WO1997010519A1

WO1997010519A1 - Locating a metal mass at a great depth

Info

Publication number: WO1997010519A1
Application number: PCT/FR1996/001385
Authority: WO
Inventors: Robert Gable
Original assignee: Bureau De Recherches Geologiques Et Minieres B.R.G.M.
Priority date: 1995-09-13
Filing date: 1996-09-10
Publication date: 1997-03-20
Also published as: AU6991196A; MA23971A1; FR2738644A1; ZA967683B; FR2738644B1

Abstract

A geophysical method for detecting a metal mass at a great depth in the ground is described, characterised in that it includes the steps of drilling a series of shallow holes, substantially at a depth of 50 to 200 meters, waiting for said holes to be in thermal equilibrium, taking at least one temperature measurement in said holes so as to establish the temperature at a selected depth, and establishing the field of temperatures at said selected depth in order to detect field anomalies characteristic of the presence of a metal mass at a great depth.

Description

LOCATION OF HIGH DEPTH METAL MASS

The present invention relates to a geophysical method for detecting metal clusters, such as in particular ores, which are buried deep in the ground.

The general evolution of mineral exploration in recent decades has been the use, increasingly intensive, of indirect exploration methods, mainly geophysical. This development is due to the significant reduction in surface deposits "easily" discovered and exploited intensively. The geophysical methods, currently the most used, are gravimetric and electromagnetic methods. These methods are based respectively on the measurement of the variations of the gravitational field and the terrestrial magnetic field, on a determined area of research, caused by the presence in this area of heaps of ores of high density and magnetic susceptibility. These methods have both advantages and disadvantages.

The gravimetric method, the most widely used, and relatively easy to implement, is a surface prospecting method which, however, in countries with marked relief, must be doubled by a precise leveling of the measurement stations which makes its implementation expensive work. In addition, the interpretation of the data presents an indeterminacy on the mass of the heap and on its depth, a large mass located at great depth can create the same gravimetric anomaly as a weaker mass located at shallower depth. Finally, this method does not allow the determination of the nature of the body having created the gravimetric anomaly to be resolved, insofar as there exist a large number of dense bodies underground (such as, for example, basic rocks) other than metallic ores. .

The electromagnetic method, for its part, is used in surface prospecting, or in drilling, and is of a relatively complex implementation. The the results it provides are moreover a delicate interpretation, insofar as the anomalies highlighted are highly dependent on the shape and orientation of the magnetic body relative to the direction of the Earth's magnetic field. This method therefore requires sophisticated data processing software

In conclusion, overall, the methods of the prior art do not make it possible to determine, on the one hand, whether the body causing the anomaly is or is not metallic and, on the other hand, whether said body occurs in large quantities. The present invention aims to remedy these drawbacks by proposing a detection method which simplifies the lifting of indeterminacy as to the nature of the body detected and as to its importance.

The present invention is based on the thermal effect called "chimney effect", according to which it has been found that there exists above a salt mass an anomaly in the distribution of temperatures due to the high thermal conductivity of this mass. The Applicant has found that it is possible by measuring, in a series of boreholes dug to a shallow depth ((of the order of 50 to 200 meters), distributed over a determined research area, the temperature existing at a given depth and by establishing a series of isotherms at this given depth, to highlight an anomaly in the temperature field which is characteristic of the presence of a thermally conductive mass and in particular of a metallic mass, buried at great depth.

According to the invention, the determination as to the nature of the conductive body detected is easily removed since it is known that the closest bodies, in terms of thermal conductivity, of metal clusters, namely salt clusters, have thermal conductivity ten times weaker than the latter. In addition, the lifting of indeterminacy is favored, insofar as we know that the salt clusters and the metallic clusters are not located in the same geological contexts.

The present invention thus relates to a geophysical method for detecting metal clusters buried deep in the ground, characterized in that it comprises the steps consisting in:

- carry out a series of shallow boreholes, substantially between 50 and 200 meters,

- wait for the thermal equilibrium of these boreholes,

- carry out, in these boreholes, at least one temperature measurement making it possible to establish the temperature existing at a determined depth,

- establish the temperature field at said determined depth, in order to detect the anomalies of this field characteristic of the presence of metal clusters at great depth.

The temperature measurements can be carried out either at said determined depth, or at any depth and, in the latter case, a processing of the temperatures taken is then carried out in order to determine the temperature at the determined depth. Of course, the temperature measurement can also be carried out continuously, and a processing of the values read (in particular of the interpolation type) will then be carried out, which will make it possible to establish the temperature at one or more determined depths.

The detection method according to the invention has the advantage of only calling for drilling shallow depths, generally of the order of a hundred meters, much less than the depth at which the metal cluster, which makes it interesting in terms of cost. The method according to the invention also has the advantage, compared to certain methods of the prior art, of not requiring, in order to carry out the measurements, to have to reseal the wellbore, which in particular makes it possible to be able to reuse this same well later. This process allows also to use existing boreholes, wells which after measurement can be used for other purposes.

Another advantage of the present invention is that it can be implemented by means of simple, non-cored boreholes, which are consequently a relatively low cost of implementation.

An embodiment of the present invention will be described below, by way of non-limiting example, with reference to the accompanying drawings in which

Figure 1 is a theoretical diagram according to the prior state of the art representing the rise of isotherms above salt masses. FIG. 2 is a plan view of a research area, in which a series of eighteen boreholes dug above a known metal mass has been made, and on which the isotherms constructed from measurements made in this series of drilling.

FIG. 3 is a theoretical diagram according to the invention combined with a schematic vertical section along a line of the research zone, consisting of four wells represented in FIG. 2, on which a series of isotherms constructed from measurements performed

FIG. 4 is a three-dimensional view of the temperature field of the research zone represented in FIG. 2 FIG. 5 is a plan view of this same research zone in which the measurements carried out on only nine boreholes have been kept

FIG. 6 is a three-dimensional view of the temperature field of the previous research zone established on the basis of the temperature measurements carried out in the nine boreholes. In Figure 1, there is shown a diagram showing a vertical section of a soil 2 inside which is disposed a salt mass 1. The diagram shown has for reference an abscissa axis representing the distance d in meters and an axis ordinates, directed downwards, representing the depth also in meters. We have drawn on this figure the different isotherms representing the temperature of the ground at different depths distributed between two hundred and eight hundred meters. It is noted that the presence in the soil 2 of the salt mass 1 disturbs, by increasing it, the temperature of the soil situated above it, thus creating an effect called "chimney effect".

The Applicant has established by series of tests and calculations that, in the case of buried metallic clusters at great depth, the disturbance created by these metallic clusters on the isotherms at shallow depth was detectable.

According to the invention, as shown in FIG. 2, 18 holes F _] to Fj g have been drilled in a search area A, B, C, D of about a thousand meters by a thousand meters, at a slightly greater than 100 meters. One can of course, if desired, also use existing wells. In Figure 3, a vertical section line is shown passing through four of these holes, namely holes F7, F8, Fi l and FI 3. These holes were drilled on a geologically known study area, located at above a metallic mass 3 ("masa valverde") made up at the same time of metallic ore 6, represented in bold hatched lines in FIG. 3, of massive sulphides and of "stockwerk" 8, represented in doubly hatched lines, c 'that is to say fractured rocks having allowed the passage of water loaded with metallic elements, this metallic heap extending appreciably between 500 meters and 800 meters. After a rest period, of the order of 30 days, the temperature was measured in each borehole at various determined depths and in particular at a depth of 100 meters, using a particularly precise temperature measurement device ( accuracy of the order of 1/100 ° C). During these measurements the boreholes remain open, which makes it possible to carry out, if desired, other series of measurements, at this same depth or at a different depth, so as to be able to construct the temperature fields which exist. at different depths. We then built a series of isothermal curves at the depth of 100 meters which we have represented, in horizontal plane in figure 2 and in plan vertical in FIG. 3. We have also focused on this representation the location of the various drillings Fι, F2, ..., Fιg.

FIG. 4 shows a representation of these same isotherms in three dimensions, in the area ABCD. This series of isotherms, and in particular its three-dimensional representation in FIG. 4, clearly shows irregularities in the temperature field at this depth of 100 meters. The Applicant has found that these disturbances occur above the metallic masses 3 constituted by the "masa valverde". The strongest disturbances appear above the zones of the metallic cluster 3 which contain the greatest quantity of metallic ore 6. It can thus be seen in FIG. 4 and FIG. 2 that the field of the isotherms is very strongly disturbed around the well Fi l, which is well confirmed in the drawing of FIG. 3.

The Applicant has also established that it was not necessary, for such an ABCD search area, to carry out such a large number of holes and that it was possible to highlight the irregularities of the temperature field with fewer holes. . To do this, as shown in FIGS. 5 and 6, the measurement data previously established was used, limiting them to nine of the eighteen previous holes, namely (FI, F3, F4, F8, F10, FI 1, F13, F14, and F17) and, on the basis of this information, we established by calculation, and we drew the isotherms corresponding to the same depth of 100 meters.

It is clearly seen, in the plan representation of FIG. 5, as well as in the three-dimensional representation of FIG. 6, that the irregularities of the temperature field established previously are found overall, although the number of boreholes is equal halfway through previous drilling.

The Applicant has thus established that the geophysical method for detecting metal clusters buried in the ground according to the invention could be implemented with only nine boreholes for a research area of approximately one square kilometer. According to the invention, it is also possible to carry out several temperature measurements in each of the boreholes, or even to carry out continuous measurements, preferably going from top to bottom in the borehole. One will then establish, by calculation, for each borehole, the temperature at at least a determined depth, so as to be able to build at least one network of isotherms at this determined depth.

It will be noted that the irregularities in the temperature field were highlighted with a distribution of the measurement boreholes which was not regular over the entire research area. It is thus found that it is possible to implement the present invention, to use both previously drilled wells for other purposes, to a greater or lesser depth and with a more or less regular spatial distribution, as wells specific materials regularly distributed and excavated for this purpose.

Claims

1. Geophysical method for detecting metal clusters buried deep in the ground, characterized in that it comprises the steps consisting in: - carrying out a series of boreholes (FI, .... F 18) of shallow depth, included substantially between 50 meters and 200 meters,

- wait for the thermal equilibrium of these boreholes,

- carry out, in these boreholes, at least one temperature measurement making it possible to establish the existing temperature at a determined depth, - establish the temperature field at said determined depth, in order to detect the anomalies of this field characteristic of the presence of deep metal clusters.

2. Method according to claim 1 characterized in that several temperature measurements are made at any depth and a processing of the temperatures recorded in the drilling series is carried out in order to determine the temperature at said determined depth.

3. Method according to claim 2 characterized in that the temperature measurements are carried out continuously.

4. Method according to claim 2 characterized in that the temperature measurements are carried out from the top to the bottom of the borehole.

5. Method according to any one of the preceding claims, characterized in that the establishment of the temperature field is carried out by drawing a network of isotherms in three dimensions.