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German-Australian Demonstration of the Application of Lotem Measurement in Coal Mining

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Published: April 97Project Number: C4029

Get ReportAuthor: Keeva Vozoff, O Engels, Greg Poole, S Lintker | Harbourdom Consulting, BHP Coal, DMT

The primary objective of the project was to show that it is possible to obtain important information about drainage and fluid content of coal seams from surface electromagnetic (LOTEM) measurements combined with reflection seismic data resistivity logs. If this can be done with sufficient resolution to map undrained zones ahead of mining, it will reduce the need for in-seam measurements of all kinds, thus reducing costs. Computer modelling showed it should be possible because of the large changes in the electrical properties of the coal seam as it is mined and drained.

A second objective of the project was to demonstrate the effectiveness of distributed multichannel electromagnetic (EM) data acquisition, and new field procedures designed around that principle. The concept is to apply seismic-like procedures to EM acquisition and processing in order to reduce the cost of acquiring EM data.

To test the concepts in a real situation, two surveys were carried out a year apart (May 1995 and May 1996) at exactly the same surface positions over an active longwall in the Appin Colliery. Between the two surveys the mine and drainage zones had shifted laterally by one longwall width, 250m.

In parallel with this project there was another in Europe, using exactly the same technology, to monitor movement of the water table in a closed anticline used to store gas in the summer. In that project, which was funded by the European Union, DMT was one of four participants and HarbourDom GmbH the prime subcontractor.

The main objectives in that project were to demonstrate and develop the distributed multichannel EM system concept. Whilst there has been no direct interchange of data, technical information has been exchanged between the two projects.

To plan the survey it was necessary to numerically model the effects on our measurements of changes in subsurface distribution of petrophysical properties. It was assumed for modelling purposes, that the only changes taking place were the 250m lateral shift of the mine and drainage zones, and that changes lay entirely within the Bulli seam itself.

The modelling predicted changes in our responses of up to 25% between the two surveys, which should have been easily observed with the anticipated performance of our equipment and data processing procedures.

The first results, from the 1995 survey, showed there was a serious design fault in the equipment which badly affected measurements when the receiver spread was too close to the transmitter. However, because of the survey procedures used we were able to use the data taken when transmitter and receivers were far enough apart, about 25-30% of the total.

The result from the second year's survey was that the effects we observed in the useable data were 4-5 times larger than predicted by the models. The likely explanation, supported by subsequent modelling, is that petrophysical properties changed (resistivities increased) not only in the seam itself, but also in a much thicker stress relief zone overlying the seam.

This was also observed in the seismic reflection records, with reflectors in the overlying 100-200m 'breaking up' as the longwall moved. New modelling done to explain our results shows that the effects can be explained by a zone of increased resistivity about 200m thick. What is less clear at this time is how the effects we are looking for can be used in the presence of the much larger effects which occur.

In terms of the second objective, data were acquired at a rate much greater than that of any other electromagnetic data acquisition system of this scale, that is with transmitter-receiver separations of 100's to 1000's of metres, and there are clear ways to improve production still further. The major limitation at present is in the time required for data processing and interpretation.

 

The Survey

A new distributed multichannel electromagnetic system was used to monitor changes in subsurface electrical properties due to mining and seam drainage. This was done through repeated surveys one year apart over the Appin Colliery near Appin, NSW.

By repeating measurements at the identical closely-spaced surface positions, and by recording very large volumes of data and applying statistical processing, it was expected that we would be able to observe the 15-25% changes in responses predicted by computer modelling. The model assumed that the only changes in physical properties taking place were within the (Bulli) seam itself.

Engineering faults in the equipment led to a loss of about 75% of the data. From the remaining data it was seen that the changes from survey to survey were 5-10 times greater than predicted in the modelling. Such changes can be explained if the electrical resistivity increases by 1-2 orders of magnitude in a zone 200m thick overlying the active region of longwall mine and drainage.

This is consistent with changes seen in reflection seismic profiles, in which reflectors break up and vanish in a zone of that dimension.

Conclusion

We conclude that the changes we were looking for are still there but there is little point in looking for them until the larger changes in the overlying region can be predicted.

Although many technical aspects of the project were successful, the main objective of demonstrating a capability to monitor a seam drainage was not attained, for two reasons. One is that the initial model, in which changes were assumed to take place only in the seam itself, was too simple.

From our results we learned that a very thick zone above the seam also changes, but do not yet know whether those changes are predictable. Since the changes also affect the reflection seismic results, it is possible that further study will suggest ways to predict their effects.

The method is in fact very sensitive but will of course require a better equipment design if it is to be used. A variety of other changes to hardware and software are also necessary if it is to be used in commercial applications.

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