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COMPILATION OF AEROMAGNETIC DATA

ALTIPLANO AND CORDILLERA OCCIDENTAL/ORIENTAL,

BOLIVIA

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A large compilation of high-resolution aeromagnetic and gravity datasets that cover portions of the Altiplano, Cordillera Occidental, and Cordillera Oriental regions of southern Bolivia is now available from EDCON-PRJ, Inc. Several datasets were recently released from the U.S. Geological Survey, and PRJ is editing and merging all of the grids to generate a total magnetic intensity map. EDCON-PRJ's proprietary processing technique is then being applied to generate a low-latitude reduction-to-pole map without distortions.

The aeromagnetic data consist of four surveys: One flown in 1991 covering approximately 25,106 line-kilometers (15,601 line-miles) in the Altiplano, Cordillera Occidental and Cordillera Oriental regions of southwestern Bolivia; another survey flown in 1988-1989 covering 30,000 line-kilometers (18,642 line-miles) in the southern parts of the Altiplano region; the third survey, flown in 1967-1968, covers three non-contiguous areas and a total of 42,000 square kilometers (16,216 square miles) in the Altiplano and Cordillera Oriental regions; and the fourth survey, flown in 1962-1963, is the easternmost of the datasets and covers 27,000 line-kilometers (16,778 line-miles) in the Cordillera Occidental. The two most recent surveys are digital while those flown in the 1960s are analog.

The 1991 survey was flown by BGM Airborne Surveys, Inc. for the Servicio Geologico de Bolivia (GEOBOL) and the Bolivian Ministry of Mines. Aero Service Corporation flew the 1988-1989 aeromagnetic survey for the Yacimientos Petroliferos Fiscales Bolivianos (YPFB) while Prakla of Hannover, Germany, flew the 1967-1968 survey, also for the YPFB. The easternmost 1962-1963 dataset was flown by Swedish General Consulting AB (SGCAB) for GEOBOL. This latter survey is both aeromagnetic and electromagnetic with a flight elevation of only 150 meters draped. See subsequent pages for survey specifications.

EDCON-PRJ has merged the modern digital data with the hand-digitized, older, published surveys to make an aeromagnetic map covering a total area of approximately 100,000 sq km. The data will be continued to a common datum, merged, and reduced to the pole to create a digital data base that can be interpreted and used for geologic mapping at scales of 1:50,000 to 1:1,000,000.

One of the surveys, the YPFB Aero Service Survey, is appropriate for basement structure mapping. To facilitate this undertaking, EDCON-PRJ is using its newly-developed neural network mapping technique which helps quantify subtle relief to a degree not previously possible. The power of the neural network pattern-recognition capability to detect subtle structural anomalies in the presence of intrabasement susceptibility variations is far superior to pre-existing computer anomaly recognition techniques.

An optional regional geologic interpretation of the other geophysical data is available with maps showing geology and recommended prospective sedimentary basins.

Aeromagnetic data permit the mapping of the igneous bodies present in this region of Bolivia because the hypabyssal intrusives that make up the plumbing of the volcanoes are more magnetic than the overlying lava flows and ignimbrite sheets. Paleozoic and Mesozoic sediments are largely non-magnetic. Prospective hydrocarbon areas occur in northwest- or northeast-trending sedimentary basins of Paleozoic through Tertiary age. Many details of basin thickness and faults in these units can be determined from the magnetic data, even under cover of some younger rocks.

Because of the low inclination of the magnetic field vector, the total magnetic intensity map is difficult to interpret for location, depth and strike of magnetic basement and igneous bodies. EDCON-PRJ is using its proprietary low-latitude reduction-to-pole software to reduce the magnetic data to the pole. The resulting filtered magnetic map can then be properly interpreted.

In addition, two large gravity datasets covering much of the same area as the aeromagnetic data have been released to the public and are available from EDCON-PRJ. One dataset containing more than 8,000 gravity stations comes from the U.S. Defense Mapping Agency (DMA). The other dataset contains more than 55,000 gravity stations in various parts of the Altiplano region. It was released by YPFB.

I. Area

The area encompassing the aeromagnetic and gravity data is in the Altiplano, Cordillera Occidental, and Cordillera Oriental, Bolivia, lying between latitudes 1630 and 2300S and longitudes 6630 and 7000W as outlined in Appendix A.

II. Data Specifications

A. Digital Data

1. GEOBOL-BGM Survey -- 25,106 line-kilometers

2. YPFB-Aero Service Survey -- 30,000 line-kilometers

B. Analog Data

1. YPFB-Prakla Survey -- 42,000 square kilometers (3 areas) (16,216 square miles)

2. GEOBOL-SGCAB Survey -- 27,000 line-kilometers (Swedish General Consulting AB) (16,778 line-miles)

C. Gravity Data

1. DMA Gravity Data -- See Appendix D for coverage.

2. YPFB Gravity Data -- See Appendix D for coverage.

The DMA gravity data is from an archive of several surveys, most of which are contained in files at the DMA Gravity Library in St. Louis, Missouri. Information concerning base stations, type of gravity meters used and other technical details can be obtained from the DMA.

Only the digital gravity data was obtained from the YPFB. Therefore, technical information regarding the base stations and gravity meters used cannot be provided by EDCON-PRJ.

III. Scope of Work

A. Aeromagnetic Data Recovery

Since the various magnetic datasets are being acquired with different specifications and parameters, data compilation consists of the following phases:

1. Digital Data Manipulation

All profile data are being edited and re-levelled as required to produce minimum distortion of the magnetic data. The resulting grid also is being edited to remove "herring-boning" and line-oriented anomalies. Grid spacing is commensurate with line-spacing, but the smallest feasible grid-spacing is being used to facilitate and enhance screen-based display systems.

2. Analog Data Recovery

Each individual data block is being recovered in digital format, gridded, and edited.

3. Data Merging

A mosaic of all the survey blocks is being constructed. Level differences and associated elevation differences between the surveys is being compensated, and a new, integrated, levelled grid of the different surveys is being prepared.

B. Reduction-to-Pole

Aeromagnetic maps at the low latitude of the study area (12) are difficult to interpret because magnetic highs are displaced from magnetic rocks that are their source. North-south trending magnetic anomalies are attenuated and east-west trending anomalies are exaggerated. Randomly distributed sources may cause an east-west anomaly pattern.

To remove the distortion caused by the low latitude, EDCON-PRJ is reducing the data to the pole (RTP). As this process is very sensitive to noise at low latitudes, an optimal reduction to the pole algorithm using spectral whitening is being applied to produce a magnetic intensity map that more directly represents the geometry of the geology of the area.

C. Gravity Data

The gravity stations from the DMA and YPFB have been compiled by the U.S. Geological Survey into a simple Bouguer gravity map. This map and a digital grid can be provided without charge to purchasers of the Basic Package of deliverables. EDCON-PRJ will provide interpretative services also. A written or verbal price quote will be provided within 24 hours of a client request for services.

IV. Deliverables

A. Mylar maps at a scale 1:250,000

  1. Total Magnetic Intensity (IGRF removed)
  2. Flight Path (BGM and Aero Service surveys only)
  3. Reduced to Pole Total Magnetic Intensity
  4. Second Vertical Derivative of RTP

B. Shadowgraphs of RTP, two sun angles at a scale of 1:500,000

C. Color map of the Reduced to Pole TMI at a scale of 1:500,000

D. Digital data

1. CD-ROM containing profiles and grid of all surveys

2. Merged grids on CD-ROM

    a. Total Magnetic Intensity

    b. Reduced-to-Pole of Total Magnetic Intensity

3. CD-ROM containing Bouguer gravity grid and list of principal facts

E. Optional Items (with purchase of Basic Package)

Operator maps (HG, VG, AS, etc.)

Additional shadowgraphs, each pair illuminations

Additional color maps at scale of 1:500,000

Interpretation

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