Stefan L. Helwig

Anatomy of the Dead Sea Transform from lithospheric to microscopic scale

Fault zones are the locations where motion of tectonic plates, often associated with earthquakes, is accommodated. Despite a rapid increase in the understanding of faults in the last decades, our knowledge of their geometry, petrophysical properties, and controlling processes remains incomplete. The central questions addressed here in our study of the Dead Sea Transform (DST) in the Middle East are as follows: (1) What are the structure and kinematics of a large fault zone? (2) What controls its structure and kinematics? (3) How does the DST compare to other plate boundary fault zones? The DST has accommodated a total of 105 km of left-lateral transform motion between the African and Arabian plates since early Miocene (similar to 20 Ma). The DST segment between the Dead Sea and the Red Sea, called the Arava/Araba Fault (AF), is studied here using a multidisciplinary and multiscale approach from the mu m to the plate tectonic scale. We observe that under the DST a narrow, subvertical zone cuts through crust and lithosphere. First, from west to east the crustal thickness increases smoothly from 26 to 39 km, and a subhorizontal lower crustal reflector is detected east of the AF. Second, several faults exist in the upper crust in a 40 km wide zone centered on the AF, but none have kilometer-size zones of decreased seismic velocities or zones of high electrical conductivities in the upper crust expected for large damage zones. Third, the AF is the main branch of the DST system, even though it has accommodated only a part (up to 60 km) of the overall 105 km of sinistral plate motion. Fourth, the AF acts as a barrier to fluids to a depth of 4 km, and the lithology changes abruptly across it. Fifth, in the top few hundred meters of the AF a locally transpressional regime is observed in a 100-300 m wide zone of deformed and displaced material, bordered by subparallel faults forming a positive flower structure. Other segments of the AF have a transtensional character with small pull-aparts along them. The damage zones of the individual faults are only 5-20 m wide at this depth range. Sixth, two areas on the AF show mesoscale to microscale faulting and veining in limestone sequences with faulting depths between 2 and 5 km. Seventh, fluids in the AF are carried downward into the fault zone. Only a minor fraction of fluids is derived from ascending hydrothermal fluids. However, we found that on the kilometer scale the AF does not act as an important fluid conduit. Most of these findings are corroborated using thermomechanical modeling where shear deformation in the upper crust is localized in one or two major faults; at larger depth, shear deformation occurs in a 20-40 km wide zone with a mechanically weak decoupling zone extending subvertically through the entire lithosphere.

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.

Near-surface properties of an active fault derived by joint interpretation of different geophysical methods - the Arava/ Araba Fault in the Middle East

The motion of tectonic plates is accommodated at fault zones. One of the unanswered questions about fault zones relates to the role they play in controlling shallow and local hydrology. This study focuses on the Arava/Araba Fault (AF) zone, the southern portion of the Dead Sea Transform (DST) in the Middle East. We combine seismic and electromagnetic methods (EM) to image the geometry and map the petro-physical properties and water occurrence in the top 100 m of this active fault. For three profiles, P-velocity and resistivity images were derived independently. Using a neural network cluster analysis three classes with similar P-velocity and resistivities could then be determined from these images. These classes correspond to spatial domains of specific material and wetness. The first class occurs primarily east of the fault consisting of ‘wet’ sand (dunes) and brecciated sediments, whereas the second class composed of similar material located west of the fault is ‘dry’. The third class lies at depth below ca. 50 m and is composed of highly deformed and weathered Precambrian rocks that constitute the multi-branch fault zone of the AF at this location. The combination of two independent measurements like seismics and EM linked by a stringent mathematical approach has thus shown the potential to delineate the interplay of lithology and water near active faults.

Vertical Dipole Electromagnetic Survey to Reduce Hydrocarbon Exploration Risk, Kakelborg - A Case Study

During the last decade electromagnetic (EM) methods have become an accepted tool to reduce offshore hydrocarbon exploration risk (Constable, 2010). As an alternative to CSEM with moving horizontal sources PetroMarker developed an offshore, time domain EM method based on vertical, stationary transmitters and receivers (Barsukov et al., 2007). In this paper we present a case study for this method over a Norwegian oil prospect. Kakelborg PL 370 lies about 25 kilometers northwest of the Statfjord Nord field in the North Sea and is operated by Wintershall Norge. A vertical-vertical time domain EM survey was conducted over the Kakelborg prospect by PetroMarker in summer of 2009. After processing and inversion of the EM data it was predicted that the prospect is dry. In July 2012 wildcat well 33/6-4 was drilled without discovering hydrocarbons.

The use of a circular electrical dipole source in hydrocarbon exploration

A circular electrical dipole creates a field consisting only of a TM mode. This results in enhanced sensitivity towards thin resistive layers. As the B-field components are zero over layered cases their measurement results in an exceptional sensitivity towards 3D structures. In field experiments over 10 hydrocarbon reservoirs unusually strong vertical magnetic field components have been observed that coincide with the known extensions of the reservoirs.

Correction to “Anatomy of the Dead Sea Transform from lithospheric to microscopic scale”

Weber, M., Abu‐Ayyash, K., Abueladas, A., Agnon, A., Alasonati‐Tašárová, Z., Al‐Zubi, H., Babeyko, A., Bartov, Y., Bauer, K., Becken, M., Bedrosian, P. A., Ben‐Avraham, Z., Bock, G., Bohnhoff, M., Bribach, J., Dulski, P., Ebbing, J., El‐Kelani, R., Förster, A., Förster, H.‐J., Frieslander, U., Garfunkel, Z., Goetze, H. J., Haak, V., Haberland, C., Hassouneh, M., Helwig, S., Hofstetter, A., Hoffmann‐Rothe, A., Jäckel, K. H., Janssen, C., Jaser, D., Kesten, D., Khatib, M., Kind, R., Koch, O., Koulakov, I., Laske, G., Maercklin, N., Masarweh, R., Masri, A., Matar, A., Mechie, J., Meqbel, N., Plessen, B., Möller, P., Mohsen, A., Oberhänsli, R., Oreshin, S., Petrunin, A., Qabbani, I., Rabba, I., Ritter, O., Romer, R. L., Rümpker, G., Rybakov, M., Ryberg, T., Saul, J., Scherbaum, F., Schmidt, S., Schulze, A., Sobolev, S. V., Stiller, M., Stromeyer, D., Tarawneh, K., Trela, C., Weckmann, U., Wetzel, U., Wylegalla, K. (2010): Correction to Anatomy of the Dead Sea Transform from lithospheric to microscopic scale. ‐ Reviews of Geophysics, 48, RG1003

A Comparison between Time Domain and Frequency Domain Inversion of Vertical-Vertical CSEM Data

Summary Vertical-vertical CSEM data collected in the Norwegian Sea with a signal optimized for time domain processing was processed in frequency domain (FD) and in time domain (TD). In FD the data was inverted using the regularized Gauss-Newton style 2D inversion code MARE2DEM. For the TD inversion a modified version of MARE2DEM was used that includes a function for the frequency to time-domain transform. A line from a recently acquired data set from the Norwegian Sea was used as test example. Results of the different processing and inversion schemes were similar. Both recovered a resistivity drop in the shallower section and an increase in resistivity in the Jurassic. An uplift of the Jurassic in the eastern part of the line was also recovered by both methods. Well log data confirms the results. Gamma Ray and resistivity logs indicate an increase of porosity in Cretaceous sediments. The TD inversion shows a more pronounced resistivity in the deeper part of the section. This may be contributed to its higher sensitivity for resistive targets at depth. Computationally the FD inversion is considerably more efficient.

A 2.5D Comparison between Two CSEM Methods

Several different CSEM technologies are currently commercially available for offshore exploration, with different characteristics and advantages. Few direct and realistic comparisons between the different methodologies have been made. In this paper we use a 2.5D finite element code capable of handling both frequency domain and time domain to investigate potential for depth penetration for two methods, horizontal source frequency domain and vertical source time domain. Synthetic data with realistic noise levels has been generated for specific simple models and then inverted. Our results show that vertical time domain solution has potential to resolve deeper targets for the assumed conditions.

Time Domain 2D CSEM Inversion with Induced Polarization

Electromagnetic (EM) methods are an accepted tool to reduce offshore hydrocarbon exploration risk. One of the companies offering such a service is the Norwegian geophysical company PetroMarker, which acquires offshore time domain CSEM data with a vertical stationary transmitter and vertical E-field sensors placed on the sea bed. In many cases a 1D inversion approach will give a good enough understanding of such data. Still, to fully exploit the information inherent in the data set and enhance the resolution of subsurface features it is necessary to account for geometrical effects such as bathymetry and the lateral extension of the resistive features through 2D or 3D inversion. Recently a new 2D time domain inversion code for CSEM data with induced polarization (IP) capability was developed based on the well-established frequency domain MARE2DEM finite element code. Accounting for induced polarization allows for a wider range of transmitter receiver offsets in a single 2D inversion run. In this paper we show some first examples of this code on synthetic and field data. It is found that a resistivity structure can fast and reliably be recovered, also in the presence of induced polarization.

Minimizing the Noise Contribution in Vertical Electric Field Measurements

The uncertainty associated with measuring the vertical electric field in marine CSEM for different receiver types was investigated. Different designs for measuring the electrical field subsea have been published by a variety of authors. Our study focusses on the measurement of the vertical field component generated by a vertical transmitter. Three different receiver types, a pendulum design that aligns the measuring dipole towards gravity, a design with three orthogonal measurement dipoles and a design based on a tetrahedron are considered. The study utilizes published noise values derived from real measurements. Based on a ratio between vertical to horizontal noise of 1 to 20, we find that the vertical antenna solution delivers the best results. The design with three-perpendicular-antennas performs well on slightly sloping sea floor but its total error in vertical field measurement is up to 5 times higher for seabed tilts up to 10 degrees. The error is found to be relatively insensitive to realistic uncertainties in the tilt measurements. The tetrahedron design elegantly compensates the Ex component as long as the seafloor is flat. On a sloping seafloor the compensation may get lost and Ez has to be derived from components that contain relatively large parts of Ex.