7th International Congress of the Brazilian Geophysical Society

Compressional wave amplitude-versus-offset analysis
(AVO) is being widely used in different geological
settings as a lithology indicator and also as a direct
hydrocarbon indicator. Ambiguities associated to
elastic parameters estimation from AVO is
commonplace in most inversion procedures used up
today. Basic approaches to determine elastic
parameters directly from the coefficients of the P-P
reflectivity approximation (Rpp) fail because of the
uncertainties associated to the third coefficient, which
depend on larger incidence angles than intercept (A)
and gradient (B) estimates. One way to get around
this problem is to use the reflectivity information
from P-SV AVO (Rps) in addition to the traditional PP
AVO. This procedure can potentially reduce the
uncertainties and improve estimation of elastic parameters.
In this paper I demonstrate how to extract elastic
parameters from P-SV AVO and the way to combine
these parameters with those extracted from P-P AVO.
A simple low-contrast approximation is derived and
is compared with a more accurate, yet more complex,
high-contrast approximation. This procedure makes
possible to access the uncertainties associated with
the parameters estimated from P-SV AVO.

AVO effects in large offset P-wave data (up to 60°) is
investigated. In addition to conventional P-wave
AVO attributes (A and B) a third term (C) is exa mined.
Background trends for crossplots of the attributes
are related to rock property relationships. Exact
expressions are given for the first 3 terms, (A, B, C),
of the expansion of Knott-Zoeppritz equations of the
reflected P-wave energy. These exact expressions are
compared to the commonly used linearized (first
order contrasts of rock properties) attributes. The
effect of variation in water saturation and porosity in
a sandstone embedded in shale is investigated in a
numerical example, based on rock physics models of
a well log. The AVO attributes show promise to distinguish
so-called fizz-gas (low saturated gas sands)
from commercial gas and differentiate porosity in
reservoir rocks.

AVO is now an established technology and has been
widely deployed as a lithology indicator and also as a
direct hydrocarbon indicator. In recent years this
technology has become a routine processing and its
application to large 3D volumes has relied on the use
of near- and far-offset stack volumes. These volumes
greatly reduce the amount of pre-stack information
that needs to be stored for standard AVO processing.
Additionally, these volumes are easily converted into
usual AVO attributes, like intercept and gradient,
which can then be interpreted in terms of anomalies
and calibrated with well logs. Reservoir
characterization studies make use not only of these
traditional AVO attributes but also impedance
volumes. The near-offset, or the intercept, stack
volume offers a natural way of obtaining acoustic
impedance volume through the use of post-stack
inversion algorithms. However, to invert far-stack
volume one needs an approach capable of estimating
impedances for a variable incidence angle. This
approach has been described in the elastic impedance
function presented by Connolly (1999). In this work
we propose an approach called reflection impedance,
which is based on constant ray parameter and a power
relationship between density and S-wave velocity.
This new method proved to be of better accuracy for
angular impedance estimation and reflection
coefficient recovery when compared with the elastic
impedance approach.

We present a systematic seismic reservoir characterization
workflow that integrates log and seismic data using an artificial neural network.
Seismic attributes are examined both qualitatively and
quantitatively to determine the best discriminators of rock
and fluid properties. These attributes are systematically
classified using an artificial neural network, the Kohonen
self-organizing map (K-SOM) algorithm. Ultimately the
classified litho-facies volume is calibrated to available well
control by applying the K-SOM technology to well-derived data.
The product is a seismic-scale rock and fluid properties reservoir model that is consistent with borehole and surface seismic data.
The workflow is applied to the characterization of a Vicksburg-age reservoir in South Texas.

When complex structures with large velocity contrasts are present, wavefield techniques can provide improved imaging (structure, amplitude, and phase) over the more traditional Kirchhoff integral methods. Wavefield techniques rely on two elements: firstly, an advanced acquisition design with complete azimuthal coverage; and, secondly, wave equation based prestack
imaging methods. Together, these two elements offer improved seismic imaging in complex structural environments by better handling of multipathing, amplitudes and phase. A wavefield
experiment was conducted last year in a complex subsalt area of the deepwater Gulf of Mexico using a full-azimuth vertical cable acquisition strategy. Here, we discuss some of the unique
challenges associated with wavefield imaging such as velocity analysis and residuals correction. Finally, comparisons will be shown between conventional Kirchhoff imaging and wavefield imaging.

This paper has three purposes. First, it offers an overview
of the nature and history of the interior and
offshore basins of Northeast Brazil and West Africa,
and their relationship to the development of the
Southern and Equatorial branches of the Atlantic.
Second, it places emphasis on the differences of the
rift systems and the control exercised by basement
structures on the conjugate margins of West Africa
and Northeast Brazil. Third, as the most important, it
challenges the idea of a mantle-plume-generated
triple junction between Northeast Brazil and West
Africa.The Benue Trough is very often cited as one of
the best examples of a failed arm of an RRR triple
junction (Burke and Wilson, 1976), formed through
the activation of a mantle plume. This would imply
in: (1) early crustal doming, warping and flexure over
a plume with crestal alkaline volcanoes; (2) formation
of three simultaneous rift arms, symmetrically orientated
at 1200 to each other; (3) if one rift arm is
aborted, the other two develop into a single ridge
(new margin) and drift begin; and (4) growth of delta
at the mouth of a failed arm. Among them, just the
last one is corroborated by the data set. Voluminous
magmatism clearly post-dated the opening of the
Equatorial Atlantic. Rifting and drifting were diachronous
among the African - Brazilian basins and the
rift arms are not symmetrically orientated, and finally,
instead of one, there are two failed arms: Benue
Trough and the onshore Potiguar basin.

The deep and ultradeep water regions offshore east Brazil is
increasingly attracting attention from exploration and
production companies. Brazil hydrocarbon production is
derived mainly from the giant deepwater fields of the
Campos Basin. By contrast, the deepwater regions of the,
adjacent to the south, Santos Basin have remained largely
under-explored. In actuality, Santos Basin is one of the
largest under-explored basins of the world. In the last two
decades, there have been several significant discoveries in the
southern portion of Santos Basin mainly on carbonate
reservoirs. Recently in 1999 a significant discovery occurred
on Upper Cretaceous turbidite reservoirs in deepwater. This
paper outlines the encouraging hydrocarbon potential of the
deepwater realm of Santos Basin by utilizing a recently
acquired, processed and interpreted 2D dataset which has
revealed numerous, largely untested, play types. These plays
include both structural and stratigraphic components. Saltrelated
large 4-way closures, including faulted and unfaulted
turtle structures and drapes over salt have been identified
along with sub-unconformity traps. All these identified play
types have examples with associated amplitude anomalies,
which are widespread throughout the basin, at varying
stratigraphic and structural levels.

The finite element method is a very powerful tool
for modeling complex problems in physical science,
engineering and geophysics. However, for three
dimensional analysis, the arduous pre-processing
stage which consists of supplying the data for the
mesh and the physical properties for each element is,
certainly, a weakness of the method. In order to
handle this disadvantage, we developed a simple
Windows graphic interface for entering the
information of the physical properties across the mesh
at no great pains. We use the 3-D Resistivity and IP
modeling problem to show how the proposed graphic
interface facilitates the access of finite element data.
Apparent resistivy and PFE pseudo-sections at 10
parallel survey lines, one dipole unit apart, are
displayed graphically to exhibit the output graphic
interface that was also developed for presentation of
the modeling results.

The coast effect on magnetotelluric (MT) data acquired
near the Brazilian SE coast in the state of São
Paulo is investigated using two-dimensional (2D)
modeling. The Atlantic ocean was modeled as a layer
of constant resistivity (0.3 ohm.m) and thicknesses up
to 2.0 km at 160 km from the coast, according to
bathymetry. Offshore Santos Basin was also included
in the model using parameters derived from boreholes
and seismic lines. The continental resistivity model
was constructed based on the electrical structure inferred
from previous MT studies. The modeling exercise
shows that the ocean-coast effects are very important
for periods greater than some seconds at distances
of tens of kilometers from the coast. Synthetic
data from the model are compared with a MT sounding
carried out in the Serra da Mantiqueira plateau.
Splitting between TE and TM modes observed at a
period of 30 s in the MT data can be explained as
being generated by electric currents induced in the
highly conductive Atlantic ocean (coast effect), about 90 km away.

An electromagnetic system was developed
using some parts of an old Slingram ABEM EM35/88
as transmitter and receiver coils and matching
transformers. Modern circuits with low noise lock-in
amplifiers and analog multiplier were used to detect
signal from a high magnetic permeability receiver
coil. In phase and quadrature voltage of lock-in
amplifiers outputs were achieved using square
waveform reference from the transmitter coil circuit.
Comparison of these voltage by a precise analog
multiplier give us the electrical apparent conductive
of the overburden layer (conductor). One third lock-in
circuit was used to adjust the spacing intercoil by
nulling the total field component.