Magnetotelluric deep-sounding experiments in Iceland

EARTH AND PLANETARY SCIENCE LETTERS 4 (1968) 469-474. NORTH-HOLLAND PUBLISHING COMP., AMSTERDAM

MAGNETOTELLURIC

DEEP-SOUNDING

EXPERIMENTS

IN ICELAND

J.F.HERMANCE

Department of Geology and Geophysics, MassachusettsInstitute of Technology, Cambridge,Massachusetts and G.D.GARLAND

Department of Physics, University of Toronto, Toronto, Ontario Received 14 June 1968

The results from magnetotelluric sounding experiments at three sites in Iceland are discussed. Two sites were in the neovolcanic zone (Askja, Burfellshraun); the third was in an adjacent area of older volcanics (Sulendur). Apparent resistivities were estimated using a series of digital band-pass filters having a selectivity of 0.3 and center periods of 50, 100, 200, 400, 800 and 1600 seconds. This method of analysis allowed the stability with time of the apparent resistivity to be considered. The sounding data show very little difference between the three stations over this range of periods, thus suggesting similar electrical structures under all the stations. The results indicate an almost constant value of about 30 ohm-meters for the apparent resistivity over the entire range of periods. On the assumption of a two-layer model, the depth to the conductive, 30 ohm-meter layer must be less than 25 km. The resistivity is a factor of two or three too high for molten lava, but may indicate a temperature enhancement in basalt or gabbro of 300 to 400°K. 1. INTRODUCTION The observations o f local anomalous behavior o f variations in the geomagnetic field in different areas o f the world have been interpreted as due to induction in zones o f enhanced electrical conductivity in the crust or upper mantle [7]. It has been suggested that since normal crustal and upper mantle materials are electrical semiconductors [8] whose conductivity has a temperature dependence o f the form exp(-E/kT), where E is the activation energy, k is Boltzman's constant, and T is the temperature, the enhanced conductivity may be the result o f enhanced temperatures which in turn may be caused b y local upwarpings of high temperature isotherms normally lying at much greater depths. In an effort to correlate surface observations o f thermal activity, such as recent intensive volcanism, with the presence o f anomalous electrical conductors at depth, Garland and Ward [4], in the summer o f 1964, operated three variometers simultaneously in a profile perpendicular to the neovolcanic zone in northern Iceland. By comparing magnetic-bay type events observed in

Iceland with average features of auroral magnetic substorm activity as determined from magnetograms recorded b y the permanent Danish observations on Greenland, Hermance and Garland [6] were able to show that the observations on Greenland and Iceland were compatible if Iceland was underlain b y a broad zone o f enhanced conductivity (0.1 mhos/m) at a depth o f around 30 km. The conclusions o f these preliminary geomagnetic variation studies were speculative in that it was assumed, as a first approximation, that the material under Iceland, after correcting for pressure, had the conductivity of molten lava (0.1 mhos/m). The geomagnetic variation analysis was used to determine the depth (30 km) to the zone o f enhanced conductivity. Therefore, in an effort to eliminate some o f the ambiguity involved in the earlier interpretations, a series of magnetotelluric deep-sounding experiments were carried out in Iceland during the summer o f 1967 at the sites indicated in fig. 1. Only the sites indicated with a cross are discussed in the present paper. The dark areas on the map represent the post-glacial zone o f neovolcanic activity. The stations Askja (ASK) and

470

J.F.HERMANCE and G.D.GARLAND

MAGNETOTELLURIC SITES

SUL

UTB

VATNAJOKULL

iilii~

0 t

50 km I

Fig. 1. Positions of the magnetotelluric sites in Iceland. They are respectively Theirstareykir (THE), Burfellshraun (BUR), Utbruni (UTB), Askja (ASK) and Sulendur (SUL).

Burfellshraun (BUR) are in the neovolcanic zone, and Sulendur (SUL) is in the adjacent older area of Tertiary flood basalts.

2. DISCUSSION OF THE EXPERIMENT

2.1. Apparatus Two components of electric field, measured with lines about 300 meters long, and three components of magnetic field, measured with a flux-gate magnetometer, were recorded at each site on battery-operated Rustrak recorders running at a chart speed of 6 inches/hour. The components were oriented parallel and perpendicular to the magnetic meridian. Usable periods extended from about one minute to an hour. Electrodes consisted of copper rods inserted in small (2 inch. diam.) porous pots f'flied with a mixture of copper sulfate and gelatin. In general the resistance of two potted electrodes placed end to end was about 500 ohms. The electric signal from the buried elec-

trodes was amplified directly, without prefiltering, by an integrated-circuit operational amplifier having an input impedance of 100 kilohms. The entire system, operating from two 12 volts storage batteries and internal mercury cells, performed quite satisfactorily as long as no source of cultural noise, such as 50 cycle power lines, were nearby. However, because of dry soil conditions at Askja, very high electrode resistances were encountered, and it was necessary to correct the measured electric field at that station.

2.2. Analysis The fields were recorded for several days at each site and selected portions of the records were later digitized at 6 see or 12 sec intervals for analysis. In an effort to obtain as much reliable information as possible from the records, a new method of treating the data was tried. A digital band-pass filter having the impulse response g(t) = 2s vrlr exp(-¼s2t 2) cos(2rrfot ) was employed.

(1)

MAGNETOTELLURICDEEP-SOUNDINGEXPERIMENTS This filter has the frequency response

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Filtering action is obtained by convolving the original digitized signal with the impulse response function of eq. (1). A schematic diagram of the chain of operations for treating the data is illustrated in fig. 2, with an actual example shown in fig. 3. An orthogonal pair of magnetotelluric components (say the electric-north and the magnestic-east) are each passed through identical narrow-band filters hating a selectivity of 0.3 and a center period of 50, 100, 200, 400, 800, or 1600 sec. The rms level of filtered output is calculated as a function of time and from the rms values, the apparent resistivity E2 Pa = 0.2 T ~

(4)

is also calculated as a function of time. Fluctuations in the value of apparent resistivity as shown in fig. 3 are greatest when the signal energy is low or when

B~ND~

there is poor correlation between events in the filtered data and events in the original records. However, by visually correlating across Calcomp-plotted diagrams of the original data, the filtered signal, the rms signal and the apparent resistivity, stable estimates could be determined. The points A, A' on the electric and magnetic records of fig. 3 show the typical type of correlation observed. Similarly for B, C, D.

3. DISCUSSION OF THE RESULTS The results of the magnetotelluric experiments at Askja (ASK), Burfellshraun (BUR) and Sulendur (SUL) respectively are shown in figs. 4, 5 and 6. The solid points in each figure represent stable resistivity estimates for the electric-north and magnetic-east orthogonal pair. The crosses represent stable estimates for the electric-east and magnetic-south orthogonal pair. Even with the filtering and selection technique employed, scatter remains, and it is not obvious that there are consistent differences such as would be produced by anisotropy between the two pairs at each station. The possibility that the variation of apparent resistivity is a function of source field polarization is being examined. Except for the greater scatter in the estimates at Burfellshraun, the average range of apparent resistivities is very similar at the three sites. At least for the periods that have so far been analyzed, the resistivity structures under the three stations appear to be quite similar. Another feature of these results is that the apparent resistivity is relatively constant,

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472

LF.HERMANCE and G.D.GARLAND

ELECTRIC - EAST

MAGNETIC - SOUTH

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Fig. 3. An example of the variability with time of the apparent resistivity estimates. For an orthogonal pair of E and H components, tracings of the original records, the filtered signals and the rms level of the filter output are shown. The time base is the same for all traces; points A, B, C, D indicate times for which stable estimates of apparent resistivity are possible. between 20 and 40 ohm-meters at all three stations from periods of a minute to periods of about 30 min. Attempts are being made to extend the short period and long period resolution of the method in order to delineate any trend in apparent resistivity outside the relatively narrow range of frequencies shown in the figures. However, it is clear that the conductivity structure beneath Iceland is very different from that determined beneath continental stations. At the latter, the apparent resistivity is normally found to increase, through the period range 50 to 1000 sec, to values over 1O0 ohm-meters, and any indication of a decrease in apparent resistivity, due to the highly conducting part of the mantle, is found only at periods greater than 1000 seconds.

Surface resistivity measurements were carried out by tJae National Energy Authority of Iceland [ 1 ] near the site at Burfellshraun (BUR), using a Schlumberger array with a maximum electrode spacing of one kilometer. Their sounding curves showed a thin highly-resistive layer of several thousand ohmmeters, about 40 meters thick, underlain by a material of resistivity 500 ohm-meters. Neglecting effects from the thin resistive layer, the combined results suggest that the resistivity structure under Iceland may consist of two layers, the upper layer having a resistivity compatible with the Schlumberger sounding (500 ohm-meters) and the lower layer having a resistivity compatible with the magnetotelluric deep sounding experiment (approximately 30 ohm-meters).

MAGNETOTELLURIC DEEP-SOUNDING EXPERIMENTS

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Fig. 4. Results of the magnetotelluric sounding experiment at Askja (ASK). The horizontal lines marked E and N show on the logarithmic scale the magnitude of corrections for electrode resistance that have been incorporated in the calculation of the electric fields and therefore of apparent resistivities.

1000 M-T SOUNDING SULENDUR • ELECTRIC-NORTH + ELECTRIC EAST

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I00 200 400 I000 4000 PERIOD, SEC. Fig. 6. Results of the magnetotelluric sounding experiment at Sulendur (SUL). The data points for the electric-east polarization were corrected for high contact resistance by the factor shown.

It is difficult to estimate the thickness of the upper layer because of the absence of data for periods shorter than 50 seconds. However, an upper limit on the depth to the second layer may be determined by assuming that a change in slope in the apparent resistivity curve occurs at a period equal to 50 seconds. Comparison with the theoretical curves of Cagniard [2] then gives an upper limit of about 25 km for the depth to the second layer. Although one might question the validity of applying Cagniard's plane wave solution to cases where the source field is in the auroral zone, theoretical calculations using the auroral electrojet model of Hermance [5] have indicated that source field effects do not change the principal features of the interpretation for the periods and conductivities under consideration.

4

4. SUMMARY AND CONCLUSIONS

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Fig. 5. Results of the magnetotelluric sounding experiment at Burfellshraun (BUR).

Electrical experiments performed during the summer of 1967 have been interpreted using a two-layer model. A resistivity of approximately 30 ohm-meters for a conductive layer under Iceland was determined

474

J.F.HERMANCE and G.D.GARLAND

from magnetotelluric deep-sounding experiments. The absence of data for periods less than 50 seconds makes it difficult to determine the depth to the zone of enhanced conductivity, but it must be less than 25 km. The similarities of the sounding data for three stations (ASK, BUR and SUL) suggest the zone of enhanced conductivity is a broad feature, perhaps a regional manifestation of the thermal state under most of Iceland, and is not locally correlated with the most recent volcanism. The resistivity of molten lava is about 4 ohm-meters [1 ]. Correcting for pressure effects by the method of Coster [3] increases this value to about 10 ohm-meters at a depth of 30 km. Although the resistivity from the magnetotelluric data (30 ohm-meters) is too large to be indicative of extensive magmatic zones under Iceland, it is compatible with a temperature enhancement on the order of 300 to 400°K for basalt or gabbro [3]. The results of the magnetotelluric experiment are therefore in essential agreement with the analysis of geomagnetic variations by Hermance and Garland [6], but provide a more quantitative estimate of the resistivity of the heated material.

ACKNOWLEDGEMENTS We are indebted to S.Bjornsson and G.Palmason of the National Energy Authority of Iceland and to T.Sigurgeirsson and P. Theordorsson of the University

of Iceland for their cooperation and encouragement during all phases of the research in Iceland. The enthusiastic assistance of Mr. Adalsteinn Hallgrimsson was of inestimable value during the critical portions of the field work and is gratefully acknowledged. The major portion of this research was carried out through a grant from the National Research Council of Canada.

REFERENCES [1 ] S.Bjornsson, Private Communication (1967) National Energy Authority, Reykjavik, Iceland. [2] L.Cagniard, Basic theory of the magnetotelluric method of geophysicalprospecting, Geophys. 18 (1953) 605. [3] H.P.Coster, The electrical conductivity of rocks at high temperature, Roy. Astron. Soc. Monthly Not., Geophys. Suppl. 5 (1948) 193. [4] J.E.Everett and R.D.Hyndman, MagnetoteUuric observations in south-western Australia, Phys. Earth Planet. Interiors 1 (1967) 49. [5] G.D.Garland and J.Ward, Magnetic variation measurements in Iceland, Nature 205 (1965) 268. [6 ] J.F.Hermance, Auroral Zone Geomagnetic Variations in Iceland, Unpubl. Ph.D. Thesis, Univ. of Toronto (1967). [7] J.F.Hermance and G.D.Garland, Deep electrical structure under Iceland, J. Geophys. Res., in press (1968). [8] T.Rikitake, Electromagnetism and the Earth's Interior (Elsevier, New York, 1966). [9] D.C.Tozer, The electrical properties of the earth's interior, Chapter 8 in Physics and Chem. of the Earth, Vol. 3 (Pergamon Press, New York, 1959). [10] K.Vozoff and Robert M.Ellis, Magnetotelluric measurements in southern Alberta, Geophysics 31 (1966) 1153.