Berthold Kriegshauser

Magnetotelluric studies of the Caldas Novas geothermal reservoir, Brazil

A magnetotelluric (MT) survey was conducted on the Caldas Novas geothermal reservoir located in the state of Goias, Central Brazil. The region of Caldas Novas is a popular tourist spot because of the occurrence of hot water springs. The purpose of the first MT survey in this area is to provide more information on the geoelectrical structure of this important geothermal reservoir. The MT method is a frequency domain technique that utilizes naturally occurring magnetic and electric signals as source to obtain a resistivity map of the subsurface. Since temperature and permeability are some of the factors controlling electrical resistivity, MT is widely utilized for surveying geothermal areas such as Caldas Novas. Data were acquired along two profiles crossing the Serra de Caldas (Caldas Mountains) with a total of 25 MTstations. Frequencies of acquisition were in the range from 0.008 to 176 Hz. Spacing between stations were usually around 5 km. Apparent resistivity and phase data from the transverse electric (TE) and transverse magnetic (TM) modes were computed for both profiles. Very high (100,000 V m) apparent resistivity values in the TM mode indicate distortion, possibly caused by 3-D resistive structures. In this work, we focus on Profile 2, which was acquired aligned at N40E, approximately following the direction of weakness N50E. Two-dimensional (2-D) inversion suggests a basin-like model with very high-resistivity block structures associated with concentric faulting below the Caldas Mountains inside a resistive basin that extends to depths of approximately 25-30 km. This model is in good agreement with gravity data and the available geological information in the area and can help delineate areas of new geothermal reservoirs. The high resistivity associated with the depression in the MT model can be interpreted as caused by the low-permeability quartzites that form the Caldas Mountains. Although the survey configuration does not allow for a full three-dimensional (3-D) inter- pretation, a 3-D resistivity model was constructed and the MT responses computed. The 3-D synthetic data explains the behavior of both the TE and TM modes in this faulted resistive environment. D 2002 Elsevier Science B.V. All rights reserved.

A Fast Inversion Method For Multicomponent Induction Log Data

Summary A new multicomponent induction tool has been developed for detecting resistivity anisotropy of the formation. This new tool comprises of three mutually orthogonal transmitter-receiver coil configurations measuring all components of the magnetic field tensor. To provide the horizontal and vertical resistivities of the formation at well site, we developed a fast, robust, and hands-off inversion method. The method describes the layered formation using equally thick thin layers. A 1-D vertically layered forward modeling code is used to generate the synthetic data. The formation resistivities are updated iteratively by the difference between the synthetic data and the measured data without computing a sensitivity matrix. In a vertical well, the horizontal resistivity is inverted from the Hzz * component first, and then the vertical resistivity is inverted from the Hxx or Hyy component. In a deviated well, we use the same methodology, but we apply a constraint to derive horizontal and vertical resistivities simultaneously. We test the method on synthetic examples and we will show that the algorithm is fast and robust.

Comparison of Global and Local Optimization Techniques in Electromagnetic Applications

In this paper we compare the performances of global and local optimization techniques in electromagnetic inversion. Global optimization schemes, e.g., genetic algorithms, Nelder-Mead simplex method, simulated annealing, are becoming increasingly popular in geophysical applications, because they search the whole parameter space for the global extremum.