AEROMAGNETIC DATA COMPILATION

ALTIPLANO, CORDILLERA ORIENTAL, AND CORDILLERA OCCIDENTAL

BOLIVIA

EDCON-PRJ has compiled aeromagnetic and gravity data covering portions of the Altiplano, Cordillera Oriental, and Cordillera Occidental in southern Bolivia. The merged grid has then been reduced to the pole. The compilation is available for immediate purchase.

The aeromagnetic data consist of two modern digital surveys flown in 1991 and 1988-89 and two analog surveys flown in 1967-68 and 1962-63. 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). Prakla of Hannover, Germany, flew the 1967-1968 survey, also for the YPFB. The 1962-1963 survey 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 this modern digital data with grids of older hand-digitized surveys. The data was continued to a common datum and merged to create a digital data base covering approximately 100,000 square kilometers that can be interpreted and used for geologic mapping at scales of 1:50,000 to 1:1,000,000. Further, the merged grid can be used to generate color maps, shadowgraphs, and gradient and derivative maps.

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 bodies. EDCON-PRJ has therefore used its proprietary, low-latitude reduction-to-pole software to reduce the magnetic data to the pole. The resulting filtered magnetic map can be properly interpreted.

An optional regional geologic interpretation of the geophysical data will be presented on maps showing geology, existing mines, mineral occurrences, and recommended prospective areas.

Ludington and others (1991) divide metallic mineral deposits of the region into two groups. The first type of metallic mineral deposit is related to Cenozoic igneous activity. Aeromagnetic data permit the mapping of this igneous activity because the hypabyssal intrusives that make up the plumbing of the volcanoes are more magnetic than the overlying lava flows and ignimbrite sheets. Bolivian polymetallic vein deposits are mostly of Miocene age. Aeromagnetic data can help to determine the distribution of Miocene rocks beneath younger cover. The second type of mineral deposit is unrelated to igneous activity, and occurs in northwest- or northeast-trending sedimentary basins of Paleozoic through Tertiary age. Many details of basin thickness, folds, and faults in these units can be determined from the gravity and magnetic data, even under cover of some younger rocks.

Porphyry gold deposits are associated with Quaternary volcanic rocks in adjacent areas of Chile. Raw magnetic data show the distribution of volcanic centers and give hints about the internal structure of calderas. After reduction to the pole, we expect that the aeromagnetic data will show the distribution of intrusive rocks beneath a cover of less-magnetic flows and ignimbrites, providing a framework for gold prospecting.

I. Area

The area of the Project is in the Altiplano, Cordillera Oriental, and Cordillera Occidental, Bolivia, lying between latitudes 1630 and 2300S and longitudes 6630 and 7000W, as outlined on Appendix A.

II. Data

The data compiled consist of a digital aeromagnetic survey flown in 1991 by BGM Airborne Surveys, Inc., shown on the index map in Appendix B, and certain other aeromagnetic surveys, shown on Appendix C.

A. Digital Data

GEOBOL-BGM Survey -- 25,106 line kilometers

Flight elevation: 5,500 and 5,200 meters, barometric

Flight direction: N30W

Line spacing: Mostly 2 kilometers; Area 3, 500/1000 meters

Date flown: 1991

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

Flight elevation: 5,200 meters, barometric

Flight direction: North-South

Line spacing: 2 kilometers

Date flown: 1988-1989

B. Digitized Analog Data

        YPFB-Prakla Survey -- 42,000 square kilometers

Flight elevation: 4,900, 5,200, and 5,300 meters, barometric

Flight direction: East-West

Line spacing: 5 kilometers, 2-2.5 kilometers in a few areas

Date flown: 1967-1968

GEOBOL-SGCAB (Swedish General Consulting AB) Survey -- 27,000 line kilometers

Flight elevation: 150 meters, draped

Flight direction: East-West

Line spacing: 1,000 or 500 meters

Date flown: 1962-1963

III. Scope of Work

A. Data Recovery and Merging

     1. Digital data manipulation

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

    2. Analog data recovery

Each individual data block has been recovered in digital format, gridded, and edited.

    3. Data merging

A mosaic of all the survey blocks has been constructed by EDCON-PRJ. In addition, level differences and associated elevation changes between the surveys have been compensated, and a new integrated, leveled grid of different surveys has been 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-wes   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 has reduced 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 was applied to produce a magnetic intensity map that more directly represents the geometry of the geology of the area.

IV. Deliverables and Price

A. Basic Package

            Deliverables:

            Digital grids of the following:

1. Total Magnetic Intensity (TMI)

2. Reduction-to-Pole (RTP)

B. Optional Items (with purchase of basic package)

1. Mylar maps of TMI (IGRF removed), scale 1:250,000, set of 11 2,200

2. Reduction-to-Pole of TMI, scale 1:250,000, set of 11 3,300

3. Flight Path (BGM and Aero Service surveys only)

4. Second Vertical Derivative of RTP

5. Any gradient or derivative map

6. Shadowgraphs of RTP, two illuminations

7. Color map of the Reduction-to-Pole TMI, scale 1:500,000

8. Bouguer gravity

9. Digital data

Tapes containing profiles of BGM and Aero Service surveys and Bouguer gravity grid and list of principal facts.

10. Interpretation