Proceedings of the 10th Intl Conf. “Problems of Geocosmos” (Oct 6-10, 2014, St. Petersburg, Russia)
TWO-DIMENSIONAL INVESTIGATION OF THE EARTH CRUST
SUBSURFACE LAYER BY METHOD OF INTERNAL SLIDING CONTACT
V.E. Kolesnikov1, A.A. Skorokhodov1
1
Geological Institute of the Kola Science Center of Russian Academy of Sciences, 184209 Apatity,
Russia, e-mail: [email protected]
Abstract. Method of internal sliding contact (MISC) is a DC investigation method of Earth crust
subsurface, which combines elements of profiling and sounding. Variation of distance between
current electrode A and receiver electrode M allows the carrying on the investigations of this kind.
Media sounding is conducted when the signal on receiver electrodes is registered and AM distance
is varied. The electrode array movement along a profile permits to carry on profile
investigations.The variation of AM distance is realized using the multi-electrode array. Electric
current is injected to the ground through current dipole AB, and multi-channel digital receiver
system records the signal from the ground. The MISC modeling and data processing was done by
using ZondRes2D software that was designed by A.E. Kaminskiy. The software permits to carry on
2D-interpretation (2D-inversion) of the DC resistivity and IP data taking into consideration
topography and prior information. The efficiency of MISC was investigated by physical modelling
in a tank installation. MISC field research equipment includes AB current dipole and receiver
system: receiver lines, switch, AD-converter and mobile PC that registers the digital signal and
saves the data obtained to HDD. Method of internal sliding contact may be useful in structural
research and in search of ore bodies.
Introduction
Method of internal sliding contact (MISC) is a DC method of investigation of Earth crust subsurface which
combines elements of profiling and sounding. It is known that registration of DC signal on receiver
electrodes with variation of distance between current electrode A and receiver electrode N gives information
about media properties variation to depth (sounding). Moving of fixed electrode distance array along a
profile permits to carry out investigations on earth-air surface (profiling). Thus simultaneous Earth crust
survey along a profile and depth investigation on each point of the profile allows to conduct two-dimensional
research of geoelectric cross-section subsurface layer.
Two-dimensional investigations of Earth crust subsurface using multi-electrode arrays were conducted at the
first time in 1970-1980s. Manual and automatic switch multi-electrode survey systems are described in the
works [Barker, 1981; Dahlin, 1989]. Further development of research technique is described in proceedings
[Griffiths, D.H., Barker, R.D., 1993; Bobachev et al., 1996]. Summary overview of development of the
method is presented in publication [Dahlin, 2001]. Method of internal sliding contact was designed by A.A.
Zhamaletdinov. Features and application of the method are described in publications [Zhamaletdinov et al.,
1976; Zhamaletdinov et al., 1995].
Field research technique
Practical MISC geological media investigations are conducted with variation of distance between A and M
electrodes in multi-electrode electrical profiling array. The array is current dipole AB and receiver array that
includes N electrode and some M electrodes on different distances from N electrode. Electric current is
injected to the ground through the current dipole; signal is registered by multi-channel receiver system.
Field research equipment which applied in Geological Institute if KSC RAS includes current dipole and
receiver system. Generator injects low-frequency pulse current to the ground through the AB current dipole.
Signal is registered on multi-channel system that connected with receiver electrode array. The system
includes switch, AD-converter and field PC that registers the signal and saves the data to HDD.
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Proceedings of the 10th Intl Conf. “Problems of Geocosmos” (Oct 6-10, 2014, St. Petersburg, Russia)
Fig.1. MISC field equipment for investigations by DC resistivity and IP methods (GI KSC RAS)
MISC data interpretation technique
The one of the means of MISC field data processing and numerical modeling of research process is
ZondRes2D software that is designed by A.E. Kaminskiy. The software permits to carry on 2D-interpretation
(2D-inversion) of the DC resistivity and IP data taking into consideration topography and prior information.
Observed data interpretation by means of ZondRes2D is multi-stage process. Full interpretation process can
be divided to three stages: data input, data interpretation, data output.
At the stage of data input some actions are performed:
1)
2)
3)
4)
data file creation;
interpretation model mesh construction;
prior information input;
inversion parameters input;
Observed data file contains information about electrode relative coordinates, apparent resistivity values,
registered signal values related to the current value, apparent polarizability values if IP measurements were
conducted, weight values for each measurement. The file can also contain the topography and media
reference model data.
Interpretation model is divided to horizontal layers according to array geometry. Lower model boundary fits
the maximum depth of investigation. Number of layers is equal to number of AM distances of array. First
layer thickness is the depth of investigation on minimal AM distance. Thickness of the rest layers is
increased so that the sum of all layer thicknesses will be equal to maximum depth of investigation.
Prior information input is well-logging and lithology data addition.
Inversion parameters are inversion algorithm, iterative process stop criteria, smoothing. The parameters are
entered immediately before inversion process start.
The second stage, or interpretation stage, begins from calculation of apparent resistivity values on each point
of profile over the model. This is forward problem solution. This solution is followed by calculation of
difference between observed and model apparent resistivity values (misfit), correction of model according to
misfit values, new calculation of model apparent resistivity. The iterative process continues until the misfit
value required to all the profile is reached.
If the inversion results don’t correspond to the prior information, it is necessary to correct the inversion
parameters. If the inversion process doesn’t bring to required misfit value, or the misfit value is decreased
very slowly, inversion parameters should also be corrected.
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Proceedings of the 10th Intl Conf. “Problems of Geocosmos” (Oct 6-10, 2014, St. Petersburg, Russia)
If the inversion results are in accordance to all requirements, they are imaged as a resistivity cross-section;
also they can be saved to HDD as ZondRes2D special file or exported to external software data file such as
text file and Microsoft Excel file.
Numerical modeling process also contains of several stages. At the first stage it is necessary to create data
file that contains information about all electrode positions over a model medium. At the second stage the
model mesh is constructed. To create the mesh it is necessary to enter horizontal and vertical relative
coordinates of mesh nodes, and also to note whether a node is matched a coordinate of any electrode on
profile. After the mesh construction the matrix of cell resistivity values is entered. The last stage is forward
problem solution.
It should be noted that the model can be constructed by two ways: addition the model information to the data
file or creation the model immediately in software window.
One of the main field data interpretation problems is non-linear electrode arrangement on terrain and, as a
consequence, distortions in array geometry. Distortions in geometric factor values are a cause of apparent
resistivity or polarizability calculation mistakes. In order to solve this problem the ZondRes2D opportunity
to appoint the weight values to observed data is used. Observed geometric factor values are calculated from
registered electrode absolute coordinates, and then the residual between theoretical and observed values for
each measurement is calculated. The reverse value of the residual is appointed to each measurement as a
weight value.
Other problem is inversion process parameters selection. This problem is solved by multiple conduction of
test inversion process with different algorithms until the required misfit value is reached. Also, knowingly
false values should be deleted from observed data.
Physical and mathematical modeling of MISC investigations
During the ZondRes2D preparation for field data interpretation, its opportunities testing and adapting were
conducted in the context of MISC investigation. The data of MISC physical modeling in test layout were
used as test data. The layout is an electrolyte-filled tank with models of structural geological objects. This
layout was used to prepare the electrical prospecting equipment EMAK-1 to field investigations and to study
the MISC investigation efficiency in search of conductive objects and structural mapping.
Fig.2. Installation for physical modeling and testing of DC resistivity and IP multi-electrode survey systems
During the physical modeling four models were created – three models of conductive steeply
dipping dike on different depths, and one model of two steeply dipping dikes on some distance
between them. Physical models was created by means of metal plates buried to the tank. Models of
single dikes were constructed to determine the maximum depth of investigation with the array. The
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Proceedings of the 10th Intl Conf. “Problems of Geocosmos” (Oct 6-10, 2014, St. Petersburg, Russia)
aim of construction two dikes model was to determine an opportunity of their separate allocation on
resistivity cross-section.
The studying MISC array is not symmetrical, therefore the location of conductivity or resistivity
anomalies related to structural elements differs from elements’ true location. To compensate the
distortions caused by array dissymmetry, profile observations in «two traverses» should be
conducted: «forward traverse», in which the array moves from relative start of profile to relative
finish, and «reverse traverse», in which the array moves from relative finish of profile to relative
start.
Observations over the physical model medium were conducted only in «forward traverse», that was
a cause a mistake in horizontal localization of structural element. The mistake evaluation was not
conducted because precise horizontal localization wasn’t aim of physical modeling.
Results of interpretation for two models, in which the structural elements is localized, by means of
ZondRes2D software are presented.
Fig.3. Model of steeply dipping dike, depth to top 1 cm. ZondRes2D interpretation results. Upper part –
observed apparent resistivity plot, medium part – calculated apparent resistivity pseudosection, lower part –
interpretation model resistivity cross-section. Structural element is marked as a white stripe.
Fig.4. Model of steeply dipping dike, depth to top 2.5 cm. ZondRes2D interpretation results.
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Proceedings of the 10th Intl Conf. “Problems of Geocosmos” (Oct 6-10, 2014, St. Petersburg, Russia)
Structural elements in other two models were not localized. Depth of model dike 4.5 cm is larger than the
maximum depth of investigation. Two model dikes on distance 2.5 cm were not separated and formed an
uniform conductivity anomaly.
In addition to physical modeling material, numerical modeling by means of ZondRes2D for «two traverses»
was conducted. Two models were created. Parameters of the models were matched to physical models. Also
one additional model of two conductive dikes was created, with distance between dikes that larger than
distance between two dikes in physical model. Background resistivity was 700 Ohm.m, elements’ resistivity
was 50 Ohm.m.
Fig.5. Forward problem solving results in ZondRes2D for model of steeply dipping dike, depth to top 1 cm.
Upper part – calculated apparent resistivity pseudosection, lower part – model geoelectric cross-section.
Fig.6. Forward problem solving results in ZondRes2D for model of steeply dipping dike, depth to top 2.5
cm.
Fig.7. Forward problem solving results in ZondRes2D for model of two steeply dipping dikes, depth to top 1
cm, distance between the dikes 9 cm.
Field data interpretation results
MISC investigations were carried out during the field excursion research in the north part of Pechenga
structure as a part of investigation works complex to search of homologues of Archaean section of Kola
Superdeep Borehole on earth-air surface.
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Proceedings of the 10th Intl Conf. “Problems of Geocosmos” (Oct 6-10, 2014, St. Petersburg, Russia)
Fig.8. Field data interpretation results in ZondRes2D for MISC investigations in Pechenga area.
Conclusions
Combination of elements of profiling and sounding in the method of internal sliding contact permits
to carry on two-dimensional investigation of Earth crust subsurface layer. This investigation allows
to identify borders of structural geological elements and to localize the elements to depth, that is
useful in structural research.
Also, two-dimensional Earth crust subsurface studies with MISC array are less time-consuming
than the same investigations with other DC resistivity method arrays.
ZondRes2D software is useful for interpretation and modeling of two-dimensional investigations of Earth
crust subsurface with MISC array or with other arrays. However, work with this software requires thorough
analysis of the data observed and operator’s active participation in interpretation process.
Gratitude
Authors of this paper express a great appreciation and gratitude to Organizing Committee of the conference
«Problems of Geocosmos» for opportunity given to submit the results of investigations, to participants of the
Conference for discussion and criticism, and to scientific advisors Abdulkhay A. Zhamaletdinov and
Aleksandr N. Shevtsov for overall aid and support and wise direction of research carrying out.
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