new equipment for the investigation of deep crustal structures using the resistivity method
TRANSCRIPT
Geoexploration, 23 (1984/85) 207-216 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
207
NEW EQUIPMENT FOR THE INVESTIGATION OF DEEP CRUSTAL STRUCTURES USING THE RESISTIVITY METHOD
RONALD GREEN, F.C. LUDBEY and MARINO
Geophysical Research Institute, UNE-Armidale, N.S. W., 2351 (Australia)
(Received 1984 April 24; revised version accepted August 8, 1984)
ABSTRACT
Green, R., Ludbey, F.C. and Marino, 1985. New equipment for the investigation of deep crustal structures using the resistivity method. Geoexploration, 23: 207-216.
A heavy alternating current generator has been developed and adapted as a dipole (150 m) source for resistivity surveys. The detecting system is a 100 m dipole, phase- locked to the input frequency of the generator. The phase information is transmitted by radio from the energizing source to the detecting system. This provides a very sensi- tive and logistically easy system to use because there are no connecting wires between the source and the detector. The distance of separation between source and detector can be several kilometres, thus providing an effectively long spread-length. Resistivity information about the crust has been obtained from three localities in New England, N.S.W., and complete information from Guyra is presented.
INTRODUCTION
The resistivity method assumed wider applicability when Ghosh (1971) was able to develop an efficient method of inverting the data. However, to obtain data about the structure at depth there remains a number of logistic problems. The foremost of these is the requirement of a large dis- tance of separation (kilometres) of the dipoles for deep penetration. In many areas, and especially in built-up areas, it is difficult to lay out cables over distances of kilometres. One solution is to use the dipole-dipole method utilizing entirely separate energizing and detecting set-ups. How- ever, a further consequence of a large distance of dipole-separation is the significant reduction in the strength of the signal at the distance of the detector. There are a number of additional considerations to be borne in mind. Direct-current systems experience the severe difficulties of firstly, the polarization of the energizing current electrodes, leading to a progres- sive decrease in signal strength, and secondly, of a slowly but naturally varying potential across the detecting electrodes. These phenomena make it difficult to obtain reproduceable results using direct-current systems, and such circumstances argue in favour of injecting heavy (20 A +) altemat-
0016-7142/85/$03.30 0 1985 Elsevier Science Publishers B.V.
208
ing current into the ground by the source dipole, and using a detector tuned to the frequency of the energizing dipole to detect and register the signal. Tuned phase-locked amplifiers are extremely efficient at pulling a signal from the noise. This paper describes a number of novel technical modifica- tions that have been made to the hardware for use in a resistivity survey unit, and changes in the field procedures, to enable significant (3 km) depths to be probed reliably, quickly and conveniently. Ogilvy (1984) has discussed recently modified equipment for down-hole I.P. and resistiv- ity measurements.
EQUIPMENT
The energizing system
The basis of the energizing system is a portable engine-driven electric- welding machine that is rated at 65 V opencircuit, and able to deliver 130 A into a welding arc. The current is delivered to the ground by means of copper pipes acting as cables, and connected to a group of steel spikes (covering a 1 m square). In this way, a low impedance dipole, with a length of 150 m can be set up. The arrangement is shown in Fig. 1. The current (= 20 A) supplied through the dipole to the ground, is measured by a clip-on Ammeter and is seen to be constant. The set-up assures that a heavy (20 A) but constant current is driven into the ground through a dipole 150 m in
Volt meter
COppar tubing = 150m , 19mm 0.D
x25 gauge x6 meters lengths
Clip on Ammeter
Flexible weldin
/
cable
Fig. 1. The signal energizing system - connection details.
209
length. The strength of the energizing dipole is therefore = 3000 A m. In Australia a SO Hz generator is used, to avoid matching with the standard 50 Hz of the electric power grid. Fig. 2 shows the rest of the equipment forming part of the energizing system. Note the second pair of electrodes, which detect the phase of the heavy current flowing between the two electrodes of the energizing dipole. By means of a 25-W radio-transmitter operating at 149.94 MHz, a reference signal is transmitted which is in phase with the heavy alternating current injected into the ground by the energiz- ing dipole. A bIoek diagram of the energizing system is given in Fig. 2. Fig. 3 shows a picture of the entire energizing system.
Fig. 2. The energizing electrode system, alternator frequency monitor and voltage con- trolled ~sciiIator~moduIator - block diagram.
The detecting system
Whereas the energizing system requires care to obtain a low resistance contact for the energizing electrodes, and is cumbe~ome on account of the weight of the heavy current generator, the detecting system is, on the contrary, highly mobile, The entire system is self-contained within a 4- wheel-drive vehicle. To make a reading the two detecting electrodes need only be pushed into the ground (separation 100 m). The distance from the energizing dipole to the detecting dipole is of the order of a few kilometres. A block diagram of the signal detector system is given in Fig. 4. The critical component of the system is the phase-sensitive detector {P.&D.) which compares the 60 Hz ground-path signal with the 60 Hz radio signal which
Fig. 3. A. The energizing system: AC energizing source, automatic relay unit, 25 Watt VHF transmitter, transmitting antenna, wattmeter, sub-carrier-oscillator (SCO). B. The receiving units of the signal detector system: Honda generator, 5305A Hewlett- Packard digital counter, radio receiver Vinten MTR 30A 25 Watt VHF transceiver, detec- tor electrodes, YEW achannel strip chart recorder type 3047, SPU, PSD Brookdeal 411, receiving antenna, connecting cables.
211
also comes from the energizing system. The use of the P.S.D. provides optimum conditions for enhanced signs-to-cute. The design also provides for a notch filter to reject the 50 Hz noise arising from the mains of the electric power grid. Fig. 5 is a block diagram giving further details of the signal processing unit which is a component of the signal detecting system. The minimum detectable signal is 10 PV, and with a 100 m long detecting dipole the system will respond to a signal as low as 100 nV/m. From a
GENERATOR DETECl”OR ELECTRODES
Fig. 4. The signal detector system and its unit connections.
Fig. 5. The signal processing unit block diagmm.
212
logistic point of view, the detecting system can be moved rapidly and easily, and measurements quickly and accurately made. For each set-up of the energizing system, several set-ups of the detecting system can be made successively, thereby recording along traverses covering several kilometres in length, in any required azimuthal direction. The fact that there is no need for cables connecting the transmitting and the detecting system, is an essen- tial characteristic of the entire system. Fig. 6 shows a recording made, where the distance of separation between the energizing source and the detector system was 1.5 km. The signal was switched ON and OFF with a one-minute cycle time. Trace 2 records the strength of the detected signal and is the basic observation in determining the apparent resistivity of the ground. Trace 1 monitors the broad-band noise across the detector dipole, and is useful in indicating the signal-to-noise ratio. Detailed circuit drawings are available by writing directly to the authors.
5 4 3 2 1 -t ~---.-
Fig. 6. The recorded signal during the current ON/OFF times. Trace 1 is the rectified and DC output reference signal (‘CURRENT ON’ marker).
RESULTS
The equipment has been used in three investigations of the thickness of geological formations in the New England, N.S.W. region. Fig. 7 is a locality map showing the test areas, and the traverse lengths. A computer program based on the Work of Ghosh (1971) and Patella (1980) has been used to invert the data. Patella (1974) and Dasgupta (1984) have also written on the conversion of dipole soundings to Schlumberger curves. The program used in this study interprets the resistivity curves in terms of the depths to the
213
interfaces and the resistivity values of the layers. By increasing the number of observations the reliability of the interpretation is improved as has been pointed by Langer (1933). With the high signal-to-noise ratio, the experi- mental results are highly reproduceable. The observed data from a 10 km
Fig. ‘7. Location map of the test areas.
Fig. 8. Apparent resistivity section from the NE traverse, Guyra.
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pole
.
216
traverse at Guyra are given in Table I. The results of the inversion of the data, are given in Fig. 8. While the sections have been depicted in terms of geological layers, it should be remembered that they are based on calcu- lated resistivity-interfaces. Nevertheless, the interfaces so determined are definite and at least consistent with what is known of the local geology.
COMMENTS
The advantage of the method used, is that the two dipoles (energizing dipole and detecting dipole) operate while physically completely separated. They are not at all required to be connected in any way by cable. Surveys are designed to invoke a minimum amount of movement of the energiz- ing system and a maximum of convenient stations on a profile, for the detecting system. The transmission by radio of the phase information about the energizing current pulses, input into the ground, enables a phase-sensi- tive-detector (P.S.D.) to be used to pull the signal out of the broad-band noise, thereby enabling accurate apparent resistivity values to be obtained even when the separation between the energizing dipole and the detecting dipole is large. Consequently, results giving information about resistivity values from much greater depths, can now be obtained more conveniently and quicker than by using alternative methods.
REFERENCES
Dasgupta, S.P., 1984. A note on the conversion of dc-depth sounding curves to Schlum- berger sounding curves. Geoexploration, 22: 43-45.
Ghosh, D.P., 1971. Inverse filter coefficients for the computation of apparent resistivity standard curves for horizontally stratified earth. Geophys. Prospect., 19: 769-775.
Langer, R.E., 1933. An inverse problem in differential equations. Am. Math. Sot. Bull., Ser. 2,29: 307-323.
Ogilvy, R.D., 1984. Down-hole IP. surveys applied to off-hole mineral exploration - some design considerations. Geoexploration, 22: 59-73.
Patella, D., 1974. On the transformation of dipole to Schlumberger sounding curves. Geophys. Prospect., 22: 315-329.
Patella, D., 1980. The quantitative interpretation of dipole soundings by means of the resistivity transform functions. Geophys. Prospect., 28: 956-960.