TREA

Electromagnetic surveying

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Information

  • Patent Grant
  • 8188748
  • Patent Number
    8,188,748
  • Date Filed
    Friday, February 9, 2007
    17 years ago
  • Date Issued
    Tuesday, May 29, 2012
    12 years ago
Information
Abstract
A method of determining the nature of subterranean strata. The transmitter antenna is towed by vessel at a distance h above the sea floor. The antenna omits a wave field whose strength is proportional to the current delivered to the antenna and this is monitored. The input voltage to the antenna is also measured and changes in antenna impedance are calculated. Changes in antenna impedance are used together with seismic information to determine the nature of the subsea strata.
Description
RELATED APPLICATIONS

This application claims priority to PCT Application No. PCT/GB2007000459, filed Feb. 9, 2007, and Great Britain Application No. 0602656.1 filed Feb. 9, 2006, the disclosures of which are hereby incorporated herein by reference in their entirety.


TECHNICAL FIELD

The present invention relates to electromagnetic (EM) surveying, in particular for seabed logging, and is concerned with providing a system for determining the resistivity or conductivity of subsea strata, especially the upper strata, e.g. at a depth of up to about 100 m or even greater.


BACKGROUND ART

Currently, the most widely used techniques for geological surveying, particularly in sub-marine situations, are seismic methods. These seismic techniques are capable of revealing the structure of the subterranean strata with some accuracy. However, whereas a seismic survey can reveal the location and shape of a potential reservoir, it can normally not reveal the nature of the reservoir.


It has been appreciated by the present applicants that while the seismic properties of hydrocarbon filled strata and water-filled strata do not differ significantly, their electromagnetic resistivities do differ. Thus, by using an electromagnetic surveying method, these differences can be exploited and the success rate in predicting the nature of a reservoir can be increased significantly.


Consequently, a method and apparatus embodying these principals form the basis of the present applicants' European Patent EP-A-1256019.


This contemplates a method for searching for a hydrocarbon containing subterranean reservoir which comprises: applying a time varying electromagnetic field to subterranean strata; detecting the electromagnetic wave field response; seeking, in the wave field response, a component representing a refracted or ducted wave; and determining the presence and/or nature of any reservoir identified based on the presence or absence of a wave component refracted or ducted by hydrocarbon layer.


A ducted wave behaves differently, depending on the nature of the stratum in which it is propagated. In particular, the propagation losses in hydrocarbon stratum are much lower than in a water-bearing stratum while the speed of propagation is much higher. Thus, when a hydrocarbon-bearing reservoir is present, and an EM field is applied, a strong and rapidly propagated ducted wave can be detected. This may therefore indicate the presence of the reservoir or its nature if its presence is already known.


Electromagnetic surveying techniques in themselves are known. However, they are not widely used in practice. In general, the reservoirs of interest are about 1 km or more below the seabed. In order to carry out electromagnetic surveying as a stand alone technique in these conditions, with any reasonable degree of resolution, short wavelengths are necessary. Unfortunately, such short wavelengths suffer from very high attenuation. Long wavelengths do not provide adequate resolution. For these reasons, seismic techniques are preferred.


However, while longer wavelengths applied by electromagnetic techniques cannot provide sufficient information to produce an accurate indication of the boundaries of the various strata, if the geological structure is already known, they can be used to determine the nature of a particular identified formation, if the possibilities for the nature of that formation have significantly differing electromagnetic characteristics. The resolution is not particularly important and so longer wavelengths which do not suffer from excessive attenuation can be employed.


The resistivity of seawater is about 0.3 ohm-m and that of the overburden beneath the seabed would typically be from 0.3 to 4 ohm-m, for example about 2 ohm-m. However, the resistivity of an oil reservoir is likely to be about 20-300 ohm-m. This large difference can be exploited using EM surveying techniques. Typically, the resistivity of a hydrocarbon-bearing formation will be 20 to 300 times greater than water-bearing formation.


Thus, an EM source such as an electric dipole transmitter antenna on or close to the sea floor induces (EM) fields and currents in the sea water and in the subsurface strata. In the sea water, the EM-fields are strongly attenuated due to the high conductivity in the saline environment, whereas the subsurface strata with less conductivity potentially can act as a guide for the EM-fields (less attenuation). If the frequency is low enough (in the order of 1 Hz), the EM-waves are able to penetrate deep into the subsurface, and deeply buried geological layers having higher electrical resistivity than the overburden (as e.g. a hydrocarbon filled reservoir) will affect the EM-waves. Depending on the angle of incidence and state of polarisation, an EM wave incident upon a high resistive layer may excite a ducted (guided) wave mode in the layer. The ducted mode is propagated laterally along the layer and leaks energy back to the overburden and receivers positioned on the sea floor. The terms “refracted” and “ducted” are used in this specification to refer to this wave mode.


In seabed logging, a signal is emitted from a towed source antenna, parallel to, and close to the sea floor; and the wavefield response detected by a number of stationary receivers, distributed on the sea floor, is recorded. The strength of the emitted signal is proportional to the current IA delivered to the antenna, and this current is accurately monitored and recorded. [If the current source is very stable, the current would, of course be constant.]


SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus and method for detecting and/or determining the nature of a subterranean reservoir.


According to one aspect of the invention, there is provided a method of determining the nature of a subterranean stratum which comprises: deploying an electric dipole transmitter antenna, preferably with its axis generally horizontal; applying an electromagnetic (EM) field to the stratum containing the reservoir using the transmitter; measuring the current and voltage at the antenna terminals during the EM transmission; and determining the nature of the stratum from changes in the measured voltage.


Changes in the nature of subterranean strata cause changes in the resistivity of the seabed as a whole beneath the sea floor. Since the resistivity of the source, antenna and sea water should remain constant, these changes in seabed resistivity alone may cause the voltage at the antenna terminals to vary and so the variations in antenna voltage will be representative of the of the changes in the nature of the strata in the seabed.


Thus, analysis of the logged receiver data may reveal the presence of a fast, low attenuation guided wave, and thus the presence of a high resistivity stratum which may be oil bearing.


High resistivity may, however, occur, not in oil bearing strata only, but also in strata consisting of e.g. solid salt or rock containing little or no water. In order to determine the nature of the high resistivity stratum, it is generally necessary to carry out a detailed analysis of the logged receiver data, in order to create a model, based on the seismic data, in which resistivity values are assigned to the various strata. From the geological knowledge of the region in question, the most likely nature of a high resistivity stratum may then be ascertained.


The accuracy of the model is limited by the receiver data, and may be improved by utilising other available information. One means of obtaining such additional information is by monitoring the input impedance of the towed transmitter antenna ZA.=VA/IA, where IA is the antenna current and IA the terminal voltage.


The input impedance of the towed transmitter antenna is determined by the following parameters:


1. The configuration of the antenna.


2. The conductivity of the sea water


3. The position and orientation of the antenna with respect to the sea floor


4. The topography of the sea floor (plane or otherwise).


5. The resistivity distribution below the sea floor.


Parameters 1-4 may be separately monitored and accounted for, and the remaining variations of ZA provide information about the resistivity distribution below the sea floor.


In one embodiment, the transmitter is located on or close to the seabed or the bed of some other area of water. Preferably, the frequency of the EM field is continuously varied over the transmission period. Preferably, the field is transmitted for a period of time for 3 seconds to 60 minutes, for example, from 10 seconds to 5 minutes. Preferably, the method is repeated at different locations.


In a more preferred embodiment, the transmitter is towed over the seabed while the EM field is being transmitted. The transmitter is preferably towed as close to the sea floor as possible. Preferably, the distance to the sea floor should be much less than the length of the antenna (e.g. 20 m to 200 m), and much less than the wavelength of the frequencies to be used.


Preferably, the transmitter includes an inertia sensor to sense the difference between a change in the sea floor height and a rise/fall of the transmitter.


In addition, the transmitted signal shape may be modified so that it contains more of the harmonics which are useful for mapping the conductivity. It will be understood that calculation of the conductivity of the upper strata of the seabed as a function of position and depth is also desirable.


Preferably, the method of the invention is carried out during a conventional EM survey using receivers to detect the transmission wavefield response. The method may also be used in conjunction with seismic surveying techniques.


The invention extends to a method for detecting different subterranean strata and is particularly applicable to the detection and identification of hydrocarbon bearing strata.


Preferably, the wavelength of the transmission is given by the formula

0.01h≦λ≦30h; or more preferably,
0.1h≦λ≦10h

wherein λ is the wavelength of the transmission through the subsea strata and h is the distance from the seabed to the strata under investigation. Preferably, the transmission frequency is from 0.01 Hz to 1 kHz, for example, from 0.1 to 20 Hz.


The invention extends to a process for the production of a survey report by carrying out a method according to the invention, and a survey report produced in this way.





BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be carried into practice in various ways and an embodiment will now be described by way of example with reference to the accompanying drawings, in which:



FIG. 1 is a schematic diagram depictive how the invention may be carried out in practice.





DETAILED DESCRIPTION OF THE DRAWINGS


FIG. 1 depicts an antenna 11 towed by a vessel 12 at a distance h above the sea floor 13. The antenna 11 emits a wavefield, whose strength is proportional to the current IA delivered to the antenna 11, and this is accurately monitored and recorded. By also monitoring and recording the voltage VA at the antenna terminals the impedance of the antenna Z. VA/IA, may be calculated. ZA is a function of the frequency, f, and when a multifrequency signal is employed, this function may be found within a range of frequencies extending from f=0 to a maximum frequency fmax, determined by the frequency spectrum of the signal, and the accuracy of the voltage and current measurements. By means of the impedance function, the following important parameters may be calculated, namely, the distance from the antenna to the sea floor, the conductivity of the sea water, and the step in conductivity at the sea floor, and possibly more detail of the conductivity variation immediately below the sea floor.


Part of the emitted signal is reflected at the sea floor, the reflection coefficient being









ρ
=



σ
1

-

σ
2




σ
1

-

σ
2







(
1
)








where σ1 and σ2 are the conductivities above and below the sea floor 13, respectively. The reflected signal induces a voltage VR in the antenna, thereby altering the voltage and the current at the antenna terminals, thus ultimately causing a change in the antenna impedance. The amplitude and phase of VA vary with the frequency f, and the distance h between the antenna and the sea floor. At sufficiently high frequencies, VA is negligible, and the impedance depends only on the frequency, and on the conductivity σ1, which may then be calculated.


Assuming σ2 constant, the reflected signal may be found from the simple model indicated in FIG. 1, as emitted from an image antenna 14 located the distance h below the (plane) sea floor, with σ2=σ1. The current of the image antenna is ρIA.

Claims
  • 1. A method of determining the nature of a subterranean stratum which comprises the steps: deploying an electric dipole transmitter antenna;applying an electromagnetic (EM) field to the stratum using the transmitter;measuring the current and voltage at the transmitter antenna terminals during the EM transmission;calculating the input impedance of the transmitter antenna from the measured current and voltage;using changes in the impedance to calculate the resistivity of the stratum; anddetermining the nature of the stratum from the resistivity.
  • 2. A method according to claim 1, in which the resistivity is combined with existing geological knowledge of the stratum to determine the nature of the stratum.
  • 3. A method according to claim 2, in which the existing geological knowledge comprises seismic data.
  • 4. A method according to claim 1, in which the conductivity of the sea water is monitored and changes in sea water conductivity are taken into account in determining the antenna input impedance.
  • 5. A method according to claim 1, in which the height of the antenna above the sea floor is monitored and changes in the antenna height are taken into account in determining the antenna input impedance.
  • 6. A method according to claim 1, in which the transmitter antenna is deployed with its axis generally horizontal.
  • 7. A method according to claim 1, in which the transmitter is towed over the seabed while the EM field is being transmitted.
  • 8. A method according to claim 1, in which the transmitter is towed at a distance between 20 m and 200 m from the seabed.
  • 9. A method according to claim 1, in which the transmitter is towed at a distance from the seabed which is significantly less than the wave length of the frequencies of the EM transmitter.
  • 10. A method according to claim 1, in which the transmitted signal shape contains harmonics which are used in the determination of the antenna input impedance.
  • 11. A method according to claim 1, in which the wave length of the EM transmitter is given by the formula 0.01h≦λ≦30h,wherein λ is the wave length of the transmission through the subsea strata and h is the distance from the seabed to the strata under investigation.
  • 12. A method according to claim 1, in which the transmission frequency is from 0.01 Hz to 1 kHz.
  • 13. A process for the production of a survey report which comprises carrying out a method as claimed in claim 1, and formulating the voltage changes together with the related conclusions into a report.
  • 14. A process for the production of a survey report which comprises carrying out a method as claimed in of claims 1 and formulating the impedance changes together with the related conclusions into a report.
Priority Claims (1)
Number Date Country Kind
0602656.1 Feb 2006 GB national
PCT Information
Filing Document Filing Date Country Kind 371c Date
PCT/GB2007/000459 2/9/2007 WO 00 12/18/2008
Publishing Document Publishing Date Country Kind
WO2007/091086 8/16/2007 WO A
US Referenced Citations (232)
Number Name Date Kind
1818331 Jakosky Aug 1931 A
2077707 Melton Apr 1937 A
2139460 Potapenko Dec 1938 A
2268106 Blau Dec 1941 A
2426918 Barret Sep 1947 A
2531088 Thompson Nov 1950 A
2766422 Carbonetto Oct 1956 A
2907389 Hitzman Oct 1959 A
2919397 Morley Dec 1959 A
2953742 Herbold Sep 1960 A
3052836 Postma Sep 1962 A
3105014 Harrison Sep 1963 A
3113316 Berry Dec 1963 A
3114875 Haagensen Dec 1963 A
3182250 Conti et al. May 1965 A
3332487 Jones Jul 1967 A
3398356 Still Aug 1968 A
3548299 Duroux et al. Dec 1970 A
3763419 Barringer Oct 1973 A
3806795 Morey Apr 1974 A
3819246 List Jun 1974 A
3836960 Gehman et al. Sep 1974 A
3959721 Roschuk et al. May 1976 A
4010413 Daniel Mar 1977 A
4041372 Miller et al. Aug 1977 A
4047098 Duroux Sep 1977 A
4077686 Bukaitz Mar 1978 A
4079309 Seeley Mar 1978 A
4094304 Wright, Jr. Jun 1978 A
4168484 Wright, Jr. Sep 1979 A
4218678 Fowler et al. Aug 1980 A
4229809 Schwalbe Oct 1980 A
4245191 Schroeder Jan 1981 A
4258321 Neale, Jr. Mar 1981 A
4258322 Rocroi et al. Mar 1981 A
4283099 Burton Aug 1981 A
4296379 Yoshizumi Oct 1981 A
4298840 Bischoff et al. Nov 1981 A
4308499 Thierbach et al. Dec 1981 A
4396885 Constant Aug 1983 A
4417210 Rocroi et al. Nov 1983 A
4446434 Sternberg et al. May 1984 A
4451789 Meador May 1984 A
4456067 Pinner Jun 1984 A
4489276 Yu Dec 1984 A
4492924 Nilsson Jan 1985 A
4494805 Washburn Jan 1985 A
4500146 Peterson Feb 1985 A
4504833 Fowler et al. Mar 1985 A
4506225 Loveless et al. Mar 1985 A
4516226 Peynaud et al. May 1985 A
4547733 Thoraval Oct 1985 A
4583095 Peterson Apr 1986 A
4594551 Cox et al. Jun 1986 A
4616184 Lee et al. Oct 1986 A
4617518 Srnka Oct 1986 A
4633182 Dzwinel Dec 1986 A
4652829 Safinya Mar 1987 A
4660645 Newlove Apr 1987 A
4677438 Michiguchi et al. Jun 1987 A
4698791 Cunningham Oct 1987 A
4760340 Denzau et al. Jul 1988 A
4791998 Hempkins et al. Dec 1988 A
4793664 Jackson Dec 1988 A
4835473 Bostick, Jr. May 1989 A
4835474 Parra et al. May 1989 A
4906575 Silver et al. Mar 1990 A
4926394 Doyen May 1990 A
4957172 Patton et al. Sep 1990 A
4986354 Cantu Jan 1991 A
4992995 Favret Feb 1991 A
5025218 Ramstedt Jun 1991 A
5032794 Ridd et al. Jul 1991 A
5039179 Chouzenoux Aug 1991 A
5043667 Schofield Aug 1991 A
5066916 Rau Nov 1991 A
3990123 Martinez Apr 1992 A
5103920 Patton et al. Apr 1992 A
5170440 Cox Dec 1992 A
5172480 Labuc et al. Dec 1992 A
5177445 Cross Jan 1993 A
5185578 Stolarczykz Feb 1993 A
5192952 Johler Mar 1993 A
5219386 Kletzmaier et al. Jun 1993 A
5280284 Johler Jan 1994 A
5357253 Van Etten et al. Oct 1994 A
5369892 Dhaemers Dec 1994 A
5373443 Lee et al. Dec 1994 A
5400030 Duren et al. Mar 1995 A
D359403 Tisbo et al. Jun 1995 S
5421646 McNamara et al. Jun 1995 A
D361446 Acard Aug 1995 S
5444619 Hoskins Aug 1995 A
H1490 Thompson et al. Sep 1995 H
H1490 Thompson et al. Sep 1995 H
D362767 Kelly et al. Oct 1995 S
D366376 Tisbo et al. Jan 1996 S
D366577 Tisbo et al. Jan 1996 S
5486764 Thompson et al. Jan 1996 A
H1524 Thompson et al. Apr 1996 H
H1561 Thompson Jul 1996 H
5563513 Tasei et al. Oct 1996 A
5564806 Keisling et al. Oct 1996 A
5581024 Meyer, Jr. et al. Dec 1996 A
5689068 Locatelli et al. Nov 1997 A
5690401 Franklin Nov 1997 A
5704142 Stump Jan 1998 A
5724309 Higgs et al. Mar 1998 A
5767679 Schroder Jun 1998 A
5770945 Constable Jun 1998 A
5777478 Jackson Jul 1998 A
5798892 Kobayashi et al. Aug 1998 A
5811973 Meyer, Jr. Sep 1998 A
5825188 Montgomery et al. Oct 1998 A
5838634 Jones Nov 1998 A
5841280 Yu et al. Nov 1998 A
5877995 Thompson et al. Mar 1999 A
5886526 Wu Mar 1999 A
5892361 Meyer, Jr. et al. Apr 1999 A
5894450 Schmidt et al. Apr 1999 A
5901795 Tsao et al. May 1999 A
5905380 Weiner et al. May 1999 A
5905657 Celniker May 1999 A
5940777 Keskes Aug 1999 A
5955884 Payton et al. Sep 1999 A
5987388 Crawford et al. Nov 1999 A
5996414 Mercado Dec 1999 A
6002357 Redfern et al. Dec 1999 A
6005916 Johnson et al. Dec 1999 A
6011557 Keskes et al. Jan 2000 A
6023168 Minerbo Feb 2000 A
6026913 Mandal et al. Feb 2000 A
6041018 Roche Mar 2000 A
6049760 Scott Apr 2000 A
6060884 Meyer, Jr. et al. May 2000 A
6060885 Tabarovsky et al. May 2000 A
6061412 Stucker et al. May 2000 A
6087833 Jackson Jul 2000 A
6101448 Ikelle et al. Aug 2000 A
6108605 Doyle et al. Aug 2000 A
6114972 Smith Sep 2000 A
6140819 Peterman et al. Oct 2000 A
6157195 Vail, III Dec 2000 A
6163155 Bittar Dec 2000 A
6181138 Hagiwara et al. Jan 2001 B1
6184685 Paulk et al. Feb 2001 B1
6188221 Van de Kop et al. Feb 2001 B1
6188222 Seydoux et al. Feb 2001 B1
6225806 Millar et al. May 2001 B1
6236211 Wynn May 2001 B1
6236212 Wynn May 2001 B1
6246240 Vail, III Jun 2001 B1
6294917 Nichols Sep 2001 B1
6339333 Kuo Jan 2002 B1
6389360 Alft et al. May 2002 B1
6415231 Hebert Jul 2002 B1
6424918 Jorgensen et al. Jul 2002 B1
6450599 Mamuyac Sep 2002 B1
6480000 Kong et al. Nov 2002 B1
6499817 Jermain Dec 2002 B2
6573855 Hayakawa et al. Jun 2003 B1
6628119 Eidesmo et al. Sep 2003 B1
6662147 Fournier et al. Dec 2003 B1
6696839 Ellingsrud et al. Feb 2004 B2
6700526 Witten Mar 2004 B2
6717411 Ellingsrud et al. Apr 2004 B2
6826483 Anderson et al. Nov 2004 B1
6834732 Haarstad Dec 2004 B2
6842006 Conti et al. Jan 2005 B2
6859038 Ellingsrud et al. Feb 2005 B2
6864684 Ellingsrud et al. Mar 2005 B2
6900639 Ellingsrud et al. May 2005 B2
6917564 Leaney Jul 2005 B2
6950786 Sonneland et al. Sep 2005 B1
6950790 Nichols Sep 2005 B1
7026819 Eidesmo et al. Apr 2006 B2
7028806 Dubinsky et al. Apr 2006 B2
7032707 Egerev et al. Apr 2006 B2
7038456 Ellingsrud et al. May 2006 B2
7038850 Chang et al. May 2006 B2
7042802 Sinha May 2006 B2
7072767 Routh et al. Jul 2006 B2
7095357 Johler Aug 2006 B1
7106065 Graf Sep 2006 B1
7123543 Vaage et al. Oct 2006 B2
7126338 MacGregor et al. Oct 2006 B2
7145341 Ellingsrud et al. Dec 2006 B2
7149672 Torkildsen Dec 2006 B2
7202669 Ellingsrud et al. Apr 2007 B2
7319330 Amundsen Jan 2008 B2
7423432 Amundsen Sep 2008 B2
7471089 Zerilli et al. Dec 2008 B2
7613570 Rosten et al. Nov 2009 B2
7664603 Rosten Feb 2010 B2
7919965 Schaug-Pettersen et al. Apr 2011 B2
8086426 El Ouair et al. Dec 2011 B2
20030048105 Ellingsrud et al. Mar 2003 A1
20030050759 Srnka et al. Mar 2003 A1
20030052685 Ellingsrud et al. Mar 2003 A1
20040000912 Conti et al. Jan 2004 A1
20040003511 Silver Jan 2004 A1
20040017137 Straub Jan 2004 A1
20040027103 Boylan Feb 2004 A1
20040046568 Unsworth et al. Mar 2004 A1
20040090234 Macune May 2004 A1
20040176910 Ellingsrud et al. Sep 2004 A1
20050035696 Parise et al. Feb 2005 A1
20050040742 Tsai Feb 2005 A1
20050264294 Constable Dec 2005 A1
20060005965 Chouzenoux et al. Jan 2006 A1
20060038570 Constable Feb 2006 A1
20060091889 Ellingsrud et al. May 2006 A1
20060103387 Amundsen May 2006 A1
20060202697 Sodal Sep 2006 A1
20070145980 Conti et al. Jun 2007 A1
20070150201 Eidesmo et al. Jun 2007 A1
20070294036 Strack et al. Dec 2007 A1
20080065330 Rosten et al. Mar 2008 A1
20080122444 Schaug-Pettersen et al. May 2008 A1
20080169817 Morrison et al. Jul 2008 A1
20080189043 Anno et al. Aug 2008 A1
20080195358 El Ouair et al. Aug 2008 A1
20080221795 Amundsen et al. Sep 2008 A1
20090134877 Schaug-Pettersen May 2009 A1
20090195251 Darnet et al. Aug 2009 A1
20090243617 Farrelly et al. Oct 2009 A1
20090265111 Helwig et al. Oct 2009 A1
20090267608 Johnstad et al. Oct 2009 A1
20090271118 Saltzer et al. Oct 2009 A1
20100045295 Mittet et al. Feb 2010 A1
20100057363 Amundsen Mar 2010 A1
20100061187 Sodal Mar 2010 A1
Foreign Referenced Citations (85)
Number Date Country
0 087 271 Aug 1983 EP
0215695 Mar 1987 EP
0 219 234 Apr 1987 EP
0368762 May 1990 EP
0481886 Apr 1992 EP
0 512 756 Nov 1992 EP
0568612 Mar 1996 EP
1094338 Apr 2001 EP
1253437 Oct 2002 EP
1253443 Oct 2002 EP
1255126 Nov 2002 EP
1512033 Dec 2003 EP
1 309 887 Mar 2004 EP
1 256 019 Jun 2004 EP
0 814 349 Nov 2006 EP
2 479 992 Oct 1981 FR
2586302 Feb 1987 FR
188676 Dec 1922 GB
1342475 Jan 1974 GB
1588495 Apr 1981 GB
2089043 Jun 1982 GB
2 155 182 Nov 1984 GB
2 155 182 Sep 1985 GB
2197078 May 1988 GB
2197952 Jun 1988 GB
220070 Dec 1989 GB
2220071 Dec 1989 GB
2 231 602 Nov 1990 GB
2 256 715 Dec 1992 GB
2296567 Jul 1996 GB
2 301 902 Dec 1996 GB
2323443 Sep 1998 GB
2333364 Jul 1999 GB
2 355 538 Apr 2001 GB
2382875 Jul 2001 GB
2 34 390 Jan 2002 GB
2384068 Jul 2003 GB
2385923 Sep 2003 GB
2390904 Jan 2004 GB
2391946 Feb 2004 GB
2395563 May 2004 GB
2399640 Sep 2004 GB
2404444 Feb 2005 GB
2411006 Aug 2005 GB
2413187 Oct 2005 GB
2413188 Oct 2005 GB
2415511 Dec 2005 GB
2421800 Jul 2006 GB
2002 244580 Aug 2002 JP
2006145360 Jun 2006 JP
WO 8101617 Jun 1981 WO
WO 8910463 Nov 1989 WO
WO-9000749 Jan 1990 WO
WO 9213172 Aug 1992 WO
WO 9215771 Sep 1992 WO
WO-9215900 Sep 1992 WO
WO 9420864 Sep 1994 WO
WO 9606367 Feb 1996 WO
WO 9633426 Oct 1996 WO
WO 9635859 Nov 1996 WO
WO 9733184 Sep 1997 WO
WO 9811455 Mar 1998 WO
WO 9828638 Jul 1998 WO
WO 9913966 Mar 1999 WO
WO 0000850 Jan 2000 WO
WO 0013037 Mar 2000 WO
WO 0013046 Mar 2000 WO
WO 0054075 Sep 2000 WO
WO 0063718 Oct 2000 WO
WO 0120366 Mar 2001 WO
WO 0133481 May 2001 WO
WO 0142818 Jun 2001 WO
WO 0155749 Aug 2001 WO
WO 0157555 Aug 2001 WO
WO 0214906 Feb 2002 WO
WO 03034096 Apr 2003 WO
WO 03036331 May 2003 WO
WO 03042718 May 2003 WO
WO 03048737 Jun 2003 WO
WO 03-048812 Jun 2003 WO
WO 03048812 Jun 2003 WO
WO 03100467 Dec 2003 WO
WO 03104844 Dec 2003 WO
WO 2004053528 Jun 2004 WO
WO 2007018810 Feb 2007 WO
Related Publications (1)
Number Date Country
20100052688 A1 Mar 2010 US