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Lecture 15

 

 

Exploration

 

Geophysical methods

 

•         The approach: Look for signs caused by physical differences in geologic formations

–        Seismic methods

–        Gravity methods

–        Magnetic methods

–        Electric methods

Seismic methods

•         Reflection approach

–        Seismic waves (sonar) propagate based on density and coherence of materials

–        Reflection of seismic waves occurs on sharp boundaries caused by substantial differences in seismic parameters

–        A signal is produced; the time it takes to arrive at a receiver can be converted into thickness of layer and type of formation

•         Most important geophysical method

•         Used for exploration in hydrocarbon and ore deposits

•         Methods on land use trucks or explosives to generate signal

•         At sea, sonar approaches from ship scan large areas of the sea floor

 

Gravity methods

 

•         Earth has a gravitational field, based on gravitational attraction between bodies

•         The latitudinal and altitudinal dependence of the field is well understood

àGravitational methods look for deviations from the expected field due to differences in density of formations

 

Other geophysical methods

•         Electrical methods: tests for conduction behavior of minerals

•         Radiation: Presence of U, Th

•         Magnetic methods:

–        Earth has magnetic field (dependent on latitude) 

–        Some minerals (e.g. magnetite, Fe3O4) are magnetic

–        deviations from expected field can give information on presence of Fe3O4 or similar minerals.

 

Geochemical methods

•         Determination of changes in composition

–        Primary signals – related to original formation process

–        Alteration (see Porphyry Copper Deposits)

–        Secondary signals: Water penetration of ore deposit might change composition of surface waters

 

Systematic exploration

 

•         Most exploration is done by companies, based on specific goals and information

•         One large-scale review was carried out in Canada, looking at MSD in the Canadian Shield (1951-1974)

•         Results: 2100 mineral occurrences were discovered, but only 40 of them were economic

•         The return of most of these deposits was small, only two very sizable deposits

•         Cash flow during exploration, development and running of a mine