CANADA-BRAZIL COOPERATION
COOPERAÇÃO BRASIL-CANADÁ

The following is the integral text of the Presentation (Preface) of the Final Report of the Airborne Geophysical Survey carried out in three pilot areas in northeast Brazil in March-April 2001. The complete report, an 82-page document in Portuguese, is also available at this site, in pdf format - 7.8 mb.

The electromagnetic-magnetic-VLF-EM helicopter-borne geophysical survey (HEM) discussed in this report was carried out as part of the Northeastern Brazil Groundwater Project (PROASNE – Brasil), a multi-partner, multi-disciplinary technology transfer project under the joint leadership of Serviço Geológico do Brasil (CPRM) and the Geological Survey of Canada (GSC) with partial funding from the Canadian International Development Agency (CIDA).  The project was approved by CIDA and Agência Brasileira de Cooperação (ABC) in April 2000 with the mandate to introduce modern technologies to northeast Brazil that could help develop and better manage the groundwater resources in that drought-prone region. This initiative was also supported by Associação Brasileira de Águas Subterrâneas (ABAS), Superintendência para o Desenvolvimento do Nordeste (SUDENE) and Programa Comunidade Solidária.

PROASNE – Brasil brings together a vast range of technical and socially motivated experts and institutions, Brazilian and Canadian, private and public, to work towards the common goal of improving living conditions in Brazil’s northeast region by providing sustainable solutions to the problems caused by drought.  Amongst the technologies that offer the best hope, airborne geophysics, specifically the frequency  domain electromagnetic methods, rank highly as a potential tool to map the distribution of groundwater in the extensive crystalline bedrock environment that underlies about 80 percent of the million or so square kilometers of NE-Brazil affected by drought.  The groundwater in that environment is slightly saline and, therefore, is expected to behave as a conductor in an induced electromagnetic field.  

The present survey is probably the first helicopter-borne, frequency domain, electromagnetic/magnetic survey ever to be conducted for groundwater under the geological (i.e. fractured aquifer targets in a Precambrian crystalline bedrock environment) and climatic conditions that characterize the northeast of Brazil and, therefore, can be considered to be pioneering this technology.  Canadian and Brazilian geophysicists worked together to determine the most appropriate survey parameters, optimize field operations, and control the quality of the products, and will continue to collaborate in the interpretation of the results and follow-up.  This will ensure that both sides learn as much as possible from each other, and that the methodology is optimized for the needs.  This technology has great potential for application in other arid and semi-arid regions of the world and should be regarded as an instrument of social assistance and development. 

Three pilot areas, one in each of  three separate states of northeast Brazil were chosen to test the methodology and evaluate its effectiveness. They are referred to by the name of the nearest sizeable town to the survey area: Juá, in the state of Ceará, Serrinha in the state of Rio Grande do Norte, and Samambaia, in the state of Pernambuco. The total surface area covered was 357.9 km². At a flight line spacing of 100m and with tie lines at every 500m, the total number of kilometres flown was 4,465.7.

On February 16, 2001, contract number 002/PR/01 was signed between Serviço Geológico do Brasil (CPRM) and Lasa Engenharia e Prospecções S.A to survey these areas using airborne electromagnetic (frequency domain EM) and magnetic (total field) methods. The funding source for the project was 50% CPRM and 50% CIDA (through GSC).

The data acquisition phase was carried out from March to May, 2001 using a Helibras model HB-350 (Scroil) helicopter equipped with the following systems:

a)  Aerodat DSP-99 5-Frequency EM system (FDEM), comprising an EM bird towed by a 30m cable with the following configuration: three horizontal coplanar coil pairs (CP1-874.3/865.0 Hz; CP2–4,865/4,834 Hz; CP3–33,645/33,086 Hz) and two vertical coaxial pairs (CX1-918,5/924 Hz; CX2-4,443/4,525 Hz). This system provides 10 samples per second for In-phase and Quadrature data for each of the five frequencies;

b)  Geometrics G-822 Cesium sensor magnetometer system, assembled in the EM bird towed 30 m from the helicopter, supplying 10 samples of the total magnetic field per second;

c)  Two channel Totem Hertz 2A VLF system, sampling the Quadrature and Total VLF field every 0.1 second from the following VLF stations: NAA – 240 kHz (Cuttler, Maine, USA) and NLK – 24.8 kHz (Seattle, Washington, USA).

The survey was concluded on May 1^st, 2001 and totaled 4,465.7 km of electro-magnetic-magnetic profiles. The flight lines (FL) and tie lines (TL) were flown 100 m and 500 m apart, respectively. Line directions were E-W (FL) and N-S (TL). A high precision Novatel 12 channel GPS system was used for positioning the geophysical data.

GEOSOFT OASIS MONTAJ 5.1 software was used for data processing, besides the proprietary pre-processing and processing routines used for the electromagnetic data that were provided by Fugro Canada. The magnetic data were corrected for diurnal variation, leveled using the tie line intersections, microleveled and IGRF removed in order to generate the residual total intensity magnetic field. EM data were corrected for drift and processed to obtain the apparent conductivity. The latter was calculated for each frequency from In-Phase and Quadrature components using the pseudo layer half space model. The VLF information was reduced to correct for interferences that are common when using this method. Reduction was also applied to determine the base level to compile the VLF In-Phase and Quadrature anomaly components.

The airborne survey produced a considerable number of electromagnetic and magnetic contour and image maps, which are presented at 1:10,000 and 1:20,000 scales. The maps also show basic planimetry, EM anomalies and conductor axes. The list of maps produced is as follows:

a)  Apparent Conductivity (EM) Map for the nominal frequency of 4,500 Hz. Conductor axes and symbols for characterization of the EM anomalies are superimposed on  this map.

b)  Total Magnetic Field (IGRF removed), Total Magnetic Field Reduced to the Pole and Analytical Signal of Total Magnetic Field Maps. Conductor axes and the symbols for characterization of the EM anomalies are superimposed on  the Total Magnetic Field Map (IGRF removed).

c)  Digital Terrain Model Map (pseudo-topography) superimposed on  the base map and flight lines (FL) and tie lines (TL)  path.

The final digital data of the airborne geophysical survey are recorded on CD-ROMs in ASCII GEOSOFT OASIS MONTAJ 5.1 XYZ format. These files contain the following information: raw and corrected data of the total magnetic field, In-Phase and Quadrature EM for the five frequencies and VLF parameters (Total and Quadrature). In addition, values of flight elevation, altitude, GPS positioning and magnetic base-station recordings are also included. Other files included contain grid data, calculated apparent conductivity for the five frequencies, and the EM anomalies.

The airborne geophysical data produced by this survey are intended to be used by the technical personnel involved in PROASNE – Brasil, for interpretation, follow-up and target selection for the drilling of test wells by state organizations involved in the program.  It is anticipated that such procedures, when fully developed and optimized, will increase significantly the rate of success of drilling new high-yielding wells in the survey areas. Other applications may include the selection of sites for Aquifer Storage and Recovery (ASR) technology currently being developed and adapted to NE-Brazil conditions by a Canadian company in collaboration with PROASNE partners in Brazil. 

One of the most important anticipated impacts of using airborne geophysics for groundwater exploration in the northeast of Brazil is that it will "open up" the territory and encourage groundwater exploration and development of the entire region, not just in close proximity to the communities as is currently the case. This should significantly increase the amount of available water resources in the region. It is clear that such an expansion of the area to be developed will require a new approach to water management, including cost effective means of bringing water from distant, often non-electrified, sources to the consumers.  To this end, PROASNE – Brasil  is promoting wider use of solar power to pump and desalinize groundwater for human consumption and agriculture.

 last modified: 2004-09-28

 Helicopter flight height was maintained at an average ground clearance of 60 m and the "bird" with the sensors was suspended 30 m below the helicopter. The average traverse line spacing was 100 m with control lines flown at 500 m intervals. In total, 4466 line km were flown in the 3 areas. A vertically mounted video camera was used for verification of the flight path which was recovered using a high-precision Global Positionning System (GPS).