APPENDIX 4.

Published in

The American Geophysical Union's

Journal of Geophysical Research

Vol.73 August 15, 1968, No 16,

pp. 5377 to 5383.

NOTE.

THE AREA "BROADLANDS" WAS DEFINED BY THIS SURVEY BEFORE ANY DRILLING TOOK PLACE, OUTLINED THE DRILLING TARGET WHICH IS NOW A PRODUCTION AREA. JOURNAL OF GEOPHYSICAL RESEARCH

VOL. 73, No. 16, AUGUST 15, 1968

Geothermal Ground Noise Amplitude and Frequency Spectra in the New Zealand Volcanic Region

G. R. T. CLACY
( Institute de Geofisica, Torre de Ciencias, Cudad Universitaria, Mexico. Also a consultant to Geotech, Dallas, Texas. 1968-1971)

Geophysics Division, D.SJ.R., Wellington, New Zealand

A survey made with a portable slow-motion tape seismograph of the Rotorua, Taupo volcanic region, showed that areas of continuous high seismic noise and low dominant frequencies were found in conjunction with aquifers.

INTRODUCTION

Owing to ground unrest in the Rotorua, Taupo, volcanic region of New Zealand, survey work by various geophysical methods has been hampered. A constant high seismic noise level is encountered for most of the region. It was decided to record this phenomenon. The instrument chosen was a continuous direct-recording tape seismograph and Willmore seismometer, which was described by Dibble [1964]. The method of measurement and the final plotting on maps were devised by the author as the survey progressed, and the total amplitude from 1 to 20 cps was plotted and contoured. The dominant continuous frequency in the same frequency range is described below.


Fig. 1. Over-all. total `noise amplitude from 1 to 20 cps relative to a predetermined level. Recording stations covering the Taupo volcanic region are shown by the solid dots. 5378 G. R. T. CLACY

Precautions were taken to ensure that any sudden increases in ground noise would be known and could be calculated for.

DISCUSSION OF RESULTS

Recordings made during 1966 are indicated by dots on Figure 1. This map also shows that the over-all contours of relative total groundnoise amplitudes, which are derived from the continuous frequency bands 1-20 cps, are logarithmic in scale and do not include manmade noise, winds, ocean microseisms, or other periodic surface seismic noise. The exclusion of noises is simple, because they are readily spotted during the frequency analysis process and because periods free of these spurious noises occur during the night.

On the amplitude basis in Figure 1, it is easy to see areas of noise and quiet and to find areas that require more detailed survey. The Broadlands area was surveyed with approximately 914.4 metres station separation. From this survey, maps (Figures 2 and 3) were prepared. Figure 2 shows fairly well defined high noise areas, and Figure 3 shows the frequency distribution over the same area. It will be noticed how the lower-frequency dominant continuous noise moves up in frequency away from the high noise source.

A further interesting survey with bore 2 running was made using the same survey stations in the Broadland area. The records of this survey are plotted in Figures 4 and 5. As one would expect, the total noise in the immediate area of the bore increased, as did the dominant frequency. The hitherto high-noise area of 6.5 increased only slightly, however, and the seismic noise from the bore decreased more rapidly toward this point. The dominant frequency pattern emerged around the bore


Fig. 2. Over-all total noise amplitude from 1 to 20 cps. The recording stations are shown by the open circles, and the bores are shown by the solid squares. Bores 1 and 2 closed in the Broadlands geothermal area during the survey.
GEOTHERMAL NOISE AMPLITUDE AND FREQUENCY SPECTRA 5379


Fig. 3. First dominant frequency from 1 to 10 cps. The recording stations are shown by the open circles, and the bores are shown by the closed squares. These contours apply to the same conditions as Figure 2.

but at higher frequency; it emerged from the bore area with increasing frequency away from the noise source.

In the Maroa region, which is a fairly young volcanic centre (Figure 6) with no obvious surface activity, a high seismic noise pattern is present but no frequency analysis has yet been made for this region.

The Waiotapu Thompson area shows an interesting noise pattern (Figure 7): small and a little more complex. The frequency spectrum (not shown) is quite complex, but the 2-cps frequency does emerge in the high-noise area. Obvious surface activity occurs where the bores are. These bores are not, however, good producers, so that the aquifer system is perhaps located where the high noise, low frequency appears.


CONCLUSIONS

The seismic-noise pattern of a geothermal region is probably a useful method for determining an underground steamproducing aquifer because the water contact with heat appears to produce a useful seismic noise pattern that can be plotted to determine the possible extent of an aquifer. In Figure 8 the emergence of 2 cps as the dominant frequency is probably due to the attenuation of the higher frequencies. As the travel paths improve, the higher frequencies become the dominant ones. The direction of the aquifer depends on the rate of emergence of the higher frequencies, as does the size of the aquifer. The mechanism in Figure 8 suggests water supported by gas vapor, as in the Liedenfrost effect. This condition could account for the 2-cps emission,
5380 G. R. T. CLACY


Fig. 4. Over-all total noise amplitude from 1 to 20 cps relative to a predetermined level. Recording stations are shown by the open circles, and bores are shown by the closed squares. Bore 2 was producing steam. Bore 1 closed in the Broadlands geothermal area during survey.


GEOTHERMAL NOISE AMPLITUDE AND FREQUENCY SPECTRA 5381


Fig. 5. First dominant frequency from 1 to 10 cps. These contours apply to the same conditions as Figure 4.


Fig. 6. Over-all total noise amplitude from 1 to 20 cps relative to predetermined level.
Recording stations are shown by the open circles in the Moroa Geothermal region.


GEOTHERMAL NOISE AMPLITUDE AND FREQUENCY SPECTRA 5383


Fig. 7. Over-all total noise amplitude from 1 to 20 cps relative to a predetermined level. Recording stations are shown by the open circles in the Thompson-Waiotapu geothermal region. The bores are shown by the closed squares.

with water in contact with lower heat around the perimeter giving the higher-frequency dominances. The cooling and regeneration of heat source will give rise to many other frequencies. I think that a multiple noise generator is necessary because of the many discrete narrow-band frequency dominances recorded at one time, though all frequency dominances are low in the centre of a good aquifer system. Therefore, high-noise and low-frequency dominance indicates a good aquifer.


Fig. 8. Proposed model of geothermal groundnoise source indicating zone of high-frequency attenuation.



Acknowledgments.

I thank Dr. T. Hatherton and Mr. C. J. Banwell for their advice and encouragement and Mr. D. J. Dickinson for his field assistance.

REFERENCES

Dibble, R. R., A portable slow motion tape recorder for geophysical purposes, New Zealand J. Geol. Geophys., 7(3), 445, 1964.

(Received November 9, 1967;
revised January 22, 1968)

Go to Download page for seismic audio files and discussion by G.R.T. Clacy

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