8th International Congress of the Brazilian Geophysical Society

The Graduate Program in Marine Geology and Geophysics at Universidade Federal Fluminense, in Rio de Janeiro, Brazil, initiated in 1987 and evolved nowadays to a full degree program in marine geology and geophysics, offering the choices in three different levels: a Diploma (specialization), a Master Degree and a Doctorate Degree. Theoretical knowledge and practical skills are obtained through the attendance of the curricular disciplines and by the involvement on research projects, mostly applying geophysical methods, to investigate on a comprehensive manner the nature and evolution of coastal and oceanic environments. This work presents a historical perspective of the Program and the main results obtained so far including a review of the current professional activities involving our former students.

Multiple attenuation is a classical problem in processing and interpretation of seismic reflection sections. A special number of SEG´s The Leading EDGE (1999) is dedicated to the subject of multiple identification and attenuation, where is noticeable the non existence of a technique to provide both identification and attenuation that can be applied to all possible cases, due to the diversity of the geology responsible for the generation of multiples. This paper presents part of the theory that combines the CRS stack and Wiener-Hopf-Levinson prediction (WHLP) operators to perform multiple attenuation for 2D curved reflectors. The strategy uses the wave front attributes to alculate space-time shift windows to introduce the necessary “periodicity” between the primary and its
multiple for calculating the WHLP operator. The operation was applied in common-source time sections, where there are no periodicity between the primary and its multiple. The operator is calculated with the true amplitude of the signal, what is expected to give it more efficiency. The combination of these two theories to obtain the multiple attenuation operator is here denominated WHLP-CRS.

An isotopic tracers study has been done in order to understand some aspects of the groundwater dynamic in the São Sebastião aquifer, in Camaçari region, Bahia. The aims were to evaluate the water residence time in the aquifer and its discharge in the hydrographic basins that flow on the East limit of the Recôncavo Sedimentary Basin, near the Salvador Fault. The 14C specific activity existent in dissolved carbonates and the 222Rn concentration were determined in ten samples of groundwater from producing wells. The 222Rn concentrations were also measured in 19 surface water samples from the hydrographic basin of Capivara Grande, Capivara Pequena e Imbassaí streams.
The range of the specific activity of 14C in the groundwater samples varied from 71,5 to 7,0 percent of modern carbon, corresponding to groundwater residence time from recent to about 20.000 years in the aquifer. It was observed stratification within the aquifer, with residence time increasing with depth. The 222Rn concentration in the samples varied from 100 to 690 dpm/L.
The 222Rn concentration in the surface water is similar to the characteristics groundwater values only in the upper part of the streams investigated. In the segment where the Capivara Grande stream flows close to the Salvador Fault, the Recôncavo Basin East border, these values are very low, and do not indicate discharge from the São Sebastião aquifer in that region.

It is presented some results from processing, interpretation and integration to the geology of airborne magnetic and gamma-ray spectrometric data from the region of Rio Tapajós, States of Pará, Amazonas e Mato Grosso, Brazil, encompassing the geologic mapped sheets of Vila Mamãe Anã and Jacareacanga), at 1.250.00 scale (CPRM, 2000) and Rio Juruena (project RADAM 1975), at 1.1.000.000 scale. Many of the filtering techniques for enhancing geophysical data presentation, including composite images were used aiming to structural and lithologic mapping. Final geophysical interpretation map was integrated to the geology producing an enhanced geologic framework.

We have derived the gain function for porous rock based on the Gassmann’s equation. We study property of the gain function for consolidated sandstone and poor consolidated sands from deep-water, Gulf of Mexico. We apply the gain function to derive pore fluid modulus as a hydrocarbon indicator from log data.

We report on GPR measurements (1000 MHz) which were carried out on a well sorted, clean sand in a laboratory experimental site. The laboratory system includes a glass box, measuring 1.0 x 0.7 x 0.7 m3, with sand saturated with water and/or hidrocarbons. Our studies investigate the influence of diferent fluids on the propagation velocity of the GPR pulse across the sandy layers, at different moistures. The GPR velocities are determined using two-way traveltime CMP reflection data, the T2 – X2 and the Dix Equation techniques. A four phases (air, water, oil and sand) equation is resented here, it was built based on CRIM method and predicts the GPR propagation velocity based on porosity, saturation and dieletric properties of each phase. The experimental values of velocity had a very good agreement with the predicted ones from the analitical equation. The velocity decreases from 15 cm/ns in a sand with3% of humidity to 5 cm/ns in a sand with 24% of humidity.

In this study we use spectral analyses over the High Resolution Airborne Magnetic (HRAM) data, acquired by Fugro-Lasa Airborne Surveys, to estimate depths do the Curie Surface in the northeastern portion of the Santos basin. Special care is taken on the investigation of the minimum size of the data window that may be used without compromising the reliability of the estimates. Additionally, we describe the regional behaviour of the depths to the Curie Surface and discuss the implication of the resulting geothermal gradients to the thermo-mechanical evolution and oil maturation in the Santos basin.

The United States Geological Survey (USGS) is embarking on an ambitious program to carry out a high-altitude areomagnetic survey of the continental U.S. and Alaska, using a modified Canberra bomber aircraft, to be flown at an altitude of 15 km (50,000 feet) with a line spacing of approximately 16 km. The main purpose of the survey is to bridge the wavelength gap between numerous low-altitude surveys and the orbital-altitude Magsat data.
The areomagnetic mapping is to be carried out simultaneously in the aircraft with a NASA-funded radar mapping project, aimed at producing high accuracy elevation data and high resolution surface imagery.
Two flight tests with a cesium magnetometer mounted in a stub at the tail of the Canberra showed that the bare aircraft can be compensated by conventional methods, albeit with some difficulties, and that with a proposed 12 foot (3.66 m) tail boom, good compensation should be assured. However, the radar instrumentation will consist of three transmitter/receivers, one in each wingtip and one in the belly, each of which could require up to 125 amperes of non-stationary DC power! The varying magnetic fields associated with these power systems could create an enormous problem for the magnetics system, possible completely masking the low-level geologic signals seen by the magnetometer. A related problem is that compensation will have to be carried out with the radar system in full operating mode and it is expected that to avoid danger of exposure to high levels of EM radiation on the ground, compensation may have to be carried out at survey altitude. At high altitudes, the aircraft’s manoeuvrability is quite limited and the necessarily slow compensation manoeuvres will have some of the same frequency content as the long-wavelength anomaly signals expected at these altitudes. These anomaly signals can be expected to degrade the compensation results.
This paper explains some of the compensation problems and shows a compensation result from the bare aircraft. I will also show the results of a study done on another aircraft, in which magnetic interference pulses from VHF radio transmissions are compensated, with the hope that the technique involved can be extended to be helpful in compensating the variable magnetic effects of the radar supply currents.

Properties of inhomogeneous plane waves propagating in a viscoelastic anisotropic medium are investigated. The slowness vector p is described by the so-called mixed speci cation. In it, the vector p is expressed in terms of two given real-valued, mutually perpendic-ular vectors (one of them specifying the direction of propagation), and of a free complex-valued parameter σ. The parameter σ must be determined so that the slowness vector p satis es a constraint relation following from an equation of motion for viscoelastic media. In this contribution, σ is determined by solving a complex-valued polynomial equation of the sixth degree. The used algorithm is quite general. It can be used for homogeneous as well as inhomogeneous plane waves propagating in elastic or viscoelastic, isotropic or anisotropic media. It is shown that the inhomogeneous plane waves propagating in anisotropic viscoelastic me- dia exhibit certain phenomena, not known from elastic anisotropic or viscoelastic isotropic media. For example, the inhomogeneous plane qP wave may propagate with the same phase velocity as one of inhomogeneous plane qS wave. It is also shown that the attenuation angles of inhomogeneous plane waves can attain values greater than /2 even for very weakly inhomogeneous plane waves.