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Geophysics |
Geophysics
investigates the earth with the help of physical methods
(e.g. measurements of electromagnetic fields) and helps to understand
the interior processes of the inner earth (e.g. magnetism,
earthquakes), surface-near processes (e.g. the impact of seismic waves
to buildings), surface-near investigation of structures for engineering
purposes (e.g. waste site investigation) and processes on the planets
of our solar system.
Mostly, non-invasive, destruction-free methods are used and large areas
can be investigated.
Geophysics supports (hydro-) geological and chemical
investigations in a very good way. In geology, e.g. drill and soil
samples can be analysed very detailed but only for a very small spot /
area.
Geophysics offers the opportunity to investigate the
space
between these sample points in a fast and large vertical and horizontal
scale. If geophysical measurements are calibrated with e.g. drill hole
information, the space between drill holes can be much better
interpreted in a geological way, in contrast to interpolations of
geological models on the basis of drill hole information alone.
Such an integrated way was used at the EU-project NORISC
(www.norisc.com). This project showed that with the help of an
interdisciplinary waste site investigation, a large reduction of time
and costs is possible. With the investigation of the underground,
geophysics locates possible "anomalies" which are given to (hydro-)
geologists and chemists for a specific, target-oriented investigation
in contrast to a standard investigation with an equally spaced sampling
grid of the area.
This interdisciplinary concept can be easily and
successfully extended to questions of larger regions, e.g. for regional
groundwater matters.
Our working group wants to intensify the relations with the other earth
disciplines and tries to calculate relevant parameters for e.g.
hydro-geology directly.
Such an example is the so called "Induced Polarisation" method which
measures the attenuation of currents within the earth. With the help of
this method, the k-factor can be determined for a larger area.
A further example are geophysical electrical resistivity / conductivity
measurements which are used to investigate the structure of e.g.
aquifers.
The following table gives an overview of the investigated fields and
related methods in geophysics.
|
physical field |
passive
methods |
active methods |
|
gravity field |
gravimetry |
|
|
magnetic field (static) |
magnetics |
|
|
electrical field (static) |
Self potential |
resistivity measurements |
|
radiation |
natural radioactivity |
VLF, RMT, EM-methods,
Induced Polaristion |
seismic waves
|
seismology |
seismics |
Table
1:
Division of
methods in
Applied Geophysics
Figure 1 shows an example of the visualisation-software
GSI3D from the
NORISC-project. Aim of the investigation was to locate the source of
the hydrocarbon pollution. The upper left shows the horizontal
distribution of electrical resistivities as a result of a 3D-inversion
in a relative depth of 3 m below the surface around the ground water
table. Green und blue represent good conductors, indicating clay and
silt, red and brown are bad conductors indicating sands. The coulored
circles show the location of chemical samples, here for hydrocarbons.
Green and yellow are non polluted areas, red and purple are about the
allowed threshold level. It should be investigated if the hydrocarbons
are related to rather clay / silt or the sand. As can clearly be seen
the source of pollution is located within the clay / silt.
During rainy seasons, the hydrocarbons are washed out and reach an
aquifer which is used for groundwater extraction.
In the lower part of the figure, geological and chemical information
(vertical columns) are projected on a vertical resistivity. Blue
indicates good conductors, red bad conductors.
The coloured columns show in the left and middle part geological
information (fawn: sand; black and dark green: clay; blue: gravel, red:
stones; white/grey: no information) and in the right part the
hydrocarbon concentration (green: < 50 mg/kg, red: 100 - 300 mg/ kg,
purple: > 300 mg/kg). There is a good correlation between geology
and geophysics in the first few meters. A comparison between
hydrocarbons and resistivities indicates that near the surface, where
only sand and fillings are present, the resistivities are unusual low
which can be a hint of bacteriological activity.
In greater depth, the relation between geology and geophysics grows
weaker because of the bad signal-noise-ration of the geophysical
measurement.
In the upper right of the figure, we see some vertical and a horizontal
result of the resistivity measurements combined with borehole data
(columns).
As a result, the remediation action was changed from a water
remediation towards the excavation of the clay as the source of
pollution.
Fig. 1 shows a
screenshot from the program GSI3D. Upper
left: horizontal distribution of resistivities in about 3 m depth
including related hydrocarbon locations (coulored circles). In the
upper right we have a 3D view at some vertical and horizontal results
from the resistivity measurements and the location of boreholes. In the
lower part we see a result from a 2D inversion of a vertical
resistivity profile. Geological and chemical borehole data is projected
on the profile.
For my Ph.D. thesis, the GSI3D is extended for a better in-field
interpretation and calibration of geophysical data. This is realized by
the ability of calibrating e.g. resistivity data from DC or RMT
measurements with geological drill log data or resistivity Direct Push
results. Thus, a more accurate geological underground model is created
than by a single method alone and the position of contaminants and
possible contamination plumes can be located with much higher accuracy.
List of add-ons for my Ph.D. for the GSI3D:
-
modules for 2D-DC-resistivity- and
2D-RadioMagnetoTelluric- (RMT) and 1D-TransientElectroMagnetics (TEM) inversions
-
modules for 2D-DC-resistivity- and
2D-RadioMagnetoTelluric- (RMT) grid creators
-
routines for loading and displaying 1D data (DC,
TransientElectroMagnetics, DirectPush), 2D data (DC resistivity & RMT ) and 3D
data (normally DC; can also be used for different kind of data)
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List of Publications
-
Perk, M., Tezkan, B., Hoerdt, A.
(2003). Interdisziplinaere Altlastenuntersuchung (das EU-Projekt NORISC). 20. EMTF-Kolloqium der Deutschen Geophysikalischen Gesellschaft,
Abtract-Band , S. 63 - 77
-
Perk, M., Tezkan, B., Hoerdt, A.
(2003). Interdisziplinäre Untersuchung einer Altlastflaeche in Köln (NORISC-Projekt).
63. Jahrestagung der Deutschen Geophysikalischen Gesellschaft. Abstract-Band,
S. 400
-
Perk, M., Tezkan, B., Hoerdt, A.
(2004). Interdisziplinary Waste Site Investigation in Balassagyarmat
(Hungary). EAGS / Near Surface Geopyhsics-Meeting. Extended Abstracts Book,
B036
-
Perk, M., Tezkan, B., Sobisch, H.-G.
(2004). Der Feld-Einsatz der Visualisierungssoftware GSI3D (Geological
Surveying and Investigation in 3D) im EU-Projekt NORISC am Beispiel der
Testflaeche Balassagyarmat (Ungarn). Schriftenreihe der Deutschen
Geologischen Gesellschaft (GeoLeipzig 2004, Abstract-Band), Heft Nr. 34, S.
386
-
Perk, M., Tezkan, B., Hoerdt, A.
(2004). Interdisziplinaere Untersuchung einer Altlastflaeche in Balassagyarmat / Ungarn (NORISC-Projekt). 64. Jahrestagung der Deutschen
Geophysikalischen Gesellschaft. Abstract-Band, UI08
-
Perk, M., Tezkan, B., Sobisch, H.-G.
(2005). Kalibierung geophysikalischer Daten auf kontaminierten
Flaechen mit Hilfe der Visualisierungssoftware GSI3D. 65. Jahrestagung der
Deutschen Geophysikalischen Gesellschaft. Abstract-Band, S. 289
-
Kremer, M., Perk, M.,
(2005). Minimierung des Restrisikos durch Altlast-untersuchungen. BEW
(Bildungszentrum für Entsorgung und Wasser-wirtschaft), Forum Bodenschutz,
September 2005
- Perk,
M., Tezkan, B., Sobisch, H.-G. (2006):
Infield-Kalibrierung geophysikalischer Daten auf
kontaminierten Flächen mit Hilfe der Visualsierungssoftware GSI3D.- 66.
Jahrestagung der Deutschen Geophysikalischen Gesellschaft. Abstract-Band.
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