GEOL 3000 With Assistance from Nigel Wattrus Seismic Reflection subhorizontal geologic structures Siesmic Refraction subhorizonal changes in density or elasticity Gravity contrasts in density sees deep into the crust Magnetics magnetic properties created by the earth s magnetic field Electromagnetic magnetic properties created by user induced field Electrical Resistivity electrical conductivity commonly related to water content or metal content Measures changes in the Earth s gravity field produced by subsurface variations in density Rock density primarily controlled by mineralogy Mafic rocks typically have unusually high densities producing positive gravity anomalies m2 m1 r G Gravitational constant 6 67 X 10 11 m3kg 1s 2 Law of Mutual Attraction Gravitational force on a mass m2 due to the Earth s mass GRAVITATIONAL ACCELERATION Combined with Newton s Law of Mutual Attraction we can define the acceleration of the m2 due to the Earth s mass m1 as 1 gal 1 cm s2 mgal 0 001 gal 1 gravity unit 0 1 mgal How a geologist sees the world How a geophysicist sees the world Changes in g due to near surface changes in mass density m g anomaly m g anomaly Timing falling objects Mass on a spring Timing pendulums RELATIVE measurement much easier to do Spring extension is proportional to the applied gravitational force k is the spring constant Factors affecting Readings Temporal Instrument drift Tides Spatial Latitude Elevation Slab effects Topographic effects Temporal Corrections periodic base station readings g latitude correction Spatial Corrections n FREE AIR CORRECTED g gmeas gn gFA BOUGUER SLAB CORRECTED g gmeas gn gFA gB gTC Sea Level gFA elevation correction 0 3086 mgal h gB slab correction gTC terrain correction Increasing depth REDUCES amplitude of anomaly and INCREASES it s width Non unique results The deep target s anomaly can be reproduced by a larger less dense shallow target USE GEOLOGIC CONSTRAINTS Units g cm3 or kg m3 Typical values Water 1 Sediment 1 7 Sandstone 2 0 Shale 2 0 Limestone 2 5 Granite 2 5 Basalt Gabbro 3 1 Metamorphic Rocks 2 6 2 3 2 6 2 7 2 8 2 8 2 7 3 0 Measures changes in the Earth s magnetic field produced by subsurface magnetic bodies Controlled by mineralogy Coulomb s Law Magnetic Field strength p1 p2 are the strengths of two magnetic poles they can be negative magnetic permeability Note similarity to Newton s Law of Mutual Attraction Force per unit pole strength exerted by magnetic monopole p2 H is magnetic analog of g Unit measure N Amp m tesla T Use nanotesla nT 10 9 T Average strength of the Earth s field is 50 000 nT Magnetic Induction Intensity of induced field is proportional to the strength of the applied external field Magnetic Susceptibility Material Susceptibility x 10 3 SI Air 0 Quartz 0 01 Calcite 0 001 0 01 Pyrite 0 05 5 Hematite 0 5 35 Illmenite 300 3500 Magnetite 1200 19 200 Limestone 0 3 Sandstone 0 20 Shale 0 01 15 Schist 0 3 3 Gneiss 0 1 25 Granite 0 50 Gabbro 1 90 Basalt 0 2 175 WOW Geology of Northeastern Minnesota