1990 Jgg Oshima Csamt

J.Geomag. Geoelectr.,42,211-224,1990

CSAMT Measurements across the 1986 C Cratersof Izu-Oshima Island, Japan

Yasuo OGAWA and Shinichi TAKAKURA

(geological Survey of Japan, 1-1-3 Higashi, Tsukuba, Ibaraki 305, Japan

(Received March 10,1988; Revised September 6,1988)

We carried out CSAMT(controlled-source audiofrequency magnetotelluric)measurements across the 1986 C craters of Izu-Oshima Island one year after theeruptions. After 2D analyses of the two profiles, we obtained basically two-layeredstructures, except for shallow resistivity variations. We found a deep conductivelayer below the resistive lava which corresponds to thermal water, compared withnearby wells. We also found shallow(

212 Y. OGAWA and S. TAKAKURA

Fig.1. Location map of grounded electric dipole with transmitter(TX)and receiver sites(solid dotswith site numbers)together with A, B, and C craters of Izu-Oshima lsland. Profiles l and 2 are fortwo-dimensional studies across the C craters. Stars denote geothermal wells.

rim. The alignments of the B and C craters are in the NNW-SSE direction, likemany parasitic volcanoes on the island(NAKAMURA,1964). These parasiticvolcanoes are indicators of maximum compressional tectonic stress(NAKAMURA,1964,1977).

There are many geophysical studies of Oshima Island including electrical andelectromagnetic ones. ONO et al.(1961)initially studied the resistivity structuredown to l km depth by use of the Schlumberger method. They interpreted theresistivity structure in relation to ground water. YUKUTAKE et al.(1985)also usedSchlumberger soundings inside the caldera and along our profile 2, so we cancompare our results with theirs to see if there was a resistivity change before andafter the 1986 eruption. YUKUTAKE et al.(1990)carried out an airborne VLF surveyover the island in which they found shallow conductivity anomalies which corre-spond to surface fracture zones and pointed out anomalies along the B and C

CSAMT Measurements across the 1986 C Craters of Izu-Oshima Island, Japan 213

craters. YUKUTAKE et al.(1987)monitored the resistivity change associated with the

1986 eruptions of the A crater by use of dipole-dipole measurements across the A

crater.

Our study focused on detailed structures across the 1986 fissures down to a l

km depth by the use of CSAMT. Unfortunately, because the B craters are hard to

access, measurements were across the C craters only. The purpose of this survey is to

analyze the detailed resistivity structure across the C craters. We thought it would be

difficult to detect the dyke structure itself, because the width of the observed dyke on

the surface was less than one meter(SOYA et al.,1987). However, we can expect to

see a dyke-like structure. If the dyke supplies enough heat to surrounding rocks, the

pore fluids of the surrounding rocks can become more conductive(YOKOYAMA etal.,1983).

2.CSAMT Measurements

We constructed a current bipole in the southeastern part of the island(Fig.1).

The current source has a moment of 8-12103 Am using a 25 kW transmitter. The

receiver sites are mainly across the C craters in the northwestern part of the island.

The source-receiver separations range from 3.5 to 7 km;if they are not large enough,

the so-called near-field phenomena occurs(GOLDSTEIN and STRANGWAY,1975;

KAUFMAN and KELLER,1983;ZONGE et al.,1986;YAMASHITA,1987). In the near-

field, the measured impedance has the minor effect of induction, but the major effect

of DC conduction. We get no deep information in the near-field, whatever low

frequencies we use. Source-receiver separation must be 3 times greater than the

depth of investigation to get a plane wave(GOLDSTEIN and STRANGWAY,1975).

Since the resistivity structure is a priori unknown to some extent, we set the current

bipole as far as possible in order to alleviate the near-field effect. Due to this

configuration, the excitation of the field was restricted in the TM mode(H-

polarization). Fortunately, all the sites except one(site 21)were free from near-fieldeffects.

Using a two-channel receiver, we measured one electric field parallel to the

current bipole and one magnetic field perpendicular to the electric field. Our

electrode span was 50 m, and the frequency range was from l to 2048 Hz. The data

above 4 Hz were excellent, but below 4 Hz they were often scattered due to wind

noise in the magnetic field, cultural noise in the electric field, and the small number

of stackings at low frequencies.

3.CSAMT Data and Inferred Resistivity Structure

3.1 Profile 1

Profile 1 lies adjacent to the southern end of the C craters. There are no

geothermal manifestations on the ground. However, we expected that there mightbe a connection of dykes between the B and C craters, because products from the B

and C craters were both andestic(NAKANO and YAMAMOTO,1987). We suspected a

buried dyke beneath this profile.

Figures 2(a)and 2(b)show the apparent resistivity and phase pseudo-sections,

respectively. See Fig.2(a)around sites 2 and 3. There is no perturbation due to the


(a)

(b)

Fig. 2. (a) Apparent resistivity pseudo-section for profile 1. Apparent resistivity less than 50 ohm-m is shaded. "C" corresponds to the suspected extension of C craters. (b) Phase pseudo-section for profile 1. Contour interval is 5 degrees. Phase more than 70 degree is shaded.

suspected dyke between the B and C craters; however, there are perturbations at sites 5 and 6. On the other hand, the contours in the phase pseudo-section (Fig. 2(b)) are mostly horizontal, meaning that the deep structure is almost one-dimensional. These two pseudo-sections show typical signatures of static distortion (BERDICHEVSKY and DMITRIEV, 1976). Shallow localized anomalies significantly disturb apparent resistivity even at low frequencies, but hardly disturb phase values at low frequencies.

Figure 3 shows the inferred resistivity structures by one-dimensional inversion at each site. Shallow localized anomalies may easily distort 1D results and false deep


Fig. 3. 1D inverted resistivity structure for profile 1. Numerals denote resistivity in ohm-m.

features may emerge. We are skeptical about the undulation of the low resistivity layer around sites 5, 6, 65 and 7 in Fig. 3.

By use of the TM mode (H-polarization) 2D modelings, we can overcome the effects of shallow localized anomalies. Moreover, it is known that TM mode 2D modeling gives good approximations, even for the 3D resistivity structure (TING and HOHMANN, 1981; WANNAMAKER et al., 1984). We can consequently obtain a reasonable resistivity cross section even if the structural strike is not long enough. Figure 4(a) shows the final model of the two-dimensional inversion. Our codes (see OGAWA et al., 1988) consist of forward calculations based on MOM's method (RODI, 1976) and inversions using singular value decomposition (SAITo, 1983). Figure 4(b) shows the fitting of calculated values to observed ones. We have excellent fittings. From the surface to the 200 m depth, there are resistivity variations; these may be due to the difference in porosity or water content of basaltic lava. Deeper than 200 m, all the sites are underlain by the conductive layer (15-20 ohm-m), whose top is 150 m above sea level.

Site 32 is located 1 km away from profile 1. The 1D inversion result shows a three layered model (Fig. 3). The depth to the bottom conductive layer (10 ohm-m) is 200 m below sea level. The resistivity of the bottom layer is similar to the conductive layer below profile 1. The depth to the conductive layer may become deeper as one goes farther away from the craters.

3.2 Profile 2 Profile 2 crosses the C craters. Figures 5(a) and 5(b) show the apparent

resistivity and the phase pseudo-sections, respectively. Site 14 is just on a volcanic vent. At this site, the apparent resistivity pseudo section (Fig. 5(a)) is very perturbed, but the phase pseudo section (Fig. 5(b)) is not. These are typical signatures of static distortion like profile 1.

Figure 6 shows the 1D inverted resistivity structure. Below site 14, the depth of the conductor and resistivity values are underestimated by 1/2 and 1 order of


magnitude, respectively, compared with surrounding sites. We may expect a conductive dyke beneath site 14 at first glance. However, we have to take the effect of localized structure into account by the use of 2D modelings.

Figure 7(a) shows the result of the 2D inversion, and Figure 7(b) shows the comparison between the observed and calculated values; the fitness is excellent. All the sites are underlain by a conductive layer (10-30 ohm-m) below sea level. Beneath site 14, the 2D model does not require a conductive dyke-like structure, but a localized shallow (


(a)

(b)

Fig. 5. (a) Apparent resistivity pseudo-section for profile 2. Apparent resistivity values less than 50 ohm-m are shaded. "C" corresponds to the C craters. (b) Phase pseudo-section for profile 2. Contour interval is 5 degrees. Phase values more than 70 degrees are shaded.

found relatively conductive volcanics (300 ohm-m) at sites 5 and 6 in the shallow

part (


Fig. 6. 1D inverted resistivity structure for profile 2. Numerals denote resistivity values in ohm-m.

the 200-300 m depth, except for the shallow anomalies. One may expect a systematic resistivity decrease with respect to the distance from a suspected dyke. We should have had such results, if the heat transfer had been efficient enough to decrease the resistivity of pore fluids in the surrounding rocks; however, we cannot see such effects from our analyses.

We obtained conductive (6-30 ohm-m) layers, below the resistive lava, which we can interpret by referring to geothermal wells between sites 7 and 32 (see Fig. 1). One of the wells was drilled down 369 m from the surface, i.e. 105 m above sea level. These wells encountered ground water at 190 to 250 m above sea level (ISSHIKI et al., 1963); this ground water level almost corresponds to the top of the conductor below profile 1. TAKAHASHI et al. (1987) reported that the resistivity values of water taken from these geothermal wells range from 13 to 17 ohm-m, and this value is compatible with our inferred resistivity for the deep conductive layer. Consequently, we think that the conductive layer is due to thermal water.

YUKUTAKE et al. (1985) showed a resistivity structure below our profile 2 from a Schlumberger sounding before the eruption. They didn't detect the deep con-ductor, as we did; this significant resistivity change reflects the thermal activity below the C craters.

We got very few sites away from the C craters. The inferred resistivity structures differ from those below the two profiles; i.e., below sites 32, 22, and 23, we got three layered structures. There is an intermediate conductive (70-100 ohm-m) layer between the top resistive layer and bottom conductive layer; the intermediate and bottom layers may correspond to a fresh water lens (Ghyben-Hertzberg's lens), and invading sea water, respectively. This is a typical distribution of groundwater on a volcanic island (e.g., ECKER,1976). The resistivity of basalt saturated with sea water is thus estimated as 10-20 ohm-m. This value is similar to that of Hawaii (30 ohm-m) inferred from Schlumberger soundings (ZoHDY and JACKSON, 1969).

Figure 9 summarizes the schematic picture relating resistivity structure to groundwater. The two profiles are lacking in this intermediate conductive layer (the fresh water lens). Hydrothermal convection may mix fresh water and invading sea water.


Fig. 8. Sounding curve for site 22 and 1D inverted structure.

Fig. 9. Schematic structure of Izu-Oshima Island obtained from the present CSAMT study.

TAKAHASHI et al. (1987) analyzed the chemical components of water samples taken from wells on Oshima Island. They thought the groundwater near the wells must be isolated and dyke-impounded, because the water samples from the wells in Fig. 1 were chemically different from other groundwater. In contrast, our CSAMT result below the two profiles requires a horizontally connected fluid path; we could not find isolated water pockets. The form of groundwater distribution may be different between the well locations and our two profiles.

Lastly, we discuss two possible reasons why we could not sense the dyke-like structure. One is that our measurements were in the TM mode only. The TM mode is rather insensitive to vertical dyke-like structures, so further study requires TE


mode excitation as well, or tensor AMT measurements. The other reason is that the dyke itself or dyke-like structure is too thin. A thin dyke could not heat surrounding rocks to reduce their resistivity.

5. Conclusion

We analyzed CSAMT data across the 1986 C craters of Izu-Oshima Island. After 2D analyses of the two profiles, we obtained basically two-layered structures, except for shallow resistivity variations. We found a deep conductive layer below resistive lava. This deep conductive layer corresponds to thermal water, comparing with nearby wells. We found shallow conductive bodies beneath the two profiles whose locations correspond to an old vent (8th century) and the 1986 vent, respectively. We think these contain meteoric water contained in fracture zones due to magmatic intrusion.

We expected to see a dyke-like structure due to heated rocks surrounding the dyke, however, we did not obtain affirmative evidences for dyke-like structures. This was partly because we used TM mode excitation, and partly because the dyke hadn't transferred enough heat to the surrounding rocks to decrease resistivity.

Away from the two profiles, we found three layered structures: (top) resistive lava, (middle) less resistive basalt saturated with fresh water, and (bottom) deep conductive basalt saturated with sea water. This is a typical hydrological structure of a volcanic island. On the other hand, the two profiles lacked the three layer structure. Fresh water and saline water may be mixed by thermal convection.

We appreciate the assistance of Drs. R. Takada, Y. Kuwahara, J.L. Oubina and H. Endo in the field measurements. We also acknowledge the Oshima Volcanic Observatory, Earthquake Research Institute, University of Tokyo for their cooperation. We would like to thank Drs. Y. Murakami, T. Uchida, J. Nakai, H. Tsu, K. Ono, T. Soya, N. Isshiki, N. Hanaoka, and M. Takahashi for helpful suggestions. Comments from two anonymous referees much improved the manuscript.

Computer facilities were at the Research Information Processing System, the Agency of Industrial Science and Technology (IBM3081K), and at the Geological Survey of Japan (SIGMA system, IBM4341).

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1990 Jgg Oshima Csamt

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