Bülent Tezkan

A review of environmental applications of quasi-stationary electromagnetic techniques

Electromagnetic (EM) techniques are the most commonly used geophysical methods in mineral exploration. However, the use of EM measurements for environmental and engineering applications like the detection of contaminant plumes or the exploration of waste sites is relatively new.The reason for the success of the application of EM methods to environmental problems lies in the variation of conductivity caused by different geometry of pore fluids and clay contents in rocks, and by the presence of organic and inorganic contaminants.Many EM methods/instruments used for mapping near surface geology exist and nowadays they play a central role in environmental geophysics. In general, these methods can be classified in two blocks: EM methods using a plane wave source of excitation and EM methods using a controlled source like a magnetic dipole or a loop source. The Very Low Frequency (VLF, VLF-R) and Radiomagnetotelluric (RMT) methods are chosen as representative methods for plane wave techniques, while horizontal loop EM methods operating in low induction numbers (EM31, EM34) and Transient Electromagnetic methods (TEM) are chosen as representatives of magnetic dipole or loop source techniques. Basic principles, advantages and disadvantages of each technique as well as their connection to specific environmental problems will be discussed.Different successful applications of these methods are reported in the literature. However, this review will focus on three major subjects: waste site exploration, detection of contaminated earth layers, and groundwater exploration. Case histories are presented illustrating the suitability of EM methods for solving such problems.

3D inversion of a scalar radio magnetotelluric field data set

A radio magnetotelluric (MT) field data set, acquired in scalar mode, over a buried waste site has been successfully analyzed using a 3D MT inversion scheme using nonlinear conjugate gradients. The results of this analysis demonstrate the utility of the scheme where more than 4800 data points collected on multiple measurement profiles have been inverted simultaneously. The resulting image clearly detects the buried waste; when receiver profiles cross pit boundaries, the image maps the lateral extent of the pit. However, the base of the pit is poorly resolved, and depends upon the starting model used to launch the inversion. Hence, critical information on whether contamination is leaching into a resistive gravel bed lining the base of the pit, as well as the deeper geological horizons consisting of brown coal, clay, and tertiary sands, is inconclusive. Nevertheless, by incorporating within the inversion process a priori information of the background media that is host to the waste, sharper images of the base of the pit are obtained, which are in good agreement with borehole data. The 3D analysis applied in this paper overcomes previous limitations in the radio magnetotelluric (RMT) method using 2D data analysis and inversion. With 3D analysis, it is unnecessary to make assumptions regarding geological strike, and near-surface statics can be accommodated in both source polarizations. Our findings also indicate that 2D MT interpretation can overestimate the pits depth extent. This may lead to the erroneous conclusion that the geological horizons beneath the pit have been contaminated.

A joint application of radiomagnetotellurics and transient electromagnetics to the investigation of a waste deposit in Cologne (Germany)

Abstract A joint radiomagnetotelluric (RMT) and transient electromagnetic (TEM) survey was carried out on a profile over an old waste deposit in Cologne to map lateral and vertical boundaries as well as inhomogeneities within the deposit itself. For the RMT method, three pairs of frequencies between 16.4 and 198 kHz, from radio transmitters located in roughly perpendicular directions, were used. The observed apparent resistivity ( ϱ a ) and phase (φ) data clearly show the lateral boundary of the deposit to be characterized by strongly decreasing ϱ a values for all frequencies. The penetration depth for the lowest frequency is about 25 m outside and only 12 m inside the deposit. The RMT data were interpreted by one- and two-dimensional inversion techniques. On the waste site there is a resistive overburden over the highly conductive waste. Inhomogeneities within the deposit could also be resolved. However, the bottom of the deposit could not be detected by the RMT method because of the low penetration depths observed over the highly conductive waste site. TEM measurements on the same profile, however, allowed to achieve exploration depths between 10 and 100 m. Interpretation using one-dimensional conductivity models indicates the lower boundary of the waste site (20 m) and completes the two-dimensional conductivity depth section of the waste deposit.

Two-dimensional radiomagnetotelluric investigation of industrial and domestic waste sites in Germany

Abstract Radiomagnetotelluric surveys were carried out on two different waste sites in order to map them and to detect the surrounding hosts. One was filled with industrial waste and the other with household refuse. Both had been recultivated. Powerful military and civilian radiostations (10–300 kHz) located parallel and perpendicular to the strike direction of both waste sites served as transmitters. Hence, apparent resistivity and phase data were observed for several selected frequencies. Using this field setup, the data were associated to the E- and B-polarisation directions of the magnetotelluric field. The lateral borders of both waste sites are located accurately in the field. They are characterized by strongly decreasing apparent resistivities observed at nearly all frequencies at the border between the waste site and the undisturbed geology. The data were quantitatively interpreted by conductivity models using a 2D inversion algorithm. The derived 2D conductivity models give information about the vertical extent of the studied waste sites and about the structure of the surrounding hosts.

Interpretation of long‐offset transient electromagnetic data from Mount Merapi, Indonesia, using a three‐dimensional optimization approach

In the years 1998, 2000, and 2001, long-offset transient electromagnetic (LOTEM) surveys were carried out at the active volcano Merapi in Central Java. The measurements investigated the conductivity structure of the volcanic edifice. Our area of interest, which is below the summit and the upper flanks, was investigated using horizontal and vertical magnetic field time derivative data from seven transmitter-receiver setups. Because of topography and a three-dimensional (3-D) underground structure, a 3-D interpretation is used. The method optimizes few parameters of a 3-D model by a stable least squares joint inversion of the data, providing sufficient resolution capability. Reasonable data fits are achieved with a nonhorizontally layered model featuring a very conductive basement below depths of 1.5 km. While hydrothermal alteration is also considered, we tentatively explain the high conductivities by aqueous solutions with relatively high salt contents. A large magma body or a small superficial reservoir below Merapis central volcanic complex, as discussed by other authors, cannot be resolved by the LOTEM data.

Signal detectability of marine electromagnetic methods in the exploration of resistive targets

We compare selected marine electromagnetic methods for sensitivity to the presence of relatively thin resistive targets (e.g., hydrocarbons, gas hydrates, fresh groundwater, etc.). The study includes the conventional controlled-source electromagnetic method, the recently introduced transient electromagnetic prospecting with vertical electric lines method, and the novel marine circular electric dipole method, which is still in the stage of theoretical development. The comparison is based on general physical considerations, analytical (mainly asymptotic) analysis, and rigorous one-dimensional and multidimensional forward modelling. It is shown that transient electromagnetic prospecting with vertical electric lines and marine circular electric dipole methods represent an alternative to the conventional controlled-source electromagnetic method at shallow sea, where the latter becomes less efficient due to the air-wave phenomenon. Since both former methods are essentially short-offset time-domain techniques, they exhibit a much better lateral resolution than the controlled-source electromagnetic method in both shallow sea and deep sea. The greatest shortcoming of the transient electromagnetic prospecting with vertical electric lines and marine circular electric dipole methods comes from the difficulties in accurately assembling the transmitter antenna within the marine environment. This makes these methods significantly less practical than the controlled-source electromagnetic method. Consequently, the controlled-source electromagnetic method remains the leading marine electromagnetic technique in the exploration of large resistive targets in deep sea. However, exploring laterally small targets in deep sea and both small and large targets in shallow sea might require the use of the less practical transient electromagnetic prospecting with vertical electric lines and/or marine circular electric dipole method as a desirable alternative to the controlled-source electromagnetic method.

Multi-dimensional Interpretation of Radiomagnetotelluric and Transient Electromagnetic Data to Study Active Faults in the Mygdonian Basin, Northern Greece

The Mygdonian Basin is located about 45 km NE of the city center of Thessaloniki in northern Greece. We conducted an electromagnetic (EM) survey that covers parts of the epicenter area between Lake Volvi and Lake Langada of the local 1978 earthquake that caused structural damage in the city center of Thessaloniki. Ambient noise measurements in the basin strongly suggest a complex 3-D tectonic setting. Hence, near-surface electromagnetic measurements were carried out to map the local fault pattern in the research area. We conducted radiomagnetotelluric (RMT) and transient electromagnetic (TEM) measurements along eight profiles, giving a total number of 443 RMT and 107 TEM soundings to study the spatial electrical conductivity distribution. In this context, RMT data are used to study the shallow conductivity structure down to about 35 m, whereas the TEM data explore the conductivity distribution down to a depth of 200 m. The 1-D and 2-D interpretation of RMT and TEM data indicate a local fault pattern in the survey area that was also studied by 3-D modeling of the RMT data.

Appraisal of a new 1D weighted joint inversion of ground based and helicopter‐borne electromagnetic data

ABSTRACT In order to couple spatial data from frequency‐domain helicopter‐borne electromagnetics with electromagnetic measurements from ground geophysics (transient electromagnetics and radiomagnetotellurics), a common 1D weighted joint inversion algorithm for helicopter‐borne electromagnetics, transient electromagnetics and radiomagnetotellurics data has been developed. The depth of investigation of helicopter‐borne electromagnetics data is rather limited compared to time‐domain electromagnetics sounding methods on the ground. In order to improve the accuracy of model parameters of shallow depth as well as of greater depth, the helicopter‐borne electromagnetics, transient electromagnetics, and radiomagnetotellurics measurements can be combined by using a joint inversion methodology. The 1D joint inversion algorithm is tested for synthetic data of helicopter‐borne electromagnetics, transient electromagnetics and radiomagnetotellurics. The proposed concept of the joint inversion takes advantage of each method, thus providing the capability to resolve near surface (radiomagnetotellurics) and deeper electrical conductivity structures (transient electromagnetics) in combination with valuable spatial information (helicopter‐borne electromagnetics). Furthermore, the joint inversion has been applied on the field data (helicopter‐borne electromagnetics and transient electromagnetics) measured in the Cuxhaven area, Germany. In order to avoid the lessening of the resolution capacities of one data type, and thus balancing the use of inherent and ideally complementary information content, a parameter reweighting scheme that is based on the exploration depth ranges of the specific methods is proposed. A comparison of the conventional joint inversion algorithm, proposed by Jupp and Vozoff (1975), and of the newly developed algorithm is presented. The new algorithm employs the weighting on different model parameters differently. It is inferred from the synthetic and field data examples that the weighted joint inversion is more successful in explaining the subsurface than the classical joint inversion approach. In addition to this, the data fittings in weighted joint inversion are also improved.

Investigation of the Azraq sedimentary basin, Jordan using integrated geoelectrical and electromagnetic techniques

The Eastern Mediterranean has been used as a passageway for human migration from Africa to the Middle East, the Balkans and Europe. The Azraq basin in the eastern desert of Jordan has been a major spot for human settlements since the middle Pleistocene. The former lake in the basin centre has developed to a hyper-saline alluvial mudflat, the Qa’ Al Azraq. In the mudflat thick sequences of alluvial sediments are deposited. Such sediment successions are promising archives used for reconstructing a paleoclimate. In order to identify geological structures and to derive suitable borehole locations in the area for a paleoclimatical reconstruction, the Transient Electromagnetic (TEM) and the Electrical Resistivity Tomography (ERT) methods were utilized. Two transects were investigated from the edge of the basin to the basin centre, crossing three geological formations. The data sets of both methods are interpreted by 1D and 2D inversion algorithms and appraised by inversion statistics. Previously uncertain geological boundaries are determined from geoelectrical models along both transects. Furthermore, a transition zone from fresh to saline groundwater is clearly detected.

Float-transient electromagnetic method: in-loop transient electromagnetic measurements on Lake Holzmaar, Germany

Lake sediments may serve as archives on paleoclimatic fluctuations, geomagnetic field variations and volcanic activities. Lake Holzmaar in Eifel/Germany is a maar lake and its lacustrine sediments provide paleoclimatic proxy data. Therefore, knowledge about the geometry and, especially, about the thickness of the sediments is very important for determining an optimum drilling location for paleoclimatic studies. We have developed a floating in-loop transient electromagnetic method field set up (Float-transient electromagnetic method) with a transmitter and receiver size of 18 × 18 m2 and 6 × 6 m2 respectively. This special set up enables in-loop transient electromagnetic method measurements on the surface of freshwater lakes that define the geometry and the thickness of sediments beneath such lakes thus helping to determine optimum drilling locations. Due to the modular design of the new Float-transient electromagnetic method field set up, this system can be handled by two operators and can easily be transported. Sixteen in-loop soundings were carried out on the surface of Lake Holzmaar. The transient electromagnetic method data could not be interpreted by conventional 1D inversions because of the 3D distribution of subsurface conductivity caused by the lakes geometry. Three-dimensional finite element modelling was applied to explain the observed transients and the 3D conductivity distribution beneath the lake was recovered by taking its geometry into account. The 3D interpretation revealed approximately 55 m thick sediments beneath 20 m deep water in the central part of the lake.