2. 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
5. 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.
7. 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
12. Gravity Mapping
• Simple corrections for • Gravimeters are
latitude and altitude basically sensitive
• Density = Lithology spring balances
• Can sense deep into • Fragile
crust • Prone to drift
• Discrete data points
• Labor intensive, low
detail
14. 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
17. Magnetic Mapping
• Corrections are • Magnetism is
complex and time electromagnetic
variable phenomenon
• No simple correlation • Instruments can be
with lithology purely electronic
• Can’t sense deep into • Can record
crust because heat continuously
destroys magnetism • Can be extremely
detailed
18. Gravity and Magnetic Mapping
• Greater sensing depth • Complex corrections
• Greater detail possible
19. Gravity and Magnetic Mapping
Gravity maps Magnetic Maps
Mechanical Instrument Instruments are purely electronic
Discrete readings Continuous readings
Less detail Great detail
Can sense to great depths Can sense only a few kilometers
deep
Simple corrections for latitude and Complex corrections in time and
elevation space
Density correlates with rock type No simple correlation with rock
type
The top image displays visible and near infrared bands 3 (.81µm), 2 (.56µm), and 1 (.66µm) in red, green, and blue (RGB). Vegetation appears red, snow and dry salt lakes are white, and exposed rocks are brown, gray, yellow, and blue. Rock colors mainly reflect the presence of iron minerals, and variations in albedo. The middle image displays short wavelength infrared bands 4 (1.65µm), 6 (2.205µm), and 8 (2.33µm) as RGB. In this wavelength region, clay, carbonate, and sulfate minerals have unique absorption features, resulting in distinct colors in the image. For example, limestones are yellow-green, and purple areas are kaolinite-rich (kaolinite is a clay mineral). The bottom image displays thermal infrared bands 13 (10.6µm), 12 (9.1µm) and 10 (8.3µm) as RGB. In these wavelengths, variations in quartz content are more or less red; carbonate rocks are green, and mafic volcanic rocks are purple (mafic rocks have high proportions of elements like magnesium and iron).
