Abstract
A long-offset transient electromagnetic (LOTEM) survey was carried
out on Merapi volcano, Indonesia, in 1998. LOTEM data have been recorded
at 41 receiver locations which cover a 10 km W-E and a 15 km S-N
profile. The signals were transmitted at four locations on the south,
west, and north sides of Merapi. The data were interpreted with one-dimensional
(1-D) inversions. In addition, two particular features were investigated
with 3-D modeling. On the south flank the magnetic field data show
strong 3-D distortions consistently over the profile which can only
be explained by a conductive near-surface structure, like a fracture
filled with conductive fluids. The simulation of topographic effects
shows that the interpretation is not significantly affected. On both
profiles the most striking feature is a conductive layer of 20 Ohm
m at depths of 500-1000 m below the surface and a thickness of 1-2
km. The cause of the increased conductivity may be different for
the summit area, the intermediate zone, and at the flanks: Below
the summit, in vicinity of the conduit, the decrease in resistivity
is produced either by hydrothermal fluids, by partial melts or rocks
altered by the hydrothermal system, or by a combination. In the intermediate
zone between the conduit and the flanks, either alteration or hydrothermal
fluids may be the source of the conductivity increase. Fluids seem
to be the most likely cause for the conductive layer at the flanks
and the west-east striking anomaly. From the resistivity of the conductive
layer and the typical porosity of volcanic rock we estimate fluid
resistivities of 0.2-1 Ohm m.
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