T 0869/90 (Electromagnetic borehole logging / TEXACO) 01-07-1992

1. The only issue which is to be considered in the present appeal is the question of inventive step.

1.1. In the Board's view, the prior art coming closest to the claimed invention is the logging apparatus as disclosed in D1, for determining the dielectric constant and/or resistivity of earth formations surrounding a borehole. The apparatus as described with reference to Figure 12 at column 16, line 51 to column 19, line 47 comprises a sonde which carries a transmitter T and four receivers R1,R2,R3 and R4 located at different distances from the transmitter. The electrical circuitry (290,390,490) illustrated is for the generalised case so that means (e.g. ratio circuit 226 and difference amplifier 224) are provided whereby amplitude ratio, that is, attenuation, and phase difference measurements are available for each of the three receiver pairs R1,R2; R2,R3 and R3,R4. Typically, attenuation measurement, Atten1,2 between receiver pair R1,R2, and phase difference measurement 3,4 between receiver pairs R3,R4 are utilised by a computing module (100) to determine the dielectric constant and the resistivity. Nonetheless, when the amplitude signal either from R3 or R4 is weak and exceeds a predetermined threshold, a phase difference measurement 2,3, and not 3,4, is combined with Atten1,2 to derive the above-mentioned parameters (see, column 19, lines 28 to 47).

In view of the above, the Board concurs with the Appellant that although the method of determining dielectric constant and resistivity described in D1 utilises signals only from three receivers in the event that the amplitude signal from the receiver R4 is weak, the apparatus disclosed is provided with four receiver coils and, as mentioned earlier, with ratio circuits (e.g. 226) and difference amplifiers (e.g. 224) for deriving attenuation and phase difference for each of the three receiver pairs R1,R2; R2,R3 and R3,R4.

1.2. The apparatus according to the invention as claimed thus differs from the above prior art only in that it is provided with only three receiver coils.

In this connection, the Appellant submitted that according to the wording of the claim, the apparatus as claimed is intended for determining dielectric constant and/or resistivity of an earth formation which is subject to invasion of fluid from the borehole; the apparatus and method disclosed in D1, on the other hand, are concerned with investigating an earth formation which is not invaded by such a fluid, so that the receivers are necessarily located further away from the transmitter, so as to receive signals from the deeper "non-invaded" earth formation, than is the case in the apparatus of the present invention. The Board, however, is of the view that the intended use of the apparatus according to the invention does not uniquely define the transmitter- receiver spacing, since the area or the diameter of the invaded zone to be investigated can vary considerably so that the distance between the transmitter and each of the receivers would also vary accordingly.

1.3. In the application in suit (see page 4, lines 5 to 20 and Figure 3) and in the Statement of Grounds of Appeal, the technical advantage of the apparatus according to the present invention is discussed in relation to a prior art logging apparatus, as disclosed in US-A-4 107 598 (D2), using only two receiver coils. The Appellant argued on the basis of this technical advantage that whereas the present invention seeks to provide higher accuracy in the determination of the dielectric constant, the technical problem addressed by D1 is to determine dielectric constant etc. with assurance that the measurements used for such determination are "looking" at substantially the same formations, i.e. having substantially the same depth. The Appellant further submitted that the problem addressed in D1 was therefore not the same as the one addressed by the present invention, so that in an unpredictable art such as the present one, a teaching of a solution to one problem cannot necessarily be expected to apply to solve an overlapping or neighbouring problem.

The Board accepts that the dielectric constant determined by the present invention is more accurate than that determined by the apparatus employing only two receiver coils. However, it is the established jurisprudence of the Boards of Appeal that for an objective assessment of inventive step, the technical problem to which the invention provides a solution is to be formulated in the light of the closest state of the art. In the present case, therefore, it is essential that the technical problem is formulated having regard to the disclosure in D1, and not the prior art document US-A-4 107 598 (D2) discussed in the application. Since in D1 the method of determining dielectric constant uses signals only from three receiver coils as in the present invention, it must be presumed to provide an accurate determination of the dielectric constant as well. This technical effect, that is, an accurate determination of the dielectric constant, is, therefore, to be left out of consideration while formulating the objective problem.

1.4. Having regard to the prior art apparatus described in D1, therefore, the objective problem confronting a skilled person can be regarded simply as providing a more compact apparatus for determining dielectric constant and/or resistivity of an earth formation, whereby considerable saving in space is achieved.

1.5. As mentioned earlier in paragraph 1.1 above, although in the embodiment of Figure 12 in D1, attenuation and phase difference measurements are available at each of three receiver pairs R1,R2; R2,R3 and R3,R4, when the amplitude signal from receiver R3 or receiver R4 is not sufficiently strong, the document teaches to use only the attenuation measurements at the receivers R1,R2 (Atten1,2) and the phase difference measurement ( 2,3) at the receivers R2,R3 for the calculation of dielectric constant or resistivity. Also it is suggested that in such a situation, portions of the circuitry providing the information, in other words the circuitry in channel 290, which is not utilised, may be omitted (see, in particular, column 17, lines 6 to 12). Thus, when the problem confronting a skilled person was to provide a compact logging apparatus for investigating an earth formation which was not deeply located, he would have regarded the omission of the receiver R4 as a logical extension of the teaching of D1.

1.6. For the foregoing reasons, in the Board's judgment, the subject-matter of Claim 1 lacks an inventive step within the meaning of Article 56 EPC.

1.7. The Appellant referred, inter alia, to the Technical Board of Appeal Decisions T 20/84, T 9/86 and T 109/87, and a Decision of the US Federal District Court in the case of Deering Milliken Research Corporation v. Beaunit Corporation (182 USPQ 421) to support his proposition that a seemingly simple solution may disguise an inventive step, and that there is a danger in assessing apparently simple solutions with hindsight. The Board accepts that an omission of a feature may, under certain circumstances, as illustrated by the above Technical Board of Appeal Decisions, involve an inventive step. However, each case has to be examined on its own merit taking into consideration the relevant technical facts, and ultimately the essential question which needs to be considered in each case is whether an omission of a feature is rendered obvious by the prior art having regard to the objectively formulated problem, as has been done in the present case.