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Remote Sensing, Rocks and Soils. Prospecting and Exploration. What we’re looking for is subtle Most rocks, even the most favorable, do not have extractable resources 1% of mineral occurrences are worth detailed study 1% of those are worth drilling 1% of those are worth mining.
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Prospecting and Exploration • What we’re looking for is subtle • Most rocks, even the most favorable, do not have extractable resources • 1% of mineral occurrences are worth detailed study • 1% of those are worth drilling • 1% of those are worth mining
Gravity • Mean value about 9.8 m/sec2 = 1 g • About 0.5% smaller at equator than poles • Map unit = gal (for Galileo) = 1 cm/sec2 • Mean gravity = 980 gal • Maps contour in mgal = 10-6 g • Modern gravimeters can detect .001 mgal variations (= 1 ppb) • A gravimeter is essentially a spring balance.
Gravity Maps • Gravity varies by latitude due to earth’s equatorial bulge and centrifugal force • Need altitude correction = 0.3 mgal/m = 3 x 10-7 g/m • Altitude only correction = Free-Air Anomaly Map • Correct for mass between you and sea level = Bouguer Anomaly Map • Correct for variations in thickness of crust = Isostatic Anomaly Map
Gravity Mapping • Simple corrections for latitude and altitude • Density = Lithology • Can sense deep into crust • Gravimeters are basically sensitive spring balances • Fragile • Prone to drift • Discrete data points • Labor intensive, low detail
Geomagnetism • Magnetic field of Earth = 40 microtesla = 40,000 nt • Varies from 25 to 70 mt • Non-axial • Not centered on the earth • Varies over a human lifetime
Magnetic Mapping • Corrections are complex and time variable • No simple correlation with lithology • Can’t sense deep into crust because heat destroys magnetism • Magnetism is electromagnetic phenomenon • Instruments can be purely electronic • Can record continuously • Can be extremely detailed
Gravity and Magnetic Mapping • Greater sensing depth • Complex corrections • Greater detail possible
