Hédison Kiuity Sato

Imaging industrial contaminant plumes with resistivity techniques

A systematic geophysical procedure has been developed and applied to image groundwater contamination caused by industrial activities in Bahia, Brazil. The procedure combines the use of conventional Schlumberger sounding with a regular scheme of sampling the resistivity stratification in depth. This is achieved by traversing an area with multiple profiles measured at selected electrode spacing. By multiple profiles we mean closely spaced partial soundings made using only six electrode spacings. Partial and complete soundings are correlated and inverted assuming horizontally stratified models within the limits covered by each electrode array. Pseudo-resistivity sections constructed from these data are inverted and adjusted using a two-dimensional finite difference algorithm. Electrical and lithological well logs are used to constrain this interpretation. The procedure was successfully applied to investigate the groundwater conditions and to outline contaminant plumes within industrial areas of the Camacari Petrochemical Center, Reconcavo basin, Bahia. The study includes cases of electrically conductive plumes generated by infiltration of inorganic aqueous effluents and a resistive plume containing hydrocarbon contaminants.

Potential field from a dc current source arbitrarily located in a nonuniform layered medium

In this paper, I evaluate the potential field due to a dc current source located anywhere within a horizontally layered space, all layers possessing exponentially varying resistivities. The solution takes the form of Hankel transforms with their kernel expressions containing functions defined by recursion formulas. The resulting expressions can be used to model any kind of resistivity array. Specializing the general solution to full‐ or half‐space models possessing exponential dependence, I find terms I interpret as primary and secondary contributions. Analytic expressions for the secondary electrical field revealed an error in the Stoyer and Wait solution for a half‐space. To test the derived n-layer solution based on recursion formulas, I modeled resistivity logs for both homogeneous layers and intercalated homogeneous and heterogeneous layers. Curves from the theory reproduced asymmetries and ripples observed in real resistivity logs.

Contribution to Oil Exploration and Development - A Successful Inductive Multi-Frequency EM Survey On-Shore Brazil

In this work we discuss the results of an experimental study performed using a multi-frequency electromagnetic method over a mature oil field in Reconcavo basin, Bahia-Brazil. Five 1.8 km transects, 200 m apart, were surveyed over a selected oil reservoir block. The processed EM data are represented as cross-sections and maps of apparent resistivity and induced polarization parameter, using a consistent plotting procedure. All the sections, controlled by well logging data, allow to recognize the following geological features: (i) the oil sandstone horizons and their trapping shales; (ii) the oil-water interface and some zones of water invasion; and (iii) lateral electric contrasts representing fault zones. These results suggest the real possibility of the use of the spectral EM method in the direct detection of hydrocarbons, as well as for monitoring the efficiency of the artificial fluid injection used for secondary recovery. Also, this experiment brings about a further development in the inductive measurement of IP and introduces, for the first time, the use of this property in oil reservoir exploration and characterization.

Wavefront sets analysis of limited aperture migration sections

Although geophysical literature is quite rich in papers on migration, there are only a few papers that treat LAM (Limited Aperture Migration). Deterministic and stochastic criteria for the reconstruction of reflection interfaces in prestack migration were derived and demonstrated on synthetic and real data examples (a real data set was acquired offshore the Brazilian coast, courtesy of Petrobras). The reconstruction of reflection interfaces in LAM depends on the relative position of “normal bundles,” as defined here, and gradients to the reflection interfaces. The derived stochastic criterion relates the variance of “normal bundles” to the variance of slowness in the medium. These criteria will help the geophysicist to identify directly distorted regions caused by LAM and those regions that are correctly migrated.

Multi‐frequency EM method for hydrocarbon detection and for monitoring fluid invasion during enhanced oil recovery

In this work we discuss the results of an experimental study performed with a multi-frequency electromagnetic method over a mature oil field in Reconcavo basin, Bahia – Brazil. Five 1.8 km transects 200m apart and extending over a block of the oil reservoir were surveyed. The processed EM data are represented as cross-sections of apparent resistivity and induced polarization parameter, using a consistent plotting procedure developed by Dias and Sato (1981). All the sections, controlled by seismic and well log data, although showing some distortions in the IP-resistivity configurations, allow to recognize the following geological features: (i) the oil sandstone horizons and their trapping shales; (ii) the oil-water interface and some zones of steam invasion; and (iii) lateral electric contrasts representing fault zones. These results suggest the real possibility of the use of the spectral EM method in the direct detection of hydrocarbons, as well as for monitoring the efficiency of the artificial fluid injection used for secondary recovery.

Geofísica elétrica na caracterização da hidrologia subterrânea na região do Aterro Metropolitano Centro, Salvador, Bahia

Resistivity and induced polarization (IP) vertical electrical sounding data, and superficial self-potential (SP) survey, allow to evaluate the underground hydrological conditions around the new Salvadors sanitary landfill. The inversion of the combined IP-resistivity data allows to differentiate zones of large shaliness within the sandstones of Barreiras Formation. They also allow to map the aquifer substratum topography and the water table configuration, as well as to infer the general pattern of the underground natural flow of water. The SP data, acquired around the first two cells already constructed for waste disposal confirm the general underground flow and identified areas of recharge and discharge around the cells. The resulting hydrological-petrophysical picture may be a useful reference frame to check eventual environmental impacts in the future, caused by the waste disposal in the area.

Multi-frequency electromagnetic method for inductive measurement of ground ip and resistivity

ABSTRACT A geophysical electromagnetic method to inductively measure the ground electrical resistivity and induced polarization has recently been tested. Its basic characteristics involve three major differences from other methods: the two electrical ground parameters are obtained through measuring magnetic field. For this purpose, a transmitter–receiver (T, R) electromagnetic system is used that operates in the frequency domain and consists of a horizontal loop as the transmitter for the perpendicular loops configuration on the ground surface; the measured function is the (T, R) inductive coupling main variation produced due to the presence of the earth, that is the magnetic field radial component; the measurements are conducted at a large number of frequencies (139 in the more advanced prototype), and the measured function is explored in the frequency interval 0.2 Hz to 1 kHz, a much broader frequency range of the induced polarization effect spectrum, than the one conventionally used in field exploration. Three major aspects are emphasized: (1) the existence of a small ‘main zone’ interior to a half‐space, which is responsible for most of the magnetic energy that the receiver measures on the half‐space surface. This permits to substitute the entire half‐space by the ‘main zone’ and, in a second step, to substitute the ‘main zone’ by an equivalent homogeneous half‐space with the electrical characteristics of such ‘main zone’; (2) the existence of a closed solution for the fields that the (T, R) system generates on the surface of a homogeneous isotropic half‐space, which provides exact functions with the two electrical parameters of interest as the variables (the apparent resistivity and relative polarization parameter); (3) the values of the electrical parameters so determined can be attributed to the central point of the ‘main zone’. Three‐horizontal layers half‐space and a conductive sphere in the free‐space are discussed as models. Four field surveys are analysed as examples and show a satisfactory performance of the method for detection of on‐shore hydrocarbon reservoirs, description of induced reservoir variations and structural features mapping at depths up to 2.5 km.

Hydrocarbon detection and reservoir imaging during enhanced oil recovery using an inductive EM multi-frequency method

Abstract We discuss the results of an experimental study performed with a multi-frequency electromagnetic method over a mature oil field in Recôncavo basin, Bahia-Brazil. Five 1.8 km transects 200 m apart and extending over a block of the oil reservoir were surveyed. The processed EM data are represented as cross-sections and maps of apparent resistivity and induced polarization parameter, using a consistent plotting procedure. All the sections, controlled by well logging data, allow to recognize the following geological features: (i) the oil sandstone horizons and their trapping shales; (ii) the oil-water interface and some zones of water invasion; and (iii) lateral electric contrasts representing fault zones. These results suggest the real possibility of the use of the spectral EM method in the direct detection of hydrocarbons, as well as for monitoring the efficiency of the artificial fluid injection used for secondary recovery. Also, this experiment brings about a further development in the inductive measurement of IP and introduces, for the first time, the use of this property in oil reservoir exploration and characterization. Introduction The electronic and computing developments are bringing notable progresses in the performance and resolution of EM methods. We have available today: (i) a high precision and versatile equipment with capacity, coupled to a powerful computer, to register and process a larger amount of field data; and (ii) good techniques for inverting and representing the EM data. The limitations associated to the resolution of EM methods and environmental and cultural noise effects, normally present in oil field, have been overwhelmed by the use of a dense and multiple data acquisition, a careful data processing and an effective integration of the available geophysical and geological informations. In a structural geological block, having dimensions of 800 m by 1,800 m by 1,000 m depth, a frequency domain EM experiment has been made (Figure 1).