1. introduction
Northeast Brazil is subjected to periodic droughts that are
often severe and cause the population to migrate from the small communities of
the interior towards the state capitals along the coast, as well as to the
major urban centres of São Paulo and Rio de Janeiro. During drought periods, the Brazilian government spends large
sums of money trucking drinking water to the small communities of the NE, and
drilling large numbers of additional wells to tap the limited groundwater
resources. In much of the Northeast, the
groundwater resources are found primarily in fractures within the crystalline
basement rocks.
Some consultants and drilling companies in the region use
geophysics to optimize well placement.
The primary geophysical methods used are electrical resistivity and Very
Low Frequency (VLF) electromagnetic methods.
The use of these techniques has been shown to improve the success rate
of drilling wells with adequate yields to 85 - 90% (Francisco Said Gonçalves, oral
communication, Natal, June 2000). In northeast
Brazil, a well is generally considered to be productive if it produces more
than 1000 liters per hour (l/hr).
In an effort to improve groundwater exploration and
management in northeast Brazil, including increasing the success rate of
drilling productive water wells, the Canadian and Brazilian Geological Surveys
have initiated a multi-year cooperation project, supported by the Canadian
International Development Agency (CIDA).
The project involves the transfer of technologies and know-how from
Canada in such areas as geophysics, groundwater modeling of fractured media,
aquifer recharge technology, groundwater geochemistry including research on the
problem of groundwater salinity, etc. The
project was ratified in April 2000, and the mission described in this report
has the distinction of being the first technical mission of the program.
The main purpose of the mission was:
(1) To improve the capacity of local institutions to use
ground geophysical techniques to investigate factors that affect the
groundwater supply (e.g. location of aquifers, water quality, flow rate, etc.)
in selected areas of northeastern Brazil;
(2) To help select pilot areas for future groundwater
mapping surveys by airborne geophysical methods, and to organize the collection
of ground orientation data that will be needed for interpreting the airborne
survey data;
(3) To help plan the geophysical component of the
Northeastern Brazil Groundwater Project by recommending additional activities
in which Canadian technologies and specialists may have an input.
After a brief stopover in Rio de Janeiro to visit local
institutions, a three-day workshop was held at the excellent facilities of the
Federal University of Rio Grande do Norte in the capital city of Natal (June 14
to 16). This was followed by field work
in Rio Grande do Norte, Pernambuco and Ceará (Figure 1). During the first two days of the workshop,
Richard Kellett and Gilein Steensma, of the Canadian firm Komex International
Ltd., presented a short-course on the applications of geophysical methods to
groundwater exploration. The third day was
reserved for presentations by Brazilian colleagues on geophysical work being
conducted by the universities, the Geological Survey of Brazil (CPRM), the
Ceará state drilling company (SOHIDRA), and private consultants in NE Brazil.
Ground geophysical field work was conducted in the three
states, to help with the selection of appropriate pilot areas over which to
conduct airborne electromagnetic surveys, and to provide data useful for the
airborne survey design and interpretation.
Ground surveys were specifically conducted to determine the contrasts in
electrical conductivity between basement rock, saturated fractured rock and
thin alluvial cover, and the magnitude of geophysical anomalies that might be expected
from target structures.
1.1
Mission Agenda
The mission began with visits to the headquarters of Companhia
de Pesquisa dos Recursos Minerais (CPRM) and the Observatório Nacional in Rio
de Janeiro, to meet the personnel and discuss geophysical work being conducted
by them in Brazil, as well as inventorying the geophysical equipment in their possession
that may be available for groundwater exploration in the Northeast. A three day short course on the use of
geophysics for groundwater exploration was then presented in Natal, followed by
ground geophysical field work in Rio Grande do Norte, Pernambuco and Ceará. Universities and government institutions
were also visited to determine the extent of the use of geophysics for
groundwater exploration in each state.
1.2
Institutions Visited
The conclusions from visits to government institutions in
Rio de Janeiro are discussed in this section.
Discussions held with individuals from institutions in Natal (Rio Grande
do Norte), Recife (Pernambuco), and Fortaleza (Ceará) are described in section
3.
CPRM, Rio de
Janeiro:
Discussions were held with Samir Nahass, Mario José Metelo,
Alexandre Monteiro, Luis Marcelo Mourão and Maria Laura Azevedo at CPRM. The principal role of the geophysics group
of CPRM in Rio de Janeiro (DIGEOF) is overseeing airborne data collection as
well as processing and compilation of airborne magnetic and spectral
radiometric data. Recently, the group
has also been involved in the field calibration of spectral radiometric data
using technologies transferred from Canada under a previous GSC-CPRM
collaboration project.
It has only been in the last few years that CPRM has used
geophysical surveys for groundwater exploration. There are approximately 10 geophysicists within the organization
nationwide who are trained to conduct such surveys, and cooperative work has
been conducted with the Universidade Federal de Bahia and with the Observatório
Nacional in Rio de Janeiro.
Ground geophysical surveys are conducted from the Rio office
by Alexandre Monteiro. These surveys
are aimed at mapping groundwater and environmental targets, including saltwater
intrusion. The bulk of the ground
geophysics work at CPRM is conducted from the Belo Horizonte office in Minas
Gerais. This group is involved in
mineral and groundwater exploration, but does not generally work in northeast Brazil. Geophysical surveys for groundwater
exploration are also conducted by the Recife and Salvador (Bahia) regional offices. Ground surveys for groundwater exploration primarily consist of
electrical resistivity surveys.
Observatório Nacional, Rio de Janeiro
Discussions were held at the Observatório Nacional (ON) with
Sergio Luiz Fontes and Irineu Figueiredo.
Research at this institution focuses on both solid earth geophysics and
exploration geophysics. ON has
conducted a variety of surveys for groundwater exploration in the state of
Piauí. These surveys have been both in
sedimentary basins and crystalline basement environments. ON is planning to locate 50 water wells in
that state this year. In addition to
groundwater studies, ON staff are involved in geothermal and deep seismology
studies. ON are in the process of
proposing a magnetotelluric program for basin definition in the Amazon
basin. They are also involved in
studies of the San Francisco river basin with Petrobras, and have an offshore
remote sensing project for environmental impact studies from offshore petroleum
extraction operations.
ON relies on federal and state sources to fund their
projects, including cooperative work with CPRM. The institute has a good selection of geophysical instruments
available for their studies. Their
staff includes 12 individuals with Ph.D. degrees, 20 graduate students and 10
technicians. ON is interested in
participating in geophysical surveys in northeast Brazil as needed, and has the
resources and equipment to work in states where geophysical expertise is limited. Strong collaborative ties have developed
between the ON and the environmental geophysics group at Leicester University
(United Kingdom).
The following is a list of geophysical equipment that ON
either has, or that is available to them:
Owned:
Sirotem MK3 Transient Electromagnetic
(TEM) system
Geonics EM34 Frequency domain
Electromagnetic (FEM) system
Geometrics G857 magnetometer
On loan:
Stratagem Controlled Source Audio
Magnetotelluric (CSAMT) system
Auslog geophysical borehole logging
system
2.
SHORT COURSE
A 3-day workshop was held in Natal, Rio Grande do Norte on
June 14, 15, and 16. The first two days
consisted of a short course on the applications of geophysical methods to
groundwater exploration in semi-arid crystalline basement terranes and other
environments. The third day consisted
of presentations by representatives of the states of Rio Grande do Norte (Prof.
Walter Medeiros), Pernambuco (Roberto Gusmão and Prof. Edilton Feitosa) and
Ceará (Francisco Said Gonçalves) on geophysical results, hydrogeological
models, and the state of the geophysical sciences in NE Brazil. Prof. Emanuel Jardim de Sá gave a
presentation on regional structural geology and Mr. Jorge Dagoberto Hildenbrand
of Lasa-Geomag, Rio de Janeiro (now part of Fugro Airborne Surveys Ltd.)
presented some general thoughts about airborne electromagnetic, magnetic and
radiometric surveys. A complete list of
short course attendants, including affiliation and email addresses, is
presented in Table 2-1.
2.1
GeophysiCs Short Course content
The short course included presentations on geophysical
methodology, applications, and case histories of the use of geophysical methods
in a variety of hydrogeologic settings.
The case studies concentrated on fractured bedrock and weathered
crystalline terranes similar to those existing in the semi-arid interior of
northeast Brazil. The course notes are
included in Appendix I. A course
syllabus is listed below.
DAY 1 DAY
2
1. Groundwater
environments 8. Electromagnetic methods II
2. Yemen case study 9. Resistivity and examples
3. Ground
penetrating radar and examples 10. Malawi case study
4. Electromagnetic
methods I 11. Gravity and magnetics and examples
5. VLF case study 12. California case study – basin
6. Seismic methods 13. Regional data
7. Seismic case
study
2.2 Presentations by
Brasilian Colleagues
Summaries of the presentations given on the third day of the
workshop are presented in this section.
2.2.1 Rio Grande do Norte
Dr. Walter Medeiros of the Universidade Federal do Rio
Grande do Norte (UFRN) presented the results of the recently completed M.Sc.
thesis of his student, Jesimael Avelino da Silva, in which two hydrogeologic
models are studied using geophysical methods.
At the Fazenda Inharé site, the location of an intermittent creek is
postulated to be fracture controlled, in what is called the “riacho-fenda”
model. In the second case (at the
Fazenda Santa Rita site) it is found that a creek is aligned with the foliation
of metamorphic rocks, and with weak planes along this foliation. The creek is aligned at an angle to the
primary fracture directions at the site (north-south). Thorough structural geological
characterization of the site combined with electrical resistivity, Very Low
Frequency (VLF), and Spontaneous Potential (SP) data support the models.
2.2.2
Pernambuco
Roberto Gusmão, from the CPRM office in Recife gave an
overview of geophysical capabilities within the organization, including
hardware and software used by the Recife office. The primary method used is electrical resistivity, and processing
includes 2D inversion using Resix2D software developed by Interpex from Golden,
Colorado, USA. Additional information
on CPRM capabilities from his presentation has been incorporated into section
1.2.
Two projects in which electrical resistivity datasets were
used to explore for groundwater east of the pilot area were also
described. In one case, groundwater was
interpreted to be primarily located in open fractures in marble. Fracture directions were predominantly
northwest-southeast and northeast-southwest in marble. In the second case, increased resistivity
zones were interpreted to be due to sands adjacent to crystalline basement, as
well as fracture zones within basement.
Both the fracture zones and sediments are potentially groundwater-bearing.
Prof. Feitosa spoke about the history of the use of
resistivity methods in Brazil, dating back to a French technology transfer
program in 1965. He also talked about
the use of the method for saltwater intrusion identification and the
qualitative use of electrical resistivity for the placement of groundwater
wells. He pointed out that the method
is not as good at finding fractures as it is at finding alteration zones
associated with fractures.
2.2.3
Ceará
Francisco Said Gonçalves from SOHIDRA mentioned that all
their wells are presently located using air photo interpretation, structural
analysis and electrical resistivity.
The group uses a modified Schlumberger array similar to the gradient
method to find zones of decreased resistivity associated with fracture zones,
and showed a number of examples from their practice. They have found that with their approach to siting wells they
have a success rate of drilling productive wells between 85 and 90%.
Dr. Mariano Castelo Branco from Universidade Federal de
Ceará (UFC) described the capabilities and equipment available at his
laboratory, the Laboratório de Geofísica de Prospecção e Sensoriamento
Remoto. Research at the laboratory
focuses on environmental problems and groundwater exploration. The group is involved in remote sensing,
electromagnetic induction surveys, resistivity surveys, ground penetrating
radar, VLF, and geophysical borehole logging.
Example of work at the lab were also shown. The department of Geology has 7 faculty members.
2.2.4 Other presentations
Two additional presentations were given that were not
directly related to geophysical capabilities in the three states, but provided
background geological information and some thoughts on airborne surveys.
A presentation on regional geology was given by Dr. Emanuel
Jardim de Sá (UFRN), which has been briefly summarized in section 4.2.
Mr. Jorge Dagoberto Hildenbrand, general manager of Lasa –
Geomag in Rio de Janeiro gave a brief presentation on acquisition of airborne
EM, magnetic and radiometric data.
2.3
General Impressions
The audience attending the short course had a wide range of
experience in geophysics, as can partly be seen by the list of participants
presented in Table 2-1. It included academics who taught many of the
subjects being covered, university students, geologists, hydrogeologists,
geophysicists, and engineers from state and federal institutions as well as
representatives of industry. As a
result, portions of the information presented were elementary for some
participants. In general, our
experience provides some guidelines for the preparation of future short
courses.
The primary interest of the audience was the case
studies. Many of the participants had a
high level of theoretical background knowledge, and it was the more practical
aspects of data collection, processing and interpretation that were of most
interest to them. Extensive
introduction to geophysical methods was therefore not required.
In order to make future short courses more appropriate to
the type of audience that were present at this workshop in Natal, the program
should include:
Very brief descriptions of methods;
Discussions of potential pitfalls of
methods with examples;
Extensive discussion of interpretation
of data, including pitfalls;
Extensive discussion of case studies and
integrated interpretation.
The case studies of most interest were those focusing on
problems in fractured crystalline basement.
However, we believe that ideas derived from case studies in other
geological settings were generally found to be useful as they provided different
approaches to dealing with certain situations occurring in the Northeast. Although the primary interest of the first
mission was electromagnetic induction methods, there was also much interest in electrical resistivity methods,
ground penetrating radar (GPR), and geophysical well logging. Several participants were also interested in
seismic methods, which are not commonly used for groundwater work in the
Northeast. Seismic methods are
applicable primarily in sedimentary basins, but can also be used to
characterize fractured bedrock.
Regarding language and the difficulties of communicating with
a mostly unilingual Portuguese-speaking audience, this was overcome largely by
one of us (GS) being fluent in Spanish, and by having simultaneous translation
available for the portions of the short course that had to be presented in
English. Simultaneous translation
proved to be extremely effective and should be available in all future courses
presented by non-Portuguese or Spanish speaking instructors. If simultaneous translation is not available,
then a bilingual member of the audience may be able to provide some
interpretation; however, this is far from satisfactory because it is extremely
slow and most often incomplete. Spanish spoken slowly is generally almost as
comprehensible to the Brazilians as Portuguese.
3. state of
geophysical sciences IN NORTHEAST BRAZIL
3.1
Federal INSTITUTIONS
Federal capabilities are discussed only as they apply to the
three states visited. CPRM is active in
the states of Ceará and Pernambuco, but not in Rio Grande do Norte. The Rio de Janeiro office of CPRM has the
capacity to handle large volumes of airborne data. They are not, however, in a position to provide integration with
ground studies and interpretation of ground and airborne datasets. As mentioned before, the Observatório
Nacional is able to support ground investigations in each of the states, as
needed.
3.2 Rio Grande do Norte
Discussions about the use of geophysics in Rio Grande do
Norte were held with Drs. Walter Medeiros, Emanuel Jardim de Sá, and Fernando
Plins, of UFRN.
Resistivity methods, VLF, and spontaneous potential (SP) are
the primary tools being used at the university for the characterization of
fractures and structure that may control groundwater distribution. The department owns an Abem SAS-300
Terrameter resistivity unit, and an Abem WADI VLF unit. A project described in the recently completed
M.Sc. thesis of Jesimael Avelino da Silva proposed two hydrogeologically different
models for sites only a few kilometers apart.
In one case, a stream is believed to be fracture-controlled (Fazenda
Inharé), and at the second site, the
stream is believed to be foliation-controlled.
In addition to the fractured bedrock investigation, the
university has started a project studying dunes in Natal, using Ground
Penetrating Radar (GPR). The dunes are
within protected areas, set aside for groundwater recharge. GPR surveys are planned for fracture
delineation in areas where conductive near-surface layers are thin or
non-existent in the dry interior of the State.
The Department owns a GSSI System 2 GPR unit and has 15 to 80 MegaHertz
(MHz) variable frequency antennae, as well as 200 and 400 MHz fixed center frequency antennae.
The university also plans to conduct research in the use of
seismic refraction and reflection methods to delineate thickness of regolith in
crystalline basement environments, delineate stratigraphy and lateral
continuity of formations in sedimentary basins, and possibly measure directions
of velocity anisotropy due to fractured crystalline basement. The Department has a new 24 channel OYO
DAS-1 seismograph with refraction cables and vertical geophones.
Electrical, VLF, SP and GPR projects at UFRN are lead by Prof.
Walter Medeiros. Prof. Fernando Antonio
Plins heads the seismic group, in addition to being the gravity and magnetic
methods expert in the Department.
UFRN also has a strong remote sensing department. Two faculty members are involved in this
work, Prof. Sebastião Milton Pinheiro da Silva and Assistant Prof. Venerando
Eustaquio Amaro, specializing in the areas of economic geography and structural
interpretation, respectively.
There is a strong interdisciplinary approach to solving
geoscientific and hydrogeological problems at UFRN, with input from
geophysicists, remote sensing specialists and structural geologists. There are no faculty members specialized in
hydrogeology, and to make up for this, the department brings in lecturers, most
often from the Universidade Federal do Pernambuco (UFPE), in Recife.
There is a significant amount of very new, good quality
geophysical equipment at UFRN, and the department is in a good position to
continue and expand research on the usefulness of the different surface
geophysical methods, and transfer this knowledge to the drilling companies and
consultants in order to increase the number of productive wells drilled. The department has about 30 graduate
students, 10 of whom are Ph.D. students.
The combination of strong remote sensing, geophysics and structural
geology groups, places UFRN in a good position to integrate the results of the
airborne geophysical surveys planned later in the year over the selected pilot area.
We have no information on the use of geophysical methods by
the private sector in Rio Grande do Norte.
The Secretaría dos Recursos Hídricos, the institution in charge of
groundwater resources in the state, presently does not appear to use geophysics
as a tool to increase the rate of drilling of productive wells. Their participation in the project and
interaction with the university may change
this.
3.3
Pernambuco
Information on the use of geophysical methods for
groundwater studies in the state of Pernambuco was restricted to conversations
with personnel from CPRM and Prof. Edilton Carneiro Feitosa, hydrogeologist at
the Universidade Federal do Pernambuco (UFPE) in Recife. Conversations with Roberto Gusmão provided further insight into the extent
of geophysical work being conducted at UFPE.
At the CPRM office in Recife, Roberto Gusmão conducts primarily electrical
resistivity surveys for groundwater exploration. He showed data using dipole-dipole arrays with six different separations
between current electrode and potential electrode pairs. Advanced processing and modeling (2D
inversion) is available through the CPRM group in Minas Gerais. Surveys in Pernambuco include work conducted
in crystalline basement, as well as in sedimentary basins. Equipment includes a resistivity meter built
at the Electronics Laboratory at the Division of Geophysics of CPRM in Rio de
Janeiro, as well as a German-made B&B model GES 1/72.
At UFPE, Prof. Feitosa, a retired hydrogeology professor,
has conducted resistivity surveys for groundwater exploration for many
years. His experience is that 35% of
wells are located using resistivity methods, and another 35% using aerial
photography. Electrical resistivity
methods were introduced in Brazil by a French Technology Transfer program in
1965. Another hydrogeology professor,
Geiluson Alves, conducts VLF surveys for groundwater exploration. Professors Valdir Manso and Paulo Correia
have conducted gravity and magnetic surveys primarily for basin definition, and
Joaquim Mota has conducted studies of magnetic susceptibility.
Of the three states, Pernambuco probably has the least
extensive capacity to conduct geophysical surveys. Applications to groundwater exploration are not used widely, with
the notable exceptions of the work of Roberto Gusmão and Prof. Feitosa.
3.4
Ceará
Of the three states visited,
geophysics is used most widely in Ceará.
Geophysical surveys are conducted by geologists at state institutions,
universities, and private consultants.
There were representatives from each of these sectors at the short
course and during the field sessions.
The extent to which geophysics and other methods are used in the state
was discussed with Clodionor Carvalho de Araújo (CPRM), Nizomar Falcão Bezerra
(FUNCEME), Prof. Mariano Castelo Branco (UFC/Geophysics), Prof. Sonia Maria
Silva Vasconcelos (UFC/Hydrogeology), José Pedro Rubens Lima (UFC/Geophysics),
David Lopez de Castro (UFC/Geophysics), Luciano Soares da Cunha (UFC/Geophysics),
Fernando Feitosa, M.Sc. (CPRM), Liano Verissimo (CPRM), Ricardo de Lima Brandão
(CPRM), Walber Cordeiro (COGER), Fernando César M. de Andrade (FUNCEME), Nelson
Palva Paulino de Sousa (FUNCEME), Francisco Said Gonçalves (SOHIDRA), and
Francisco de Assis Capristano (SOHIDRA)
The primary methods used by the state drilling company
(SOHIDRA) and private consultants are electrical resistivity and VLF. The Fundação Cearense de Meteorología e
Recursos Hídricos (FUNCEME), which is the state meteorological, remote sensing
and water resources agency, also is involved in these surveys, particularly
during drought years. Present
statistics kept by SOHIDRA are that 10 to 15% of drilled wells are dry when
sited using a combination of electrical resistivity, aerial photography, and
structural analysis.
Electrical resistivity equipment used by SOHIDRA is built by
TECTROL, a company based in São Paulo.
Resistivity surveys conducted by SOHIDRA consist of placing current
electrodes at a fixed distance of 100, 200 or 300 m apart, and collecting data
with potential electrodes spaced 20-m apart in the central half of the spread
(a modification of the gradient array method).
The data are converted to apparent resistivity values and displayed as a
function of potential electrode midpoint position. Wells are placed on local minima in the calculated apparent
resistivity values. Direct current (DC)
soundings are performed on selective targets.
Interpretation is based on a qualitative assessment of the sounding
curves and curve matching.
A consultant (Walber Cordeiro) attended the field trip and
provided an Abem WADI VLF system which he uses for routine well siting.
FUNCEME has 2 geophysicists who undertake both contaminant
and groundwater exploration projects.
The focus is primarily research.
The group has a Wadi VLF system, an Abem 300C resistivity meter, and a
Stratagem CSAMT system. The latter is
currently being used by Observatório Nacional.
The Laboratório de Geofísica de Prospecção e Sensoriamento
Remoto (LGPSR) at the Department of
Geology of the Universidade Federal do Ceará (UFC) has a wide selection of
geophysical instrumentation. These
include a resistivity unit, an Abem WADI VLF unit, a Robertson borehole logging
system, a Geonics EM34 terrain conductivity meter, a gravimeter, and a GSSI
System 2 ground penetrating radar system with 15 to 80 MHz variable frequency
antennae and 200 and 400 MHz fixed frequency antennae.
The primary line of research at LGPSR is groundwater
exploration and contaminated site characterization. The department currently has 7 faculty in geology and
physics. Research at LGPSR focuses on
groundwater investigations using integrated geophysical, remote sensing and
geological studies. The Department is
in a good position to further research in groundwater exploration, and transfer
this technology to local consultants and drilling companies.
4. Ground
geophysical surveys in proposed pilot areas
Ground geophysical surveys, primarily electromagnetic
surveys, were conducted in, or near proposed pilot areas in each state. The
purpose of the ground geophysical surveys was to determine the electromagnetic
response of basement and potential water-bearing structures (fractured zones
and zones of thicker regolith). The
specific questions that were being addressed were: (1) whether the conductivity contrasts between basement and
potential water-bearing structures were large enough to be observed in airborne
electromagnetic data; and (2) what is the width of the water-bearing
structures.
In this section, we present a brief overview of models
proposed for water-bearing structures.
We then discuss criteria used for site selection, present ground
geophysical survey results, and make recommendations for the airborne surveys
that will be conducted in the pilot areas.
4.1 rationale
for site selection
Several potential pilot areas were identified in each state
by CPRM staff with variable input from local government agencies and the universities. The final selection was based on a
combination of both technical suitability and social needs (i.e. the number of
households and the extent to which they are affected by drought). Due to cost considerations, it was necessary
to limit the size of the pilot areas for the airborne surveys in each state to
approximately 100 sq. km. The
coordinates (UTM and Lat-Long) for each of the pilot areas are given in Table 4-6.
Maps for each area are shown in Figures 3 (Rio
Grande do Norte), 15 (Pernambuco) and 21 (Ceará).
Technical suitability was determined by examining the
structural geology and hydrogeology of the proposed areas, in combination with
the ground geophysical measurements made during the field mission, and data
from previous studies.
Social suitability of a field area was determined by the
presence of one or more villages in need of groundwater, and was determined in
consultation with the water resources institutions of each state.
4.2
groundwater-bearing structures
Expected water-bearing structures in the region include
fault zones, elevated water tables behind groundwater barriers, and groundwater
accumulation in thin regolith and alluvial deposits along creeks. Fractures are primarily associated with
regional tectonic stresses associated with east-west Precambrian compression,
northeast-southwest Cretaceous compression, north-south Tertiary compression
and present-day east-west maximum horizontal stress direction (Jardim de Sá,
oral communication, Natal, June 2000).
The complex structural history of northeast Brazil resulted in primarily
northeast and northwest trending fracture zones (Figure 1).
An interesting model has been postulated by Lages et al. (in
press), in which Paleozoic and Tertiary sandstone-filled grabens are postulated
to exist in crystalline basement. Such
structures, if confirmed, could provide significant sources of groundwater, and
should be considered when interpreting the airborne electromagnetic data.
4.3 Rio
grande Do norte
In Rio Grande do Norte, the university (UFRN) has been
conducting integrated groundwater exploration studies incorporating geophysical
methods and structural geologic mapping.
Three potential pilot areas had been identified by the Secretaría de
Recursos Hídricos and the university prior to commencement of fieldwork, and
ground geophysical surveys were conducted at two of the three areas. The areas are located near Santo Antonio,
Santa Cruz and Equador (Figure 2).
Ground geophysical measurements and survey results are
presented below. Field work
participants in Rio Grande do Norte are listed in Table 4-1.
Ground Geophysical
Survey Results
Measurements of apparent conductivity were made using a
Geonics EM34RT Terrain Conductivity
Meter (EM34) at the Fazenda Santa Rita and Fazenda Inharé sites and near
Equador, in the western portion of the state (Figure 2). A description of the method and technical
specifications of the EM34 are presented in Appendix II.
Fazenda Santa Rita:
A map of the Fazenda Santa Rita site is presented in Figure 4, showing the location of the EM34 profile
collected at that site, profile RN01. This profile is coincident with UFRN profile 7 (da Silva,
1999). There is a productive water well
drilled into bedrock at a distance of 27 m along this profile. A dry well is located at a distance of 80 m.
EM34 data along profile RN01 were collected with coil
separations of 10, 20, and 40 m in both the vertical and horizontal dipole
modes (Figure 5).
Measurement spacing was one half the coil spacing, and data points were
linearly interpolated to 5 m spacing in the 20 and 40 m dipole data for display purposes. Also shown in Figure 5
is a schematic interpretation of the data.
No numerical inversion was performed on these data.
Lateral variations in the
horizontal dipole datasets indicate that measurements in the southern portion
(south of -115 m) and northern portion (north of +80 m) are located over
crystalline basement, as observed in the field. The conductivity of basement is 5-10 mS/m. The central portion of the profile has
increased conductivity values at the smaller coil spacings reflecting the
presence of alluvium (75 mS/m). These
values decrease with increasing inter-coil spacing as the shallow basement
becomes an increasingly more significant portion of the measured volume. Interference is observed in the data (particularly from the 10 m coil
spacing) from fences that crossed the profile at –25 m and 65 m.
Fracture zones in the crystalline basement are interpreted
at –75 m and +40 m, as observed by the characteristic negative excursion in the
vertical dipole data and general increase in the horizontal dipole conductivity
values. A third fracture zone has been
tentatively interpreted near –20 m, although the presence of a fence at this
location makes this interpretation tenuous.
This scenario shows one of the problems that must be faced when
interpreting electromagnetic data in areas where there is cultural interference
such as fences, power lines, and buried metallic pipes.
The southward dip in the southernmost fracture zone, and
northward dips in the two other interpreted fracture zones is based on slight
asymmetry of the vertical dipole anomalies.
Foliation at the site dips to the south. The direction of mapped surface
fractures is generally north-south. The
single profile that was collected at this site cannot provide further
information to differentiate between the foliation-controlled and fault
controlled models for the site.
A pseudosection constructed from 17 Schlumberger soundings
collected by UFRN along profile 7 is presented in Figure 6
along with the 20 m vertical dipole EM34 data.
The central, shallow part of the section has lower resistivity,
following the pattern observed in the EM34 data. Higher resistivities are observed in the shallow part of the
section on the south side of the profile, but not on the north side, as the
EM34 data similarly indicates.
Decreased resistivity values at depth are observed at –20 m and +80
m. A fence exists near –20 m (Figure 5) and it is possible that the slight decreases
observed in the 20 m and 40 m vertical dipole data reflect the same
structure. The schematic cross-section
devised by UFRN for the profile (da Silva, 2000) is shown in Figure
7.
Fazenda Inharé:
A map showing the location of three geophysical profiles
collected at the Fazenda Inharé site is presented in Figure
8. The location of the creek is
dominantly fracture-controlled and the targets of the survey were fractures
zones passing through both a highly productive and a dry well.
Profile RN02 (Figure 9), placed
approximately along UFRN profile 9, was located close to a highly productive well
(7000 l/hr from a depth of 80 m). A 10
cm wide cataclasite zone observed along a N20W, 75NE dipping fault was also
crossed by this profile. The profile
shows typical low conductivity associated with bedrock (<10 mS/m) in the
vicinity of the well. Conductivity
values increased over the alluvium in the creek, and continued to increase over
the farmland on the western bank of the creek.
Profile RN03 (Figure 10) shows two
possible fracture zones near 90 m and 140 m.
Profile RN04, collected along the riverbed (Figure
11), shows low conductivity values corresponding to outcrops along the
river up to a distance of 60 m. A
single fracture zone is observed at 110 m, and slight asymmetry is interpreted
to be due to northeastward dip of this structure, which is interpreted to be a
fracture zone associated with the cataclasite.
Equador:
The location of the EM34 profile collected in the Equador
area is shown in Figure 12. The site is in an area with significantly more topography than
those previously discussed. Significant
deformation and fracturing of crystalline basement is evident in outcrop, and a
number of highly productive wells exist at the site. The EM34 profile collected at the site is presented in Figure 13. A
single fracture zone is interpreted at a distance of 30 m. A line joining two of the productive wells
would intersect the profile at a distance of 80 m, suggesting that the
wells probably produce from different
fractures. One of these fractures may
be that observed at 30 m. Very low
conductivity values (<5 mS/m) indicate the absence of a weathered near-surface
layer. A single measurement was made on
an outcrop of mica schist approximately 400 m west of the profile. Similarly low conductivity values were
observed.
Conductivity
Anisotropy:
Measurements of conductivity anisotropy were made in a
weathered mica schist not far from the Equador site. EM34 data were collected with a 10 m vertical and horizontal
dipole configuration. The data are
presented in Table 4-2 as a function of
the angle with the direction of foliation.
Significant conductivity anisotropy is evident from these data, with maximum
values parallel to foliation (8.2 and 7.3 mS/m for vertical and horizontal
dipole measurements, respectively).
Minimum values of conductivity were measured perpendicular to foliation
(3.9 and 2.1 mS/m for vertical and horizontal dipole measurements,
respectively).
It is important to identify electrical anisotropy when
processing and interpreting the airborne EM data. Leveling of the data during processing could be affected by
anisotropy, and variations in airborne EM measurements can be expected as the
angle between flight line and structure varies.
4.4 Pernambuco
CPRM has conducted extensive hydrogeologic and structural
geologic work in the Moxotó area in central Pernambuco, as well as some
geophysical surveys east of the proposed study area. The area encompasses the
Upper Moxotó River drainage basin (Figure 14). Within the basin, eight different locations
were visited as possible candidates for the pilot area in Pernambuco. A ninth site, in the sedimentary Jatobá
basin on the north side of the Moxotó drainage basin was not visited due to
lack of time. Participants in fieldwork
in Pernambuco are listed in Table 4-3.
The Recife division of CPRM has developed a significant
hydrogeologic database that includes well locations, construction information,
and hydrogeochemistry. The database
also defines drainage basins, topography,
surface geology and structure of crystalline basement, and thickness and extent
of alluvial cover. Well construction
and hydrogeochemistry data have been compiled by Manuel Júlio da Trindade Gomes
Galvão. Sergio Guerra used remote
sensing and geoprocessing to derive a geostatistical analysis of areas within
the Moxotó river drainage basin most favorable for the existence of
water-bearing fractures and lineaments.
The hydrogeologic settings of these sites include alluvial
aquifers, which produce good quality water during wet periods, groundwater
barriers in the form of faults and diabase dikes, and fractured crystalline
basement, which produces significant yields of saline water. Reverse osmosis desalinization plants are
effective in producing potable water, but were found to be non-functional in
the majority of cases due to a lack of maintenance and spare parts.
Possible targets for the airborne EM survey include fracture
delineation, mapping alluvial zones (extensive ground work has already been
conducted in this area by CPRM) and possible increased saturated thickness of
salty groundwater behind barriers. The
selected pilot location (Figure 15), near the villages
of Fazenda Nova and Caiçara (just north of the larger town of Samambaia), has
all three types of targets.
Geologically, the two communities are separated by a SW-NE
trending quartzite ridge. A SSW-NNE
trending left-lateral strike slip fault crosses and offsets the ridge near the
location of the road between the two villages.
On the north side of the quartzite ridge there is a zone of thickened
alluvium. Highly productive wells
(14,000 l/hr) exist on the north side of the ridge, whereas in Caiçara only dry
wells have been drilled. The water supply
for Caiçara is primarily from surface water sources which are inadequate to
supply the population during periods of drought.
The quartzite ridge, and the fault cutting it, appear to act
as a groundwater barrier to the outflow from a drainage basin to the
north. The highly productive wells on
the north side suggest that groundwater may be pooled in the alluvium. Since groundwater is saline, such pooling
should be detectable with both surface and airborne EM surveys. The fault itself, or any associated fracture
zones, could also be good targets for geophysical surveys.
Ground Geophysical
Survey Results
EM34 profiles were collected across the fault (PE01) and
from the outskirts of Fazenda Nova onto the alluvial section (PE02). Profile locations are shown in Figures 16 and 17,
respectively. EM34 terrain conductivity
data along profiles PE01 and PE02 are presented in Figures
18 and 19.
Both 20 m and 40 m vertical and horizontal dipole data were collected
along the two profiles. Also shown in
these figures is a schematic interpretation of the data.
Along profile PE01 there are very low conductivity values to
the northeast (< 5mS/m) that are typical for crystalline bedrock. The duplicated measurements between
distances of 360 m and 500 m reflect overlap of two profiles away from, and between a fenced section of the road
along which data were collected. The
lower conductivity values on both vertical and horizontal dipole data
correspond to measurements made away from the fence. The response of the fence is more significant on the vertical
dipole data than on the horizontal dipole data, particularly near a distance of
380 m. To the SW of this area,
conductivity values increase on the horizontal dipole data, indicating a
thickening alluvial section. Within
this zone, there are three anomalies in the vertical dipole data that have
negative conductivity values, as would be expected over a vertical conductive
sheet. These are interpreted as
fracture zones, and the anomaly near 280 m is associated with a fault trace
inferred from the alignment with a small extensional basin containing water and
marking a topographically low point.
Individual fracture sets have been marked at distances of 120 m, 220 m
and 280 m, coinciding with negative anomalies in the vertical dipole data. However, the entire fracture zone, between
approximately 80 m and 320 m, is probably related to the mapped fault. The indicated directions of dip are based on
asymmetry in the vertical dipole data.
This asymmetry is not significant, and should be considered with some
caution, particularly since the vertical dipole data are more sensitive to
misalignments of the coils.
Profile PE02 (Figure 19), collected
over the alluvial section near Fazenda Nova shows that the saturated alluvium
has conductivity values generally above 20 mS/m on the horizontal dipole data,
with values between 30 and 40 mS/m observed on the 20 m coil separation data
between the beginning of the line and 90 m, and between 220 m and the end of
the line. Decreased conductivity values
in the central portion of the profile are interpreted as a zone of shallower
bedrock. An alternative interpretation
could be that the decreased conductivity values in the central portion of the
line reflect increased water quality (decreased salinity), but there is no
independent information to verify this interpretation. There is anecdotal information that the
alluvial aquifer produces the least saline water in wet years. A possible fracture zone is located at a
distance of 225 m.
Two vertical soundings were conducted near a dry well in
Caiçara, and the results are presented in Table
4-4. The UTM coordinates of the
well are 639230E, 9086549N. One
sounding was conducted next to the well, and a second 26 m away from the well,
to ensure that the metal casing of the well did not interfere with the
measurements.
The vertical dipole results away from the well show a
decrease in conductivity as the coil separation is increased from 10 to 20 m,
followed by an increase in conductivity as the coil separation is increased
from 20 to 40 m. Vertical dipole data
are better coupled to horizontal layering, and probably reflect stratigraphy
near the well. The horizontal dipole
data indicate a monotonic decrease in conductivity with depth.
4.5 Ceará
In the state of Ceará (Figure 20)
the participants in the field trip included representatives from academia and
government institutions, private industry, as well as representatives of local
government, schools, and private individuals of the town of Juá, where the
field work was conducted. A list of
fieldwork participants is presented in Table
4-5.
The field work in Ceará included EM34, VLF and resistivity
surveys. These methods are commonly
used by the state drilling company (SOHIDRA), the university, and by private
consultants.
CPRM has been gathering geologic and hydrogeologic data in
the pilot area near Juá, with ongoing measurements of groundwater chemistry and
electrical conductivity, the development of detailed structural maps, and
interpretation of remote sensing images at a scale of 1:25000.
The geological complexity which adds to the difficulty of
locating groundwater in fractured crystalline basement is exemplified by the
hydrogeochemical work described in Vidal Silva et al., 2000. In this article geochemical analyses in two
wells drilled at distances of 5, 20, and 40 m from the retaining dam of a small
reservoir near Juá are discussed.
Geochemical analyses suggest that there is hydraulic connection between
the reservoir and the well 5 m from the dam, but that no such connection exists
with the well 20 m from the dam.
Ground Geophysical
Survey Results
The pilot area in Ceará had basically been selected on the
basis of social needs and potential for geophysical targets, and only needed
confirmation that the geophysical targets were resolvable. The pilot area is near the town of Juá, in
central Ceará, as shown in Figure 20. Maps showing the location of geophysical
profiles collected in the Juá area are presented in Figures
22, 23 and 24.
Four profiles were collected in the Juá area. Profile locations were chosen in the field
based on aerial photographs and a structural map of the area prepared by José
Roberto de C. Gomes, of the regional CPRM office in Fortaleza. The profile locations were selected to cross
structure and lithologic boundaries.
Profile CE01 is a one kilometer long profile, collected
across identified faults and fractures, to estimate the scale of the structures
and their geophysical response. The
EM34 data and a schematic interpretation are presented in Figure
25. Two broad fracture zones are
observed on the western and eastern ends of the line, respectively. The vertical dipole data becomes negative
several times in these zones. A
narrower fracture anomaly is observed near a distance of 620 m. Resistivity data collected along profile CE01
by SOHIDRA geologists are also shown in Figure 25,
after conversion to conductivity values.
The anomaly near 620 m is evident on the eastern side of the resistivity
data as a zone of slightly elevated conductivity values relative to background
values that are less than 5 mS/m.
Differences in the conductivity values between the two surveys reflect
variations in depth of penetration.
An important point supporting the use of airborne EM data,
made by Fernando Feitosa of CPRM, is that the broader fracture zones often do
not show up on aerial photographs. However, even slightly saline water in the same fracture zones
make them excellent targets for airborne EM surveys. Another point, made by Francisco Said Gonçalves, of SOHIDRA, is
that narrow fracture zones, such as that observed near a distance of 620 m,
usually turn out to be effectively dry holes (< 1000 l/hr).
EM34 data collected along profile CE02 using a 20 m coil
spacing and both vertical and horizontal dipole orientations are presented in Figure 26. This
profile was acquired along a road, and the data show a clayey roadbed thinning
eastward and disappearing beyond a distance of 400 m. A possible fracture is interpreted at a distance of 90 m, though
it must be noticed that the vertical dipole data do not become negative.
A dipping fracture is also interpreted at a distance of
about 150 m. The interpretation of the
clayey roadbed is made based on the high conductivity values observed along the
western side of the profile, as well as on the data of profile CE03R (Figure 27), collected across the road, and discussed
later. Also plotted in this figure are
conductivity data calculated from a
resistivity survey collected by SOHIDRA with an AB spacing of 100 m and MN
spacing of 20 m. The resulting
conductivity values are lower than the EM34
conductivity at the 20 m coil spacing.
It appears that the 100 m AB spacing has a larger depth of investigation
than the 20 m EM34 data. There is a small anomaly that is approximately
coincident with the interpreted fracture near 90 m. Discrepancies in the location of the resistivity survey
conductivity maximum and the vertical dipole minimum are likely due to the
different geometry of acquisition.
Profile CE03R was collected perpendicular to profile CE02. EM34 data along this profile (Figure 27) were collected in vertical and horizontal
dipole mode with a 20 m coil separation.
Conductivity values along CE03R are above 20 mS/m on the horizontal
dipole data, increasing to above 40 mS/m near the road crossing. Possible fractures are evident and centered
at 130 m, and near 560 m and 660 m. The
increasing conductivity values on the northern end of the line may be
associated with a lithologic change that takes place across a mapped shear
zone.
The last profile, CE04R, was collected outside, but not far
from the study area, at Fazenda Fumo (Figure 24), near
a highly productive well (8500 l/hr) that had been sited on the basis of a
SOHIDRA resistivity survey. EM34 data
collected with intercoil spacings of 20 m and 40 m are presented in Figure 28 along with calculated conductivities from the
resistivity survey having AB spacings of 200 m and 300 m.
Fractures are interpreted near 60 m, 135 m and 180 m, with a
possible fracture zone near 230 m. The
fracture into which the well was drilled is located at 135 m. There is slight asymmetry in the vertical
dipole data measured with the 20 and the 40 m coil spacing, supporting
northwestward dip of the structure.
5.
recommendations
In the following sections, recommendations are made about
airborne survey design, interpretation and data integration phases of the
project, post-interpretation surveys and other work that is of interest to the
geophysical community in northeast Brazil, and which may become part of future
missions.
5.1
airborne survey design
The airborne electromagnetic surveys recommended for the pilot areas should be frequency domain surveys with measurements at five different frequencies ranging from less than 1 kHz to more than 30 kHz, up to 56 kHz if available. Both co-axial and co-planar coils are recommended to image the steeply dipping conductors and the more f