6th International Congress of the Brazilian Geophysical Society

Here I present one version of Hamilton based ray-tracing equations solution (Cerveny, 1987) (Popov,
1996) aiming at increasing precision as well as reducing and controling the computation’s effort. The
errors in numerical evaluating the differential ray-tracing equation can be classified in two kinds. First
the error in interpolating the velocity field and its derivatives. Second the error in evaluating the next
ray’s coordinates and slowness vector by the Runge-Kutta method. The first one has well-known solutions
like fourth order precision Bi-cubic spline interpolation (William et al., 1994). The second, the
fourth order precision Runge-Kutta method, needs the time step. If we apply a constant time step, it
requires the use of a high-density coordinate’s sampling when used for a non-homogeneous medium.
The reason is that the precision of ray-tracing is directly related to the curvature radio of the ray and the
last with the gradient of the square slowness. Then to obtain a high precision ray-tracing we need to decrease
the size of the time step in regions where there are large variations of the square slowness field
or on the other hand increase where there are no such variations. The result is an adaptive algorithm,
which automatically adapts the size of the ray coordinates interval to the necessity of the medium by
geophysical control. Despite the existence of the “adaptive control for Runge-Kutta method” (William
et al., 1994) it’s based in numerical considerations. Here I developed an alternative based on solid
physical arguments most properly useful for wave propagation purposes which can easily control the
accuracy of ray’s coordinates and in consequence the ray’s amplitudes (Lambaré, G. and Lucio, P.S.
and Hanyga A., 1996) and time of processing by just changing a parameter.

A gravity investigation is conducted in a coastal area in the northeastern Ceará State (Brazil). The objective of the survey is the detection of the regional geological structures which control the accumulation of underground water. This investigation consists of 1668 gravity stations, including previous surveys both in land and marine areas. The Bouguer gravity map is dominated by regional effect caused by the mantle uplift in the continental passive margin. To display more clearly the gravity anomalies caused by upper crustal heterogeneities, the local regional gravity field is removed by using a polynomial surface fitting. The resulting residual gravity anomaly map permits us to associate the gravity anomalies with known precambrian geologic units, granitic batholiths and surface structural features. The gravity lineaments are concordant with the NE-SW structural trends. In the northwestern of the studied area an elongated relative gravity low, visible in the residual map, indicates a NWSE secundary deformation trend, underlying of the sedimentary cover. These results will be prioritized and will be used to guide a more detailed geophysical survey.

Nas duas últimas décadas verificou-se uma significativa redução do ozônio atmosférico global, criando uma
preocupação sobre o possível aumento da radiação UV-B na superfície terrestre. Entretanto, poucas medidas
de UV na superfície estavam disponíveis até meados de 1980, e a maioria destas concentrava-se no hemisfério
norte. Descreve-se neste trabalho a rede de detectores de UV do INPE e alguns resultados. O INPE vem
operando desde o início da década de noventa, vários detectores UV dos tipos radiômetros UV-Biometer, GUV e
Espectrofotômetros Brewer no Brasil, Bolívia, Chile e Antártica, constituindo uma base de dados com uma
significativa distribuição geográfica, de latitudes equatoriais, médias e polares. Esta distribuição espacial é
importante para o estudo do comportamento da radiação ultravioleta solar face ao quadro de mudanças
atmosféricas associado aos fenômenos da redução global do ozônio.

A three dimensional body is designed to simulate the valley shaped topography. Transient responses are
calculated for topography at source location and at receiver location respectively. Modeling results are shown
by relative anomalies in contours. The behavior of the topographic effect to transient eletromagnetic field
responses can be characterized by local anomaly for topography at receiver location, and by equivalently an
extra source with strong high frequency excitation for topography at source location. Learning from the
modeling results, it is emphasized that the ground wire source shall be set up at place as uniform as possible
for reliable deep sounding measurements.

Older 2D seismic data in the Falc6n basin, Zulia and Falcon states, northwestern Venezuela, indicates the
probable existence of a fold and thrust belt in northwestern Venezuela. This belt is likely the result of the collision of
the Caribbean Plate with the South American Plate during the Paleogene. A second pulse during the Lower Miocene
is also recorded on 2D seismic profiles near the coast of Falcbn. Southwest vergent thrusts are evidenced in the
Cocuiza basin, on the Dabajuro high and the Coro platform. Younger south vergent thrusts with large fault
propagated folds are seen within the Urumaco trough. Although some of the low-angle thrusting is due to wrench
movements along the Oca fault system, much of the thrusting can be attributed to compression rather than
transpression during plate collision.

A series of modern submarine canyons were examined in the south-eastern Brazilian margin in order to identify
the different styles of canyon filling and their relationship with sedimentary processes and sea-level changes.
Such study should be considered as a predictive tool in order to estimate the presence of reservoir-like
deposits. Four seismic facies related to intra-canyon depositional patterns were individualised in this study:
clinoform prograding lateral accretion (Cp), cahotic fill (Ch), parallel-to-divergent aggradation (Pd) and
horizontal strata (Pl). Relationship between these styles and sea-level changes was attempted and results
indicate a complex interaction between hydrodynamic conditions, position of sediment source and sediment
availability. Coriolis Effect seems to play a minor role in controlling the downcanyon flow/erosion processes.
Lateral accretion is mainly related to the southward flowing Brazil Current. Aggradation and horizontal strata are
related to a relative decrease in the Brazil Current action and to an increase in gravity flows activity.

The principal aim here is show a transient thermal-resistivity log dependence. The induction devices (ILM and
6ff40), common used in log interpretation, may have some errors and new corrections will be needed. This
features are presented when multilayer transient temperature is evaluated and applied to resistivity model. A
simple eight meters deep formation is simulated with 2d-thermal and resistivity model, composed by shale and
sandstone reservoir, where multiphase radial fluid flow was performed. Due mud circulation, temperature
dependence of apparent resistivity formation are evaluated and compared with the resistivity when wellformation
temperatures are the same (150oC). Errors into 1% to 12% are expected to medium and deep induction
log information with this model.

Equatorial spread F (ESF) and plasma bubbles are plasma irregularity phenomena that are known to occur in the
equatorial- and low-latitude ionospheric F region at evening and nighttime. Those irregularities cause
scintillation that disrupts trans ionospheric radio propagation, up to the GHz frequency range interfering on
communication systems including the Global Positioning System - GPS. Although the general characteristics of
spread F and bubbles, concerning their variation with season, solar cycle, and magnetic activity, are fairly well
understood, we do not know much about the cause of the day-to-day variability, a crucial point when prediction
is concerned. In this paper, we analyze ionospheric data from two low latitude stations in Brazil, in either
presence or absence of bubbles, in order to try to establish the conditions under which bubbles shall occur. We
conclude that the F layer critical frequency (foF2) in the afternoon-sunset hours (or equivalently the F layer
maximum density, NmF2) is a useful parameter to predict the occurrence of the bubble.

The influence of electrode depths in electroresistivity
survey is studied. Generally, the investigators don’t care
about this detail but it can introduce significant departures
on the apparent resistivity values obtained with
small electrode spacings. To check the influences, theoretical
analysis is performed using simulated vertical
electrical sounding with undergrounded Wenner array.
Field data using Schlumberger array are collected to
demonstrate the existence of such effects. Computed
apparent resistivity values obtained with the same field
array parameters, separations and depths, are successfully
fitted to the field data, and a horizontally layered
model is obtained. The studies with two layer models,
shown that the effects of undergrounded electrodes are
more expressivewhen the second layer is more conductive
than the first one. Ambiguities arise in the reversed situations.