1990-JGG-Oshima-CSAMT.pdf

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J . Geomag. Geoel ectr. , 42, 211-224, 1990

CSAMT Measurements across the 1986 C Cratersof I zu-Oshi ma I sl and, J apan

Yasuo OGAWA and Shi ni chiTAKAKURA(geol ogi calurvey of J apan,-1-3H gashi ,sukuba, baraki 05, J apan

(Recei vedarch 10, 1988; evi sed eptember6,1988)

We carri edout CSAMT(control l ed-source audi ofrequencymagnetotel l uri c)measurements across the 1986 C cratersof I zu-Oshi ma I sl andone year af ter heerupti ons. f ter 2D anal ysesof t he two prof i l es,e obtai nedbasi cal l ywo- l ayeredstructures,xcept for shal l owresi sti vi tyari ati ons.e found a deep conducti vel ayerbel owt he resi sti veava whi ch corresponds to thermal water, compared w thnearby wel l s.We al so ound shal l ow(


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212 Y OGAWA and S TAKAKURA

Fi g. 1. Locati onmap of groundedel ectr i ci pol ew thtransmt ter TX)andecei veri tes sol i dotsw thsi te umbers)together thA, B, and C cratersf zu-Oshi ma sl and.rof i l esand 2 are ortwo-di mensi onaltudi escross heC craters.tars enotegeothermal el l s.ri m The al i gnments of the B and C craters are i n the NNWSSE di recti on, l i kemany parasi t i c vol canoes on the i sl and NAKAMURA, 1964). These parasi ti cvol canoes are i ndi catorsof maxi mum compressi onal tectoni c st ress NAKAMURA,1964, 1977).

There are many geophysi cal studi es of Oshi ma I sl and i ncl udi ng electri cal ndel ectromagneti c ones. ONO et al . 1961)i ni ti al l y studi ed the resi sti vi ty tructuredown to l km depth by use of the Schl umberger method. They i nterpreted theresi sti vi tytructure n rel at i onto ground water. YUKUTAKE et al . 1985)al so usedSchl umberger soundi ngs i nsi de the cal dera and al ong our profi l e 2, so we cancompare our resul tsw th thei rs to see i f there was a resi sti vi tyhange before andafter he 1986 erupti on. YUKUTAKE et al . 1990)carri ed out an ai rborne VLF surveyover the i sl and i n whi ch they found shal l ow conducti vi ty anomal i es whi ch corre-spond to surface f racture zones and poi nted out anomal i es al ong the B and C


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CSAMT Measurements across the 1986 C Craters of I zu-Oshi ma I sl and, J apan 213

craters. YUKUTAKE et al . 1987)moni tored the resisti vi ty change associ ated w th the1986 erupti ons of the A crater by use of dipol e-dipol e measurements across the Acrater.

Our st udy focused on detai l ed structures across the 1986 f i ssures down to a lkm depth by the use of CSAMT. Unfortunatel y, because the B craters are hard toaccess, measurements were across t he C craters only. The purpose of this survey i s toanalyze the detai l ed resisti vi ty structure across the C craters. We t hought i t woul d bedi f f i cul t t o detect the dyke structure i tsel f , because the w dth of the observed dyke onthe surface was l ess than one meter SOYA et al . , 1987). However, we can expect tosee a dyke-l i ke structure. I f the dyke suppl i es enough heat to surroundi ng rocks, thepore f l ui ds of the surroundi ng rocks can become more conducti ve YOKOYAMA etal . , 1983).

2.CSAMT Measurements

We constructed a current bi pole i n the southeastern part of the i sl and Fi g. 1).The curr ent source has a moment of 8-12103 A m usi ng a 25 kW transmtter. Thereceiver si tes are mainl y across the C craters i n the northwestern part of the i sl and.The source-r ecei ver separati ons range f rom 3. 5 to 7 kmi f they are not l arge enough,the so-cal l ed near- f i el d phenomena occurs GOLDSTEI N and STRANGWAY, 1975;KAUFMAN and KELLER, 1983;ZONGE et al . , 1986;YAMASH TA, 1987). I n the near-f i eld, the measured i mpedance has the mnor effect of i nducti on, but the major effectof DC conducti on. We get no deep i nformati on i n the near- f i eld, whatever l owf requencies we use. Source-recei ver separati on must be 3 ti mes greater than thedepth of i nvesti gati on to get a pl ane wave GOLDSTEI N and STRANGWAY, 1975).Si nce the resisti vi ty structure i s a pri ori unknown to some extent, we set the currentbi pole as far as possi bl e i n order to al l eviate the near- f i el d ef fect. Due to thi sconf i gurati on, the exci tati on of the f i el d was restri cted i n the TM mode H-pol ari zati on). Fortunatel y, al l the si tes except one si te 21)were f ree f rom near- f i el def fects.

Usi ng a two-channel recei ver, we measured one electri c f i eld paral l el to thecurrent bi pole and one magneti c f i eld perpendi cul ar to the electri c f i eld. Ourelectr ode span was 50 m and the f requency range was f rom l to 2048 Hz. The dataabove 4 Hz were excel l ent, but below 4 Hz they were of ten scatt ered due to w ndnoise i n the magneti c f i el d, cul tural noise i n the el ectr i c f i el d, and the smal l numberof stacki ngs at l ow f requencies.

3. CSAMT Data and I nferred Resisti vi ty Structure

3. 1 Profi l e 1Profi l e 1 l i es adj acent to the southern end of the C craters. There are no

geothermal mani festati ons on the ground. However, we expected that there mghtbe a connecti on of dykes between the B and C craters, because products f rom the Band C craters were both andesti c NAKANO and YAMAMOTO1987). We suspected aburi ed dyke beneath thi s profi l e.

Figures 2 a)and 2 b)show the apparent resisti vi ty and phase pseudo-secti ons,respecti vel y. See Fi g. 2 a)around si tes 2 and 3. There i s no perturbati on due to the


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214 Y. OGAWA and S. TAKAKURA

(a)

(b)Fig. 2. (a) Apparent esistivity seudo-sectionor profile1. Apparent esistivityess han 50 ohm-m s

shaded. C correspondso thesuspectedxtension fC craters. b) Phasepseudo-sectionor profile1. Contour nterval s 5 degrees. hasemore han 70degree s shaded.

suspected dyke between the B and C craters; however, there are perturbations atsites 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(BERDICHEVSKYnd DMITRIEV, 1976). Shallow localized anomalies significantlydisturb apparent resistivity even at low frequencies, but hardly disturb phase valuesat low frequencies.

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


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CSAMT Measurements across the 1986 C Craters of Izu-Oshima Island, Japan 215

Fig. 3. 1D nvertedesistivitytructure or profile1.Numerals enote esistivityn ohm-m.

features may emerge. We are skeptical about the undulation of the low resistivitylayer around sites 5, 6, 65 and 7 in Fig. 3.By use of the TM mode (H-polarization) 2D modelings, we can overcome theeffects of shallow localized anomalies. Moreover, it is known that TM mode 2Dmodeling gives good approximations, even for the 3D resistivity structure (TING andHOHMANN, 1981; WANNAMAKER t al., 1984). We can consequently obtain areasonable 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 (seeOGAWAet 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 haveexcellent fittings. From the surface to the 200 m depth, there are resistivityvariations; these may be due to the difference in porosity or water content of basalticlava. Deeper than 200 m, all the sites are underlain by the conductive layer (15-20ohm-m), whose top is 150 m above sea level.Site 32 is located 1 km away from profile 1. The 1D inversion result shows athree 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 theconductive layer below profile 1. The depth to the conductive layer may becomedeeper as one goes farther away from the craters.3.2 Profile 2Profile 2 crosses the C craters. Figures 5(a) and 5(b) show the apparentresistivity and the phase pseudo-sections, respectively. Site 14 is just on a volcanicvent. 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 staticdistortion like profile 1.

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


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216 Y. OGAWAnd S. TAKAKURAmagnitude, respectively, compared with surrounding sites. We may expect aconductive dyke beneath site 14 at first glance. However, we have to take the effectof 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 thecomparison between the observed and calculated values; the fitness is excellent. Allthe sites are underlain by a conductive layer 10-30 ohm-m) below sea level. Beneathsite 14, the 2D model does not require a conductive dyke-like structure, but alocalized shallow


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CSAMT Measurements across the 1986 C Craters of Izu-Oshima Island Japan 217


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(a)

(b)Fig. 5. (a) Apparent resistivity pseudo-section for profile 2. Apparent resistivity values less than 50ohm-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 shallowpart (


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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 asystematic 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 todecrease the resistivity of pore fluids in the surrounding rocks; however, we cannotsee such effects from our analyses.We obtained conductive (6-30 ohm-m) layers, below the resistive lava, whichwe 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 (ISSHIKIet al.,1963); this ground water level almost corresponds to the top of the conductor belowprofile 1. TAKAHASHI t al. (1987) reported that the resistivity values of water takenfrom these geothermal wells range from 13 to 17 ohm-m, and this value iscompatible with our inferred resistivity for the deep conductive layer. Consequently,we think that the conductive layer is due to thermal water.YUKUTAKEt al. (1985) showed a resistivity structure below our profile 2 froma Schlumberger sounding before the eruption. They didn t detect the deep con-ductor, as we did; this significant resistivity change reflects the thermal activitybelow the C craters.

We got very few sites away from the C craters. The inferred resistivity structuresdiffer from those below the two profiles; i.e., below sites 32, 22, and 23, we got threelayered structures. There is an intermediate conductive (70-100 ohm-m) layerbetween the top resistive layer and bottom conductive layer; the intermediate andbottom layers may correspond to a fresh water lens (Ghyben-Hertzberg s lens), andinvading sea water, respectively. This is a typical distribution of groundwater on avolcanic island (e.g., ECKER,1976). The resistivity of basalt saturated with sea wateris thus estimated as 10-20 ohm-m. This value is similar to that of Hawaii (30ohm-m) inferred from Schlumberger soundings (ZoHDY and JACKSON,1969).Figure 9 summarizes the schematic picture relating resistivity structure togroundwater. The two profiles are lacking in this intermediate conductive layer (thefresh water lens). Hydrothermal convection may mix fresh water and invading seawater.


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220 Y OGAWA and S TAKAKURA


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

Fig. 9. Schematicstructure of Izu-Oshima sland obtained from the presentCSAMT study.

TAKAHASHI et al. 1987) analyzed the chemical components of water samplestaken from wells on Oshima Island. They thought the groundwater near the wellsmust be isolated and dyke-impounded, because the water samples from the wells inFig. 1 were chemically different from other groundwater. In contrast, our CSAMTresult below the two profiles requires a horizontally connected fluid path; we couldnot find isolated water pockets. The form of groundwater distribution may bedifferent between the well locations and our two profiles.Lastly, we discuss two possible reasons why we could not sense the dyke-likestructure. One is that our measurements were in the TM mode only. The TM modeis rather insensitive to vertical dyke-like structures, so further study requires TE


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222 Y. OGAWAnd S. TAKAKURAmode excitation as well, or tensor AMT measurements. The other reason is that thedyke itself or dyke-like structure is too thin. A thin dyke could not heat surroundingrocks to reduce their resistivity.

5. ConclusionWe 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 belowresistive lava. This deep conductive layer corresponds to thermal water, comparingwith nearby wells. We found shallow conductive bodies beneath the two profileswhose locations correspond to an old vent (8th century) and the 1986 vent,respectively. We think these contain meteoric water contained in fracture zones dueto magmatic intrusion.We expected to see a dyke-like structure due to heated rocks surrounding thedyke, however, we did not obtain affirmative evidences for dyke-like structures. Thiswas partly because we used TM mode excitation, and partly because the dyke hadn ttransferred enough heat to the surrounding rocks to decrease resistivity.Away from the two profiles, we found three layered structures: (top) resistivelava, (middle) less resistive basalt saturated with fresh water, and (bottom) deepconductive basalt saturated with sea water. This is a typical hydrological structure ofa volcanic island. On the other hand, the two profiles lacked the three layerstructure. 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 he Oshima Volcanic Observatory,Earthquake Research Institute, Universityof Tokyo for their cooperation. We would like tothank 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 anonymousreferees much improved he manuscript.Computer facilitieswere at the Research Information Processing System, he AgencyofIndustrial Science and Technology (IBM3081K), and at the Geological Survey of Japan

(SIGMA system, IBM4341).

REFERENCESBARTEL,.C. and R.D. JAcossoN,Resultsof a controlled-sourceudiofrequency agnetotelluricsurvey t the Puhimau hermal rea, KilaueaVolcano,Hawaii,Geophysics,2,665-677, 987.BERDICHEVSKY,.N, and V.I. DMITRIEV,istortion f magnetic nd electricalieldsby nearsurfaceinhomogeneities,cta Geodat.,Geophys. t Montanist.Acad. Sci. Hung. Tumus,11, 447-483,1916.ECKER,.,Groundwaterehaviourn Tenerife, olcanicsland Canary sland,Spain), . Hydrology,8,73-86,1976.GOLDSTEIN,. A. and W. D. STRANGWAY,udio-frequencyagnetotellurics ith a grounded ipolesource,Geophysics,0,669-683, 975.ISSHIKI,., Geology f the OShimadistrict,Quadrangle eries, cale1:50,000, eol.Surv.Japan, 133pp., 1984 inJapanesewith English bstract).ISSHIKI,., K. NAKAMURA,. HAYAKAWA,. HIRASAWA,. YUKUTAKE,. ARAI, nd B. IWASAKI,Structureof calderaof Oshimavolcano, zu, as revealed y drilling,Kazan,8, 61-106,1963 in


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CSAMT Measurements across the 1986 C Craters of Izu-Oshima Island, Japan 223Japanese with English abstract).

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Island area, Bull. Geol. Surv. Japan, 42, 719-730,1987 in Japanese with English abstract).TING, S. C. and G. W. HOHMANN, ntegral equation modeling of three dimensional magnetotelluric

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