The invention relates to resistivity imaging and, more particularly, to such imaging as it is carried out in connection with geological surveys.

In WO 91/17437 there are disclosed methods and apparatus for assessing geophysical characteristics by measuring resistivity of a core sample. First and second arrays of electrodes are applied e.g. at opposite ends of the sample and current passed between them in controlled manner. A further array of electrodes is applied to the sample to measure the potential at different points on the sample due to the current passing between the current electrodes. From this is inferred the resistivity of the sample at different positions and this can be interpreted in terms of the composition of the sample.

In WO 91/06854 there are disclosed methods and apparatus for use in assessing the physical state of ground materials. Electrodes are positioned about a survey site some of which are selected as current electrodes and controlled current passed between them, others selected as potential electrodes for measurement of the potential at various positions with respect to another position or other positions. As in the core sample measurements of WO 91/17437, the resistivity of the ground is inferred at different positions in the survey site and this can be interpreted in terms of the geophysical features of the site.

The methods and apparatus described in WO 91/17437 and WO 91/06854 are developments of long established techniques for examining core samples and making ground surveys.

Certain problems arise in connection with the interpretation of the measurements made by such methods and apparatus, principally due to the presence of hidden, anomalous features in the sample or the survey site.

The present invention provides improved methods and apparatus that overcome such problems and in fact reveal such anomalous features.

The invention comprises a method for resistivity imaging in a possibly heterogeneous medium comprising distributing electrodes in an array in a measuring region of the medium and making a map of the region showing the positions of the electrodes, selecting electrodes as current electrodes and measuring electrodes and passing current between the current electrodes and measuring, using the measuring electrodes, the potential field generated in the region by the passing of the current between the current electrodes, calculating and plotting on the map the direction of the current flow at each measuring electrode and/or at intermediate points whereby to produce a map of current flow in the region, evaluating from the map the actual current density at a location in the region, evaluating a homogeneous current density based on an assumption that the medium is homogeneous throughout the region, calculating a first apparent resistivity of the medium at said location on the assumption of a homogeneous medium and calculating a corrected apparent resistivity by multiplying the said first apparent resistivity by the ratio of homogeneous to actual current density at said location.

The actual current density may be calculated by counting the number of current flow lines per unit area of said location.

The map may be made by data processing equipment and held as data values in memory. The evaluations and calculations may be effected by programming in a data processor.

The actual current density as evaluated may be displayed as a set of current flow lines on the map of the region.

The method may be applied to a medium comprising a core sample, measurement of the core sample being carried out for example as described in WO 91/17437, or it may be applied to assessing the physical state of ground materials by a field survey such as may be carried out as described in WO 91/06854.

The invention also comprises apparatus for resistivity imaging in a possibly heterogenous medium comprising a plurality of electrodes adapted to be distributed in an array in a measuring region of the medium, mapping means for a map of the region showing the positions of the electrodes, current means for passing electric current between electrodes selected as current electrodes, and potential measuring means for measuring the potential between electrodes selected as measurement electrodes, calculating and plotting means for calculating and plotting on the map the direction of the current flow at each measuring electrode and/or at intermediate points whereby to produce a map of current flow in the region, and for evaluating from the map the actual current density at a location in the region and a homogeneous current density based on an assumption that the medium is homogeneous throughout the region, and for calculating a first apparent resistivity based on an assumption that the medium is homogeneous throughout the region and a corrected apparent resistivity by multiplying the said first apparent resistivity by the ratio of homogeneous to actual

current density at said location, and display means for displaying said corrected apparent resistivity.

Said mapping means may comprise memory means holding map information as data values, and the evaluating and plotting means may comprise a programmed data processor.

Said display means - which may comprise a video display unit or an X-Y plotter - may be adapted to display actual current density as a set of current flow lines on a map of the region. The electrodes may, in the case of core sample examination, be provided as described in WO 91/17437 or, in the case of site survey, as described in WO 91/06854.

Embodiments of methods and apparatus for resistivity imaging in a possibly heterogeneous medium will now be described with reference to the accompanying drawings, in which :

Figure 1 is a map current showing flow lines between two current electrodes in a homogeneous medium;

Figure 2 is a map showing current flow lines between two current electrodes in a heterogeneous medium;

Figure 3 is a map of a measuring region showing the calculation of electric field lines;

Figure 4 is a diagrammatic section through a heterogeneous medium, showing electrode distribution and the measurement apparatus;

Figure 5 is a resistivity map derived from measurements using prior art techniques;

Figure 6 is a map showing current flow lines assumed from potential measurements on the basis that the medium contains no hidden anomaly;

Figure 7 is a map showing actual current flow lines calculated from the potential field determined as illustrated in Figure 3; and

Figure 8 is a resistivity map produced from the flow line map of Figure 7.

The drawings illustrate a method and apparatus for resistivity imaging in a possibly heterogenous medium. The apparatus is essentially illustrated in Figure 4 and comprises electrodes lie, 11m distributed in an array in a measuring region 12 of the medium.

The medium in this case can be assumed to be a core sample with electrodes lie selected as current electrodes arrayed on both ends of the sample and electrodes 11m selected as measurement electrodes arrayed on the cylindrical surface of the sample.

The set-up is similar to that described in WO 91/17437 with a current source 13 and a switching arrangement 14 which can switch current between selected pairs of the electrodes lie and control the current flow so as for example to be the same for each selected pair of electrodes lie. The measurement electrodes 11m are connected via a multiplexer 15, a signal processor 16, amplifier 17 and A D converter 17 to a data processor 18 with memory 19 and video display unit and/or X-Y plotter 21.

Figure 1 illustrates current flow between two electrodes lie in a homogeneous medium. Current flows along lines C which are in fact field lines indicating the direction of the electric field produced by the potentials on the electrodes lie. The spacing of the field lines C is indicative of the current density.

Figure 2 illustrates the situation when the medium is not homogeneous, but comprises a region R- of low resistance with regions R ₂ of high resistance. Current flows more easily through the low resistance region R, and the current or field lines C are closer together there.

It should at this point be noted that Figures 1 and 2 are not specific to any particular type of medium and could as easily represent core sample media as a ground survey site, and it is also important to note that in the latter case the diagram could be a plan section or a vertical section, in which information about different depths depends upon taking measurements between electrode pairs with different surface spacing rather than using electrodes penetrating to different depths - although additional information from the latter would clearly be useful, it is not in general essential, as, for the most part, depth information is readily gleaned from surface measurements, and more especially using the techniques of the present invention.

Figure 3 is a map of part of a region of medium in which a current is flowing as a result of the presence of current electrodes outside the region. At a point pi of the region, measurements from the measurement electrodes indicate an electric field E which resolves into components Ex,Ey in the x,y directions. The Ex,Ey components might be measured for example by measurement electrodes at mesh locations Pι,p ₂ ,p ₃ or otherwise, the resultant direction θ given by tan ^"1 (Ex/Ey) being the field direction and hence the direction of current flow. A current line segment length appropriate to the degree of accuracy which is required is chosen and the field line plotted along direction θ for the next length of segment. The process is repeated for the next segment, and so on.

In this way is built up a map of the electric field in the medium.

In Figure 4, the heterogeneous medium there depicted includes regions R, of low resistance, regions R ₂ of high resistance. Two of the R, regions are not manifest on the surface of the sample and are therefore hidden to conventional surface measurements.

A current flow map made by conventional techniques is illustrated in Figure 5, and comprises essentially parallel current flow/field lines. By plotting the actual current lines as explained above, however, a map is obtained as illustrated in Figure 6, and it will at once be apparent that there is considerable correspondence between the two figures.

Maximum information is obtained from maps of resistivity, rather than current flow. The conventionally obtained resistivity map is shown in Figure 7, the corrected map

produced according to the techniques of the present invention in Figure 8. The resistivity at each point is calculated according to the relationship.

Resistivity (new) = Resistivity (old) x K

where K = Homogeneous current density Actual current density

The technique can be applied, as already remarked, equally well to core sample investigations as to field surveys. In either case the improvements over conventional techniques by way of inexpensive sample preparation in the case of core sampling, and by way of ease and flexibility of measurement in either case that are attributable to WO 91/17437 and WO 91/06853 are still fully available.

The calculations and evaluations involved can be built into programming of the computational equipment already provided for in those earlier patent applications.