Entwicklung einer 2D FE Vorwärtsrechnung für DCR/IP und RMT mit unstrukturiertem Gitter und Ergebnisse der Feldmessungen auf einem Erzkörper in der Türkei 2013

In der geophysikalischen Erzexploration ist es von Interesse, die Leitfähigkeit sowie die Ausmaße eines potentiellen Erzkörpers abzuschätzen. Durch die gemeinsame Anwendung und Interpretation von Gleichstromgeoelektrik (DCR), Induzierter Polarisation (IP) und Radiomagnetotellurik (RMT) soll ein verbessertes Modell im Vergleich zur Einzelinterpretation der drei Methoden erreicht werden. So soll das aus der Joint-Inversion von DCR und RMT Daten erhaltene Leitfähigkeitsmodell als Startmodell für die 2D IP Inversion der Aufladbarkeit verwendet werden.Auf dem Poster wird ein Algorithmus zur 2D Finite Elemente DCR/IP Vowärtsrechnung mit unstrukturiertem Gitter, welcher zur Zeit an der Universität zu Köln entwickelt wird, vorgestellt. Darüber hinaus fand im August 2013 eine Messkampagne auf einer potentiellen Kupfererz Lagerstätte in der Türkei statt, bei der DCR-, time-domain IP- und RMT-Daten aufgezeichnet wurden. Es werden 2D Inversionsergebnisse der gesammelten Daten vorgestellt, welche mit konventioneller 2D Intepretationssoftware angefertig wurden. Die Anwesenheit des Erzkörpers wird deutlich durch Bereiche mit einem geringen spezifischen Widerstand bzw. einer erhöhten Aufladbarkeit in den Ergebnismodellen angezeigt.Diese Arbeit ist Teil des von BMBF und TÜBITAK finanzierten Projekts „Zeidimensionale Joint Interpretation von Radiomagnetotellurik- (RMT), Gleichstromgeoelektrik- (DCR) und Induzierten Polarisations-Daten (IP): Ein Beispiel aus der Erzexploration“

Development of a 2D DC/TDIP Inversion Algorithm for Ore Exploration Purposes: Results from a Copper Ore Site in Turkey

An important matter of interest for the ore exploration with geophysical methods is the determination of the electrical resistivity and chargeability of the subsurface on the one hand and the assessment of the dimension of a potential ore deposit on the other hand. To make use of the advantages of electrical and electromagnetic methods, we applied the Direct Current Resistivity (DC) and Time-Domain Induced Polarization (TDIP) methods together with the Radiomagnetotellurics (RMT) method on a copper ore site in Turkey. While the DC method is sensitive to high resistive areas, the RMT method is sensitive to low resistive areas. Thus, the joint application of DC and RMT is expected to result in an improved picture of the resistivity distribution of the subsurface in contrast to the application of a single method. The TDIP method, on the other hand, is qualified to detect areas with disseminated sulfidic ores due to large electrode polarization effects which result in large chargeability anomalies. Since the presence of chargeable material effects the effective resistivity of the ground, it is advantageous to use the information of DC and RMT results as starting model for the TDIP inversion.On the poster we present the current state of the 2D DC/TDIP inversion algorithm that is being developed by the University of Cologne. It is a smoothness constraint inversion with an implemented forward algorithm that uses the finite element method with an unstructured mesh. The 2D inversion results from RMT and DC/TDIP data obtained during the survey on a sulfidic copper ore deposit in Turkey are presented. The presence of an ore deposit is indicated by areas with low resistivity and significantly high chargeability in the inversion models.This work is part of the BMBF/TÜBITAK funded project “Two-dimensional joint interpretation of Radiomagnetotellurics (RMT), Direct Current Resistivity (DCR) and Induced Polarization (IP) data: an example from ore exploration.

2D Inversion of DCR and Time Domain IP data: an example from ore exploration

Ore deposits often appear as disseminated sulfidic materials. Exploring these deposits with theDirect Current Resistivity (DCR) method alone is challenging because the resistivity signaturescaused by disseminated material is often hard to detect. The Time-domain Induced Polarization(TDIP) method, on the other hand, is qualified to detect areas with disseminated sulfidic ores dueto large electrode polarization effects which result in large chargeability anomalies. By employingboth methods we gain information about both, the resistivity and the chargeability distribution ofthe subsurface.On the poster we present the current state of the development of a 2D smoothness constraintinversion algorithm for DCR and TDIP data. The implemented forward algorithm uses a FiniteElement approach with an unstructured mesh. The model parameters resistivity and chargeabilityare connected by either a simple conductivity perturbation approach or a complex conductivityapproach.As a case study, the 2D inversion results of DCR/TDIP and RMT data obtained during a survey ona sulfidic copper ore deposit in Turkey are presented. The presence of an ore deposit is indicatedby areas with low resistivity and significantly high chargeability in the inversion models.This work is part of the BMBF/TUEBITAK funded project ``Two-dimensional joint interpretation ofRadiomagnetotellurics (RMT), Direct Current Resistivity (DCR) and Induced Polarization (IP) data:an example from ore exploration''

Exploration of a Copper Ore Deposit in Elbistan/Turkey Using 2D Inversion of the Time-Domain Induced Polarization Data by Using Unstructured Mesh

We present the results of a direct current (DC)resistivity and time-domain induced polarization (TDIP) surveyexploring a copper ore deposit in Elbistan/Turkey. The ore depositis elongated below a valley and is of disseminated form with sulfidecontent. DC and IP data were acquired using the pole-dipole arrayon eight parallel profiles crossing the valley perpendicularly. Thelength of each profile was 300 m with an inter-profile distance ofabout 50 m. The data were interpreted by a newly developed 2DDC/TDIP inversion algorithm. The finite element algorithm uses alocal smoothness constrained regularization on unstructuredmeshes. The finite element forward solution, as well as the inverseproblem, is solved by an iterative preconditioned conjugate solver.The depth of investigation (DOI) was determined from cumulativesensitivities of the 2D inversion algorithm results. Because of thedissemination of the ore, the 2D inversion of the DC data wasambiguous: However, due to the sulfide content, a strong chargeabilityanomaly associated with the ore body was detected. Weshow that chargeability anomalies can generally be detected in theabsence or presence of corresponding resistivity anomalies. Thishighly chargeable structure was confined in lateral direction.Although the lower boundary of the structure could not be resolvedby the applied field set-up, a rough estimation of it could be derivedat a depth of 90 m using synthetic modeling analyses. The 2Dchargeability models are consistent with existing boreholeinformation

Two-dimensional inversion of magnetotelluric/radiomagnetotelluric data by using unstructured mesh

We have compared structured and unstructured grid-based 2D inversion algorithms for magnetotelluric (MT) and radiomagnetotelluric (RMT) data in terms of speed and accuracy. We have developed a new 2D inversion algorithm for MT and RMT data by using a finite-element (FE) method that uses unstructured triangle grids. We compare the inversion results of our unstructured grid-based algorithm with those of the conventional algorithm, which uses either a structured FE or structured finite-difference (FD) numerical solution technique. The imaging of the surface topography and the underground resistivity structures by the new algorithm requires fewer elements than those that use FE and FD structured grids. We also find that when unstructured grids are used, the quality of the mesh is increased and the numerical errors are significantly reduced. Thus, the program runs faster and can simulate the complex surface topography in a more stable setting than the classic inversion algorithms. Furthermore, we implement a new smoothing matrix format for the unstructured triangle grids for the inversion procedure. We use two samples of synthetic data for the MT and RMT frequencies as well as a sample of field RMT data collected across a fault zone for comparison. In our synthetic data experiment, we find that the resistivity values and the boundaries obtained from the inversion of the unstructured mesh are closer to those of the true a priori synthetic model. Results of the synthetic and field data verify the computational advantages (speed and accuracy) of our inversion algorithm with respect to the conventional structured grid-based inversion algorithms

A Comprehensive Study of Local, Global, and Combined Optimization Methods on Synthetic Seismic Refraction and Direct Current Resistivity Data

Most geophysical inversions face the problem of non-uniqueness, which poses a challenge in the mapping and delineation of the subsurface anomalies. To tackle this challenge, a combined local and global optimization approach is considered for jointly inverting two-dimensional direct current resistivity (DCR) and seismic refraction (SR) data that aim to estimate the corresponding physical model parameters. In this combined approach, the output of the local optimization method is used to determine the search space and tuning parameters for the global optimization algorithm. The multi-objective genetic algorithm (non-dominated sorting genetic algorithm) was utilized to jointly optimize the objective functions of two different methods. Because the genetic algorithm is a population-based optimization method, it requires numerous forward calculations. To deal with the expected high computational cost associated with this approach, parallel computing was utilized for the forward function evaluations to reduce the run time of the entire process. The proposed approach was tested using synthetic two-dimensional resistivity and velocity models that had three different types of anomalies (dyke, positive, and combined positive and negative). The results showed an improvement in the anomaly delineation in the output of the combined local and global optimization method compared with the local optimization method. Additionally, similar synthetic models were tested using only the single objective global optimization algorithm (conventional global optimization), which showed promising anomaly delineation

A Comprehensive Study of Local, Global, and Combined Optimization Methods on Synthetic Seismic Refraction and Direct Current Resistivity Data

Most geophysical inversions face the problem of non-uniqueness, which poses a challenge in the mapping and delineation of the subsurface anomalies. To tackle this challenge, a combined local and global optimization approach is considered for jointly inverting two-dimensional direct current resistivity (DCR) and seismic refraction (SR) data that aim to estimate the corresponding physical model parameters. In this combined approach, the output of the local optimization method is used to determine the search space and tuning parameters for the global optimization algorithm. The multi-objective genetic algorithm (non-dominated sorting genetic algorithm) was utilized to jointly optimize the objective functions of two different methods. Because the genetic algorithm is a population-based optimization method, it requires numerous forward calculations. To deal with the expected high computational cost associated with this approach, parallel computing was utilized for the forward function evaluations to reduce the run time of the entire process. The proposed approach was tested using synthetic two-dimensional resistivity and velocity models that had three different types of anomalies (dyke, positive, and combined positive and negative). The results showed an improvement in the anomaly delineation in the output of the combined local and global optimization method compared with the local optimization method. Additionally, similar synthetic models were tested using only the single objective global optimization algorithm (conventional global optimization), which showed promising anomaly delineation

Application of Combined Local and Global Optimization Algorithms in Joint Interpretation of Direct Current Resistivity and Seismic Refraction Data: A Case Study of Dammam Dome, Eastern Saudi Arabia

The main geological structures in the Dammam Dome are defined by integrating geophysical measurements and applying new methodological approaches. Dammam Dome is characterized by a well-developed fracture/joints system; thus, high complexity of the subsurface is expected. Direct Current Resistivity (DCR) and Seismic Refraction (SR) geophysical survey aimed to map the Dammam Dome’s near-surface features. The geophysical data were acquired along two profiles in the northern part of Dammam Dome. To maximize the results from conducting DCR and SR measurements over a complex area, a combined local and global optimization algorithm was used to obtain high-resolution near-surface images in resistivity and velocity models. The local optimization technique involves individual and joint inversion of the DCR and SR data incorporating appropriate regularization parameters, while the global optimization uses single and multi-objective genetic algorithms in model parameter estimation. The combined algorithm uses the output from the local optimization method to define a search space for the global optimization algorithm. The results show that the local optimization produces satisfactory inverted models, and that the global optimization algorithm improves the local optimization results. The joint inversion and processing of the acquired data identified two major faults and a deformed zone with an almost N–S direction that corresponds with an outcrop were mapped in profile one, while profile two shows similar anomalies in both the resistivity and velocity models with the main E–W direction. This study not only demonstrates the capability of using the combined local and global optimization multi-objectives techniques to estimate model parameters of large datasets (i.e., 2D DCR and SR data), but also provides high-resolution subsurface images that can be used to study structural features of the Dammam Dome