Bull. Volcanol. Soc. Japan
Vol. /0 (,*++) No. . · /, pp. +/-ῌ+0*

LetterControlled Source Audio-frequency Magnetotelluric (CSAMT) andTime Domain Electromagnetic (TDEM) Resistivity Measurements
at Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan
Yoshihiko GOTOῌ and Akira JOHMORIῌῌ
(Received February +*, ,*++ ; Accepted August 3, ,*++)
Controlled source audio-frequency magnetotelluric (CSAMT) and time domain electromagnetic (TDEM)
resistivity measurements were performed in April ,**2 at the Noboribetsu Geothermal Field, Kuttara Volcano,
Hokkaido, Japan. Both sets of measurements were carried out using a high-precision electromagnetic system,
Geo-SEM, controlled by GPS (Global Positioning System). The ,ῌ, km survey area covered the entire
geothermal ﬁeld and included 00 measurement sites. Interpretation of the CSAMT and TDEM data revealed thesubsurface resistivity structure shallower than +,.** m below sea level (b.s.l.). The most prominent feature of theresistivity structure is a region of low resistivity (῍+* W · m) beneath the geothermal ﬁeld. The low resistivity
varies in lateral extent and outline at di#erent depths. At ,** m above sea level, it comprises two domains withlong axes oriented NNWῌSSE. From * to ,** m b.s.l., the two resistivity lows combine, forming a large
semicircular low of +,/**ῌ+,/** m in lateral extent. From .** to 0** m b.s.l., the low resistivity has an irregularoutline and includes linear low-resistivity zones trending NNWῌSSE and ENEῌWSW. From 2** to +,.** m b.s.
l., a linear NNWῌSSE-trending region of low resistivity is apparent in an irregular overall pattern of resistivity.
In a NῌS vertical cross-section, the region of low resistivity extends vertically for more than +,.** m. We
attribute the low resistivity beneath the geothermal ﬁeld to the presence of conductive clay minerals produced byhydrothermal alteration, which was in turn induced by high-temperature geothermal ﬂuid ascending along
fractures.
Key words : resistivity structure, controlled source audio-frequency magnetotelluric (CSAMT) survey, time
domain electromagnetic (TDEM) survey, Noboribetsu Geothermal Field, Kuttara Volcano+. Introduction
Resistivity surveying provides valuable informationon underground geological structures at active volcanoes
and within geothermal ﬁelds (e.g., Risk et al., ,**- ;Aizawa et al., ,**2 ; Srigutomo et al., ,**2). Suchdata are particularly useful for understanding hydrothermal systems, as the resistivity of volcanic rocks
shows a marked change with the presence of alteration
minerals and thermal waters. Resistivity data are
therefore essential for scientiﬁc studies of volcanic activity and for geothermal exploration.
We performed controlled source audio-frequency magnetotelluric (CSAMT) and time domain electromagnetic (TDEM) resistivity measurements at Noboribetsu
Geothermal Field, Kuttara Volcano, Hokkaido, Japan.
The CSAMT method employed here used electric currents at frequencies from + to 2+3, Hz. The TDEM
῎College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology,
Mizumoto-cho ,1ῌ+, Muroran, Hokkaido */*ῌ2/2/, Japan.
῎῎Neo Science Co., Ltd., Tarui .ῌ,ῌ-*, Sennan, Osakamethod employed here was the long o#set transient
electromagnetic method (LOTEM ; Strack, +33,). TheNoboribetsu Geothermal Field is one of the major geothermal ﬁelds in Japan, and resistivity measurements area promising tool for studying subsurface geologic structures within the geothermal ﬁeld and for examining its
hydrothermal systems. Sixty-six receiver stations weredistributed in a ,ῌ, km area covering the whole geothermal ﬁeld, and processing of the CSAMT andTDEM data revealed the subsurface resistivity structure
shallower than +,.** m below sea level (b.s.l.). This
paper describes the results of the resistivity measurements and discusses the nature of subsurface geologic
structures beneath the geothermal ﬁeld.
,. Noboribetsu Geothermal Field
The Noboribetsu Geothermal Field lies in the western/3*ῌ*/,+, Japan.
Corresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp