extending through the electrode seat .

The pore diameter of the filter may be smaller than the diameter of the filling material particles. Loss of filling material by emigration through the filter is thereby

prevented.

The closable cut-out extending through the electrode seat may be filled with epoxy. A satisfactory sealing of the electrode cable and connecting point is thereby provided.

The understanding the present invention will be facilitated by consideration of the following detailed description of a non-limiting example of a preferred embodiment which is visualized in the accompanying drawings, in which like numerals refer to like parts, and in which:

Figure 1 shows the cross section of a sensor prepared for storage, it being provided with a hydrophobic material and a lid which is to prevent drying; and

Figure 2 shows, on a larger scale, a side view of a

preferred design of an electrode.

In the figures, the reference numeral 1 indicates a sensor casing, there being, inside the sensor casing 1 and

projecting up from a first end portion Ia, a column 2

provided with an electrode seat 2a arranged to receive and fix a sintered electrode 3. The electrode 3 is provided by a pressed and sintered, fine-grain, homogeneous mixture of silver and silver chloride and it is provided with an

electrode wire in the form of a silver wire 4 arranged in electrically conductive contact with the sintered material of the electrode 3 and projecting, by a free end, from the electrode 3. The electrode wire 4 connects the electrode 3 to an external sensor cable 6 via a coupling 5 provided by soldering and arranged in a cut-out 2b extending from the electrode seat 2a to an external end surface of the first end portion Ia of the sensor casing 1.

The external sensor cable 6 is arranged to be connected to suitable measuring or recording means (not shown) .

The cut-out 2b provides a possibility of installing and connecting the electrode 3. After installation, this cut-out 2b is filled with a suitable sealing substance 7, for example epoxy.

In the cavity between the first end portion Ia and a

microporous filter 9 which is arranged near a second end portion Ib of the sensor casing 1, the sensor casing 1 is substantially filled with a particulate filling material 8, for example fine-grain quartz (SiO₂) or granules of aluminium oxide ((X-Al₂O₃) which is chemically inert to sea water among other things, mixed with silver chloride particles.

The microporous filter 9 has a tight fit against the internal wall surface of the sensor casing 1 and is secured to the sensor casing 1 by means of a mounting ring 10. The pore diameter of the filter 9 is smaller than the diameter of the filling material particles 8.

For storing purposes, a hydrophobic material 11 is arranged in the second end portion Ib of the sensor casing 1, on the external side of the filter 9. The purpose of this

hydrophobic material 11 is to prevent the filling material 8 from drying out during storage. In addition, a cover 12 is arranged on the open top of the sensor casing 1, arranged to close the sensor casing 1 in a fluid-sealing manner for storing purposes .

In the spaces between the particles of the filling material 8 is filled a saturated salt solution 13, for example a NaCl or KCl solution. This salt solution 13 fills the sensor casing 1 all the way up to the hydrophobic material 11 on the outside of the filter 9.

The shape of the sensor casing 1 accommodating the electrode 3 may be arbitrary. In figure 1, the sensor casing 1 is shown as a cylindrical vessel. The sensor casing 1 may be made of a plastic, organic material. However, polyethylene or other similar plastic that is chemically inert with respect to sea water and salt solutions is preferred.

The electrode 3 is mounted in the electrode seat 2a, the electrode wire 4 is passed through the cut-out 2b and

connected to the sensor cable 6. Then the cut-out 2b is filled with the sealing substance 7. Then the sensor casing 1 is filled with the filling material 8. To the filling

material 8 is then added a saturated NaCl or KCl solution. The microporous filter 9 is arranged to keep the filling material 8 in place in the sensor casing 1 after the cover 12 has been removed. The filling material 8 is chemically inert to sea water and said salt solution 13.

The size of the electrode 3 is essential to its inherent- noise level, drift and lifetime. The smaller the area of the part of the electrode 3 in contact with the filling material 8, the higher the level of noise and drift and the shorter the lifetime because of the continuous dilution of silver chloride in a salt solution. The size of the sensor therefore depends on the measurement conditions, the durability in use and storage time.

In a preferred embodiment, the sensor proposed here has a an external diameter of 50 mm, a length of 250 mm, and the electrode seat 2 projects 50 mm up from the internal end surface of the sensor casing 1. The pore diameter of the filter 9 is 0.5 microns, but a different pore size is also possible .

The diatomaceous earth and silver chloride are used as the solid filling material in the well-known sensor according to Webb et al . (1985) based on an Ag-AgCl electrode in the ratio 6:1. Beside silicon oxide (89 %) and aluminium, there are iron (Fe) and calcium (Ca) compounds which may form complex compounds by interaction with a NaCl and KCl solution; this results in a change in the self-potential of the electrode and an increase in the noise.

Another sensor for an electric field (Y. Hamamo et al.) which is used today consists of a sintered silver / silver chloride electrode with an addition of silver oxide with no other filling material. This sensor is used for submarine

magnetotelluric investigations with a 60^"1 Hz sampling rate. This sensor has quite good long-term stability but can have quasi-periodical fluctuations with a potential difference for a short time (a fraction of a second) . This effect is

extremely undesirable in CSEM hydrocarbon prospecting.

In the present invention, a pure, chemical aluminium oxide (Ci-Al₂O₃) in a granular form with an average diameter of less than 100 microns, intended in particular for column

chromatography, or fine-grain silicon oxide (SiO₂) with a particle diameter of less than 100 microns is proposed as the filling material. A characteristic feature of this filling material consists in its chemical inertia, which results in the absence of any reaction with a NaCl or KCl solution and the suppression of quasi-periodical fluctuations in a short- time potential difference. Granules of the polyethylene type of a size smaller than 100 microns may also be used as the chemically inactive filling material.

After mounting of the sintered electrode 3 on the electrode seat 2a, filling of the filling material 8 and NaCl or KCl solution 13, the sensor is placed on a vibration-testing machine and subjected to vibrations to remove the gas, for example air, that might be present in the solution and the filling material. During vibration, the largest particles of the filling material 8 move towards the bottom of the sensor casing 1, whereas the fine particles of the filling material 8 are deposited higher up in the sensor casing 1. The height of the column 2 is chosen on the basis of the ratio of large and small particles in the filling material 8 to provide a substantial degree of fine particles around the electrode 3.

Removal of the gas from the sensor can also be effected by vacuum pumping. The combination of vibration and vacuum pumping allows acceleration of the process of producing and stabilizing the sensor.

Series of pressure tests at ~100 bars have proved that a sensor provided with a pressed and sintered silver / silver chloride electrode 3 in accordance with the present invention endures this pressure with marginal changes to its inherent- noise level and drift. The stabilization time at the changeover from storage to operative measurements in sea water is several times shorter than for the similar sensors used in lead / lead chloride electrodes.

The electric field sensor proposed here, together with amplifiers mounted within the sensor or attached to it (not shown) , can be used for electric field measurements in a scheme with a so-called active electrode. In that case, the inductive noise excited by movable cables connecting the sensor to the measurement amplifier is abated.

List of references

U.S. patent documents

2008/0246485 Al 09/2008 Hibbs et al .

6842006 01/2005 Conti et al .

5770945 06/1998 Constable

5041793 08/1991 Thompson

4324680 04/1982 Kubota et al

4178339 12/1979 Powell et al

3111478 11/1963 Watanabe

Other patent publications

WO 01/57555 Al 09/2001 Ellingsrud et al , WO 02/14906 Al 02/2002 Ellingsrud et al . WO 03/025803 Al 03/2003 Srnka et al .

WO 03/034096 Al 04/2003 Sinha et al .

WO 03/048812 Al 06/2003 MacGregor et al .

Norwegian patent documents

NO 323889 Bl 01/2006 Barsukov et al .

Russian patent documents

RU 1067456 1984 Bogorodsky et al

RU 1357900 1987 Bogorodsky M.

RU 1497601 1989 Bogorodsky et al Other publications

Bogorodsky V. M. and Bogorodsky M. M. , 1995, Methods and

technical means for measuring quasi constant natural electric fields. In: Proceedings of the Workshop

"Electrodes", April 24-29 1995, Garchy, Edited by G.

Clerc, F. Perrier, G. Petiau and M. Menvielle, 1996.

Filloux J. H., 1987. Instrumentation and experimental methods for oceanic studies. In Geomagnetism (Ed. J. Jacobs), vol 1, London Academic Press, Ch. 3, pp. 143-248 Hamano Y., Yukutake T. and Segawa J., 1984. Development of new silver-silver chloride electrodes for ocean bottom electrometer, Proc . Conductivity Anomaly Symp., Japan, pp. 251-255 (in Japanese) .

Hiroaki Toh, Tadanori Goto, and Yozo Hamano, 1998. A new

seafloor electromagnetic station with an Overhauser magnetometer, a magnetotelluric variograph and an

acoustic telemetry modem. Earth Planets Space, 50, pp. 895-903.

Perrier F. E., Petiau G., Clerc G., Bogorodsky V., Erkul E., Jouniaux L., Lesmis D., Macnae J., Meunier M., Morgan D., Nascimento D., Oettinger G., Schwarz G., Toh H., Valiant J., Vozoff K., and Yazici-Cakin 0. , 1997. A one-year systematic study of electrodes for long period

measurements of the electric field in geophysical

environments, J. Geomag. Geoelectr. , 49, pp. 1677-1696.

Petiau G. and Dupis A., 1980. Noise, temperature coefficient and long time stability of electrodes for telluric observations, Geophys . Prosp., 28, pp. 792-804. Petiau G., 2000. Second Generation of Lead-Lead Chloride

Electrodes for Geophysical Applications, Pure and Applied Geophysics 157, pp. 357-382.

Webb S. C, Constable S. C, Cox CS. and Deaton T. K., 1985. A seafloor electric field instrument, J. Geomag.

Geoelectr., 37, pp. 1115-1129.

Claims

C l a i m s

1. A sensor for marine geophysical prospecting, in which a sensor casing (1) is provided as a vessel with an open top (Ib) , the sensor casing (1) accommodating an electrode (3) attached to an external sensor cable (6) arranged to transmit signals from the sensor to associated recording means, and the sensor casing (1) being filled with a particulate filling material (8) mixed with an electrolyte (13) formed of a saturated salt solution, c h a r a c t e r i z e d i n that the sensor casing (1) is made of an organic material;

there is a columnar electrode seat (2, 2a) projecting from an internal first end portion (Ia) ; - an electrode (3) provided by at least a pressed and sintered, fine-grain, homogeneous mixture of silver and silver chloride is arranged on the end portion (2a) of the electrode seat (2) projecting upwards ;

- the salt solution (13) is an aqueous solution of sodium chloride (NaCl) or potassium chloride (KCl) ; the filling material (8) is provided by a chemically inert, particulate material mixed with silver chloride particles; and

- the second end portion (Ib) of the sensor casing (1) is provided with a filter (9) arranged to retain the particulate filling material (8) and convey fluid between the interior of the sensor casing (1) and its surroundings .

2. The sensor according to claim 1, c h a r a c t e r i z e d i n that the sensor casing (1) has the shape of a cylinder.

3. The sensor according to claim 1, c h a r a c t e r i z e d i n that a space between the filter (9) and the open top (Ib) of the sensor casing (1) is arranged to receive a hydrophobic material which has been

5 impregnated with an aqueous solution of sodium

chloride (NaCl) or potassium chloride (KCl) .

4. The sensor according to claim 1, c h a r a c t e r i z e d i n that the second end portion (Ib) of the sensor casing (1) is arranged to be closed temporarily io with a fluid-tight cover (12) .

5. The sensor according to claim 1, c h a r a c t e r i z e d i n that the electrode (3) has the form of a conical, smoothed pin.

6. The sensor according to claim 1, c h a r a c t e r i - i5 z e d i n that the electrode seat (2) has a length determined by the grain distribution in the filling material (8) .

7. The sensor according to claim 1, c h a r a c t e r i z e d i n that the filling material (8) is provided

20 as a granulated aluminium oxide ((X-Al₂O₃) in a mixture with silver chloride particles.

8. The sensor according to claim 1, c h a r a c t e r i z e d i n that the filling material (8) is provided as a fine-grain quartz (SiO₂) in a mixture with silver

25 chloride particles.

9. The sensor according to claim 1, c h a r a c t e r i z e d i n that the connection of the sensor cable (6) to the electrode (3) extends through a closable cut-out (2b) extending through the electrode seat (2).

10. The sensor according to claim 1, c h a r a c t e r i z e d i n that the pore diameter of the filter (9) is smaller than the diameter of the filling material particles (8) .

11. The sensor according to claim 1, c h a r a c t e r i z e d i n that the closable cut-out (2b) extending through the electrode seat (2) is filled with epoxy (7) .