Carlos A. Mendonça

Forward and inverse self-potential modeling in mineral exploration

Natural self-potential (SP) fields observed in the vicinity of conductive orebodies can be explained in terms of electrochemical reactions in which the conductors participate. Battery-like models assume that a buried conductor creates an anode-cathode pair by conveying a flow of electrons to oxidizing areas in the shallow subsurface from reducing areas at depth. For conductors with invariant composition (behaving as an inert electrode), a quantitative formulation is already available. Numerical Greens functions are used to allow one-step evaluation of SP fields from an inert electrode model. The model is used to simulate geoelectric targets in mineral exploration and to test a procedure to obtain current source terms by inverting an SP data set. Data inversion is constrained by charge conservation and prescribes source terms at the target surface. A background resistivity model is assumed to be known and is used to recognize interfaces and evaluate numerical Greens functions in forward and inverse modeling. The inversion procedure is applied to interpret 2D data from two gold deposits of the Yanacocha district, Peru.

The equivalent data concept applied to the interpolation of potential field data

The equivalent layer calculation becomes more efficient by first converting the observed potential data set to a much smaller equivalent data set, thus saving considerable CPU time. This makes the equivalent‐source method of data interpolation very competitive with other traditional gridding techniques that ignore the fact that potential anomalies are harmonic functions. The equivalent data set is obtained by using a least‐squares iterative algorithm at each iteration that solves an underdetermined system fitting all observations selected from previous iterations and the observation with the greatest residual in the preceding iteration. The residuals are obtained by computing a set of “predicted observations” using the estimated parameters at the current iteration and subtracting them from the observations. The use of Cholesky’s decomposition to implement the algorithm leads to an efficient solution update everytime a new datum is processed. In addition, when applied to interpolation problems using equiva...

Interpolation of potential-field data by equivalent layer and minimum curvature: A comparative analysis

Interpolation using only the observations at discrete points is an ill‐posed problem because it admits infinite solutions. Usually, to reduce ambiguity, a priori information about the sample function is introduced. Current interpolation methods in mineral exploration introduce only the constraints of continuity and smoothness of the interpolating function. In interpolating potential‐field anomalies, the constraint that the sampled function is harmonic may be introduced by the equivalent‐layer method (ELM). We compare the performance of the ELM and the minimum curvature method (MCM) in interpolating potential‐field anomalies by applying these methods to synthetic magnetic data simulating an aeromagnetic survey. In the case the anomaly flanks and peak are undersampled, the ELM performs better than the MCM in recovering the anomaly gradients and peak. In the case of elongated linear anomalies, the ELM recovers the exact linear pattern, but the MCM introduces spurious oscillations in the linear pattern. Also,...

Automatic determination of the magnetization-density ratio and magnetization inclination from the joint interpretation of 2D gravity and magnetic anomalies

The Poisson theorem establishes a linear relationship between the gravity and magnetic potentials arising from common dense and magnetized bodies with constant magnetization–density ratio and magnetization direction. For geological formations satisfying such constraints (i.e., the Poisson conditions), this theorem provides suitable relationships between the gravity and magnetic anomalies that are useful in interpreting the related data sets. In such applications, both magnetization–density ratio (MDR) and magnetization direction can be estimated, thus helping the subsurface geological mapping from potential field data acquired on the earths surface. However, no existing method is fully automatic, which has hampered extensive use in routine applications. Such a drawback follows the adoption of equations that, although obeying the Poisson theorem, relate particular components of the gravity and magnetic fields, thus requiring either a known magnetization direction or the implementation of iterative procedu...

Programs to compute magnetization to density ratio and the magnetization inclination from 3-D gravity and magnetic anomalies

Vector field formulation based on the Poisson theorem allows an automatic determination of rock physical properties (magnetization to density ratio-MDR-and the magnetization inclination-MI) from combined processing of gravity and magnetic geophysical data. The basic assumptions (i.e., Poisson conditions) are: that gravity and magnetic fields share common sources, and that these sources have a uniform magnetization direction and MDR. In addition, the previously existing formulation was restricted to profile data, and assumed sufficiently elongated (2-D) sources. For sources that violate Poisson conditions or have a 3-D geometry, the apparent values of MDR and MI that are generated in this way have an unclear relationship to the actual properties in the subsurface. We present Fortran programs that estimate MDR and MI values for 3-D sources through processing of gridded gravity and magnetic data. Tests with simple geophysical models indicate that magnetization polarity can be successfully recovered by MDR-MI processing, even in cases where juxtaposed bodies cannot be clearly distinguished on the basis of anomaly data. These results may be useful in crustal studies, especially in mapping magnetization polarity from marine-based gravity and magnetic data.

Investigações GPR nos distritos mineiros de Santa Bárbara e Bom Futuro: Província Estanífera de Rondônia

Investigacoes GPR - Ground Penetrating Radar foram realizadas nos distritos mineiros de Santa Barbara e Bom Futuro (Provincia Estanifera de Rondonia), visando averiguar a aplicabilidade do metodo para localizacao de depositos estaniferos primarios e secundarios, bem como otimizar os custos envolvidos na etapa de prospeccao. Os perfis de reflexao GPR foram adquiridos com antenas de 25, 50 e 100 MHz, o que permitiu investigar a subsuperficie ate 30 metros de profundidade. Em relacao aos depositos primarios das minas de Santa Barbara e de Bom Futuro, o metodo GPR permitiu identificar zonas com predominância de corpos de greisens. No deposito secundario aluvionar da mina de Bom Futuro, o metodo GPR permitiu localizar uma estrutura de paleocanal na base do sistema deposicional sedimentar. Os resultados obtidos apresentaram uma excelente concordância com as informacoes geologicas provenientes das sondagens, demonstrando essa nova aplicabilidade do metodo GPR para a exploracao mineral, na qual podera ser extendida para ambientes similares do Craton Amazonico. Alem disso, os custos envolvidos no processo exploratorio poderao ser otimizados, caso as investigacoes GPR sejam realizadas antes da execucao de sondagens.

Inversion of gravity-field inclination to map the basement relief of sedimentary basins

This paper presents a method to map the basement relief of homogeneous sedimentary basins that does not require the knowledge of the basin density contrast. To reach this task, the proposed method relies on the invariance of the inclination of the anomalous gravity field with the density contrast caused by models constituted by two homogeneous media. This invariance occurs because the density contrast appears as a constant factor in both vertical and horizontal gravity components, therefore being canceled out when these components are divided during the evaluation of the field inclination. For such media, the field inclination is independent of the density contrast, thus allowing the source geometry reconstruction even when the density contrast is unknown. As the inclination is rarely measured, the gravity anomaly (i.e., the field vertical component) is initially used to compute the horizontal component of the gravity field by applying a suitable linear transform. The field inclination is estimated from both components and then used to invert the source geometry by fitting the inclination values under the geologic constraints attributed to the causative sources. In this process, the density contrast is not required nor introduced as an unknown parameter in the formulated inverse problem. Moreover, it can be estimated later by solving a new inverse problem where the source geometry determined from the inverted inclination is fixed and the constant density contrast is determined by fitting the gravity anomaly. This paper applies such ideas to map the basement relief of a sedimentary basin and to estimate its density contrast. The inversion is implemented by a random search procedure that excludes extreme models, and imposes constraints that the unknown interface is smooth everywhere and assumes known depth values at isolated points investigated by wells. The proposed technique is tested with synthetic noisy data from homogeneous and heterogeneous basin models and is applied to invert a gravity profile from the Reconcavo Basin, Brazil. The results from the real data application are compared with well data and previously published results.

Integrated earth resistivity tomography (ERT) and multilevel sampling gas: a tool to map geogenic and anthropogenic methane accumulation on brownfield sites

Soil gas emissions of methane and carbon dioxide on brownfield sites are usually attributed to anthropogenic activities; however geogenic sources of soil gas are often not considered during site investigation and risk management strategies. This paper presents a field study at a redeveloped brownfield site on a flood plain to identify accumulations of methane biogas trapped in underlying sediments. The investigation is based on a multidisciplinary approach using direct multi-level sampling measurements and Earth resistivity tomography . Resistivity imaging was applied to evaluate the feasibility of identifying the size and spatial continuity of soil gas accumulations in anthropogenic and naturally occurring deposits. As a result, biogas accumulations are described within both anthropogenic deposits and pristine organic sediments. This result is important to identify the correct approaches to identify and manage risks associated with soil gas emissions on brownfield and pristine sites. The organic-rich sediments in Quaternary fluvial environments of São Paulo Basin in particular the Tietê River, biogas reservoirs can be generated and trapped beneath geogenic and anthropogenic layers, potentially requiring the management of brownfield developments across this region.

Resistivity and induced polarization monitoring of biogas combined with microbial ecology at a brownfield site

AbstractThe accumulation of biogenic greenhouse gases (methane, carbon dioxide) in organic sediments is an important factor in the redevelopment and risk management of many brownfield sites. Good practice with brownfield site characterization requires the identification of free-gas phases and pathways that allow its migration and release at the ground surface. Gas pockets trapped in the subsurface have contrasting properties with the surrounding porous media that favor their detection using geophysical methods. We have developed a case study in which pockets of gas were intercepted with multilevel monitoring wells, and their lateral continuity was monitored over time using resistivity. We have developed a novel interpretation procedure based on Archie’s law to evaluate changes in water and gas content with respect to a mean background medium. We have used induced polarization data to account for errors in applying Archie’s law due to the contribution of surface conductivity effects. Mosaics defined by cha...

The face‐current concept and its application to survey design in electrical exploration

This paper presents a new method to identify the regions over a 3D geoelectrical structure that produce major contributions to the electrical potential established in response to a dc source at the ground surface. The measured potential is represented by a sum of a known primary potential (due to a homogeneous half space) plus an unknown potential caused by conductivity inhomogeneities. Because the primary potential is continuous everywhere, the interfaces with a conductivity contrast act as sources or sinks of currents in order to maintain the continuity of the current density related to the primary flux. These disturbing face currents are responsible for the generation of the secondary potential, and mapping them over a given structure allows us to assess the regions where the secondary potential is generated. In general, the face currents vanish away from the source according to the decay of the primary electric field. For this reason, deeper investigations can be expected when using pole sources becau...