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Titel |
The resistivity structure of the North Alex Mud Volcano as derived from the interpretation of CSEM data |
VerfasserIn |
Sebastian Hölz, Marion Jegen |
Konferenz |
EGU General Assembly 2010
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 12 (2010) |
Datensatznummer |
250040309
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Zusammenfassung |
Active mud volcanoes, where changing salinities of pore fluids, large temperature gradients
and occurrences of free gas are frequently observed, should potentially exhibit
significant variability in their internal resistivity structure. This is due to the fact
that the bulk resistivity is mainly determined by the porosity of sediments and the
electrical resistivity of the pore filling contained therein. The resistivity variations
may be derived from controlled source electromagnetic (CSEM) measurements.
CSEM systems consist of an electric dipole transmitter producing a time varying
source field and electric dipole receivers, which measure the earth´ s response to this
signal.
For a RWE Dea funded investigation of fluid and gas leakages at the North Alex Mud
Volcano (NAMV) - a comparatively small target with an area of about 1km2 - we have
developed a new high resolution CSEM system. The system consists of several
autonomous electric dipole receivers and a lightweight electric dipole transmitter, which
can be mounted on a small remotely operated underwater vehicle (ROV). The use
of a ROV allows for a precise placement of the transmitter, which is a necessary
prerequisite for the investigation of such a small target. Furthermore, electromagnetic
signals may be transmitted from different directions with respect to the stationary
receivers, allowing for a 3D-style tomographic experiment. In this experiment, ten
receivers were deployed over the surface of NAMV at a total of 16 receiver locations.
During three successful dives with a Cherokee ROV (Ghent University, Belgium), the
transmitter was deployed at a total of 80 locations. Here we present first quantitative
results consisting of apparent resistivity estimations from the CSEM time domain
data for each transmitter-receiver pair. The apparent resistivity map shows that the
NAMV indeed has a heterogeneous resistivity structure with apparent resistivities
varying by at least a factor of two: low apparent resistivities (- 0.8Ωm) are found
towards the center of the MV, whereas higher apparent resistivities (- 1.6Ωm)
prevail away from the center. In a second step, we interpret the time-domain data
based on 1D inversions. Good data fits can be achieved by models containing 2-3
layers. Generally, the models indicate low resistivities at the surface, which can be
associated with penetrating salt water and/or high temperatures. Toward greater
depths, increasing resistivities presumably are due to a combination of compaction of
sediments (i.e. reduced pore space), an increased presence of fresh water and possible
occurrences of free gas. For some 1D models, the increase in resistivity exceeds a
factor of 10 or more and layer interfaces are indicated down to depths of up to
70m.
The derived resistivity variations observed at the NAMV will be interpreted in
conjunction with temperature (Feseker, this session), fluid flow (Brückmann et al., this
session) and seismic data (Bialas et al., this session) acquired. Temperature variations
measured in the upper few meters are related to fluid flow, where high temperatures are
indicative of upwelling fluids of low salinity and low temperature of either a downward flow
of saline fluids or no flow activity. This type of surface measurement constitutes an
integrative fluid flow gauge, which we can resolve vertically with our resistivity
models. Seismic data yield a background structure to our resistivity model. New
analysis of seismic data shows that seismic activity may also be linked to fluid
flow activity, which we aim to match with resistivity variations and oscillations,
which were observed in the electric and magnetic fields (Lefeldt et al., this session). |
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