Edson E. S. Sampaio

Apparent resistivity and spectral induced polarization in the submarine environment

Relatively few investigations have employed electrical methods in the submarine environment, which may be promising for mineral deposits or threatened by environmental problems. We have measured the electric field using both disk and bar electrodes in the sea water at three different levels: sea surface, seven meters deep, and sea bottom at a depth of ten meters, employin ga2m spacing dipole-dipole array with 7 array spacings of investigation, and 13 values of frequencies at steps of (2 N hertz, N = –2, –1, 0, 1, 2,.....10). The measurement allowed the analysis of the electric field as a function of frequency and spacing, and of the spectral induced polarization. Modelling and interpretation of the apparent resistivity yielded a good fit with previous drilling data. Analysis of the spectrum of the complex apparent resistivity and the comparison with equivalent circuits, provided information about the grain size, the mineral composition and the major induced polarization phenomenon occurring below the sea. Therefore the result of the present research show the feasibility of measuring the variation of seawater resistivity in situ, as well as the resistivity of sea bottom sediments.

Electric field of a horizontal antenna above a homogeneous half‐space: Implications for GPR

Inverse modeling and interpretation of subsurface structures depend on accurate knowledge of the undisturbed field. This is especially true in the analysis of radargrams, in which it is difficult to resolve the upper homogeneous medium from the less shallow scatterers. The available forward models based on plane-wave and ray approximation are not accurate enough for this task. To improve resolution capabilities, we determine the undisturbed field using exact expressions for the electric field of a sine-shaped ground-penetrating radar (GPR) signal antenna above a homogeneous half-space. In the frequency domain it consists of the sum of two improper integrals with complex integrands. Each integrand contains a kernel multiplied by a Bessel function of the first kind and of order zero or one. In the general case these integrals do not have a solution in closed form, and their integrands are poorly convergent. Therefore, to solve the integrals we must use a special formalism involving integrals around branch points. When we assume that both the transmitter and the receiver are on the boundary of the half-space, there exist analytic solutions for the first integral without further restrictions and for the second integral for two special cases: free space and half-space, neglecting displacement currents. We check our corresponding numerical results against these analytic solutions. In the time domain we represent the electric field as a function of transmitter-receiver offset and time. For a purely dielectric half-space the backtransformation of the first integral is analytical under the assumed simplification, allowing us to check the numerical results obtained with a fast Fourier transform (FFT) algorithm. These results allowed us to design radargrams for five different models of a homogeneous earth, and they are fundamental for interpretation and further research of GPR modeling.

Zero-order time domain scattering of electromagnetic plane waves by two quarter spaces

The zero-order term of the time domain scattered electric field of an electromagnetic plane wave normally incident upon the surface of two quarter spaces is determined. The general solution is a development from a previous exact and complete solution in the frequency domain. The zero-order term of the scattered electric field has been computed in the upper medium (z < 0). The incident wave in the frequency domain assumes the same function for three cases: (1) The conductivity vanishes everywhere; (2) only the conductivity of the upper medium is zero; and (3) the three media are conductors. Case 1 helps to understand cases 2 and 3. Case 2 is applicable to geophysical exploration. For cases 1 and 2 a causal time function decaying exponentially with time at every point above the fault (z < 0) describes the waveform of the incident plane wave. The zero-order term of the scattered field has been computed above the fault. At x = 0 it reduces to a closed expression for case 1 and to a single integral for the other two cases. In the three cases it contains an integral of a Hankel function for x ≠ 0. The computation of the high-frequency part of the inverse Fourier transform for x ≠ 0 employs asymptotic expressions for the Hankel function using analytical techniques of the geometrical theory of diffraction for cases 1 and 2. For case 3 the inverse Fourier transform may have two possible contributions: either from the residue at a single pole or from the integral along a branch cut in the ω plane. The wave front of the scattered field is well defined in shape, phase, and amplitude. Its amplitude is discontinuous at x = 0, and varies smoothly but presents a sharp jump for ∣ x ∣<<∣ z ∣. For ∣ x ∣ = O(z), there is a numerical noise that oscillates at 100 MHz.

Scattering of electromagnetic plane waves by a buried vertical dike

The complete and exact solution of the scattering of a TE mode frequency domain electromagnetic plane wave by a vertical dike under a conductive overburden has been established. An integral representation composed of one-sided Fourier transforms describes the scattered electric field components in each one of the five media: air, overburden, dike, and the country rocks on both sides of the dike. The determination of the terms of the series that represents the spectral components of the Fourier integrals requires the numerical inversion of a sparse matrix, and the method of successive approaches. The zero-order term of the series representation for the spectral components of the overburden, for given values of the electrical and geometrical parameters of the model, has been computed. This result allowed to determine an approximate value of the variation of the electric field on the top of the overburden in the direction perpendicular to the strike of the dike. The results demonstrate the efficiency of this forward electromagnetic modeling, and are fundamental for the interpretation of VLF and Magnetotelluric data.

Interpretation of airborne and ground magnetic and gamma-ray spectrometry data in prospecting for base metals in the central-north part of the Itabuna-Salvador-Curaçá Block, Bahia, Brazil

AbstractInterpretation of airborne and ground magnetic and radiometric geophysical data is fundamental to provide support for geologic mapping and exploration of mineral resources. This is particularly important of the Itabuna-Salvador-Curaca Block component of the Sao Francisco Craton in the state of Bahia, Brazil, which bears important base metal mineralization in ultramafic intrusions. The interpreted airborne data covered part of the northern segment, and the ground data comprised an area of 8.75  km2 in the northern segment of that Archean block. The radiometric interpretation was only qualitative, and the magnetic interpretation was qualitative and quantitative. The qualitative interpretation defined distinct configurations of the geophysical signatures. So, it furnished possible regional and local geologic models and also helped to infer the distribution of magnetization and radiometric content in the rocks. The quantitative interpretation started with Werner deconvolution for a preliminary identif...

Magnetic anomalies of two‐dimensional bodies in a magnetic half‐space

The presence of magnetization in country rock modifies the anomaly caused by magnetic bodies. Such a modification is distinct from the self‐demagnetization effect of the body, and the concept of susceptibility contrast is not adequate to explain it. We can achieve an exact understanding of the problem by solving the potential function in three media: air, magnetic country rock, and magnetized body. This paper sets up the solution of this problem when the magnetized body is a circular cylinder with an infinitely long horizontal axis, for both a horizontal and a vertical inducing ambient field. It expresses the solution of Laplace’s equation in bipolar coordinates for the potentials in the form of Fourier series. Analysis of the vertical, horizontal, and total magnetic anomalies shows that neglect of country rock magnetization reduces the apparent causative body dimensions.

Analysis of the risk of karst collapse in Lapão, Bahia, Brazil

On October 2008, the large extent of dissolution of limestone in the Irecê Basin resulted in the occurrence of minor ground shakes and the enhancement of fractures along the surface of the ground and on the walls of buildings in the urban and periurban area of the city of Lapão. After preliminary geological and geotechnical studies to analyse the danger of collapse of underground caves, we have carried out gravity and electro-resistivity profiles in order to map the concealed caves and to help the determination of the level of groundwater within them. The interpretation of the results provided useful information both for the immediate safety of the population and for subsequent hydrological and geotechnical work. We have defined five risk areas based on the variations of the Bouguer and of the electrical resistivity data, by correlating the distribution of gravity lows with the presence of caves and the less resistive zones with the presence of brine water underground. Dissolution of limestone in a karst area resulted in ground shakes and enhancement of fractures. Gravity and electro-resistivity profiles mapped concealed caves, determined groundwater level, and defined five risk areas by correlating gravity lows with caves and conductive zones with brine water, providing information for future hydrological and geotechnical work.

Electromagnetic fields at the sea bottom induced by a line of immersed electric dipoles

The analysis of electromagnetic fields caused by alternate or transient electric currents flowing along a cable in sea water has several applications. It supports the interpretation of electromagnetic geophysical data and safety procedures against the threat of sea mines. The approach to the problem employs a magnetic vector potential in the frequency domain due to a pulse source electric dipole, and performs Laplace and Hankel transforms and integration along the cable, to describe the variation of the magnetic induction field due to an electric dipole of finite length. The result is applicable to shallow or deep sea water environments, adaptable to any transmitting current waveform and useful for wave-field separation. The prospects relate to a horizontal receiving coil at the sea bottom and simulate: a minesweeper campaign with a current source at the sea surface or a geophysical survey with a current source close to the sea floor. Therefore, the present analysis may serve: to define parameters in counter-sweeping of submarine mines; to map the conductivity of sediments under shallow waters for the prevention and control of contamination; and as a first approach in the characterization of offshore mineral and oil economic deposits.

Analysis of offshore applications of a magnetic tail for profit, safety, and environmental protection

An alternating electric current flowing along a cable of finite length situated between the surface and the bottom of a salty water layer generates a magnetic field whose analysis has several applications. It is useful in naval engineering for protecting ships from the threat of sea mines. It supports the analysis of submarine geophysical data collected either to define the degree of pollution of the sea bottom in shallow waters, or to prospect and monitor hydrocarbon reservoirs. The approach to the problem starts from an electric dipole in the frequency domain to describe the time variation of the vertical component of the magnetic induction field due to a line of alternating electric dipoles. The prospecting surveys relate to: a current source at the sea surface and a horizontal coil at the sea bottom to simulate a minesweeper campaign; and a current source and a horizontal coil at the sea bottom to simulate a submarine geophysical survey. The model is applicable to shallow or deep sea water environments. The analysis, modelling, and interpretation of the data can serve at least for three purposes: to define parameters for the magnetic sweeping of submarine mines; to map the conductivity of the submarine soil in shallow waters for the prevention and control of contamination; and as a first approach in the characterization of offshore oil reservoirs.

Time domain scattering of EM plane waves by vertical faults

The zero-order term of the time domain scattering of the electric field of a plane wave normally incident on the surface of two quarter spaces is determined. This zero-order term has been computed above the fault, (z < 0), where the conductivity is zero. The results are adapted from a previous exact and complete solution in the frequency domain. The wave form of the incident plane wave is given by a causal time function that exponentially decays with time at every point above the fault. The zero-order term reduces to a single integral for x = 0, and contains an integral of a Hankel function for 0. The high frequency part of the inverse Fourier transform for x 0 is computed employing asymptotic expressions for the Hankel function using analytical techniques of the geometrical theory of diffraction. The wave front is well defined in shape, in phase, and in amplitude. The amplitude of the scattered field is discontinuous at x = 0, it varies smoothly but presents a sharp jump for x 2, and it oscillates at about 40 MHz for x 2.