Use of Geophysics in Geologic MapsGeophysical Data allows Buried Geology to be interpreted from its Physical PropertiesCommon Geophysical Methods and their applicationsGravityPhysics of GravityGravitational Acceleration - gGravity AnomaliesMeasuring Gravity AnomaliesGravitymetersGravity CorrectionsLocal vs Regional AnomaliesAffect of Source DepthDensity of Common Earth MaterialsMAGNETICSPhysics of MagnetismSlide 16Magnetic SusceptibilitiesMagnetic AnomaliesUsing Geophysics for Geologic IntepretationsSlide 20GEOL 3000With Assistance from Nigel WattrusSeismic Reflection Seismic Reflection – subhorizontal geologic structuresSiesmic Refraction Siesmic Refraction - subhorizonal changes in density or elasticityGravityGravity - contrasts in density (sees deep into the crust)MagneticsMagnetics – magnetic properties created by the earth’s magnetic field ElectromagneticElectromagnetic – magnetic properties created by user-induced field Electrical Resistivity 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 densityRock density – primarily controlled by mineralogy. Mafic rocks typically have unusually high densities producing positive gravity anomaliesG = Gravitational constant = 6.67 X 10-11 m3kg-1s-2Law of Mutual Attractionrm1m2Gravitational force on a mass m2 due to the Earth’s massGRAVITATIONAL ACCELERATIONCombined 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/s2mgal = 0.001 gal1 “gravity unit” = 0.1 mgalHow a geologist sees the worldHow a geophysicist sees the worldChanges in g due to near surface changes in mass/density• +m > +g anomaly• -m > -g anomalyTiming falling objectsTiming pendulums• RELATIVE measurement – much easier to do!• Spring extension is proportional to the applied gravitational force• k is the spring constantMass on a springFactors affecting ReadingsTemporalInstrument driftTidesSpatialLatitudeElevation“Slab” effectsTopographic effectsTemporal Corrections– periodic base station readingsSpatial Corrections◦FREE-AIR CORRECTED g = gmeas – gn + gFA◦BOUGUER SLAB CORRECTEDg = gmeas – gn + gFA - gB + gTCgn – latitude correctiongFA – elevation correction= -0.3086 mgal . hgB – slab correction gTC – terrain correction Sea LevelIncreasing 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/m3Typical values:◦Water 1◦Sediment 1.7 – 2.3◦Sandstone 2.0 – 2.6◦Shale 2.0 – 2.7◦Limestone 2.5 – 2.8◦Granite 2.5 – 2.8◦Basalt/Gabbro 2.7 – 3.1◦Metamorphic Rocks 2.6 – 3.0Measures changes in the Earth’s magnetic field produced by subsurface magnetic bodiesControlled by mineralogy.p1 p2 are the strengths of two magnetic poles – they can be negative!= magnetic permeabilityNote similarity to Newton’s Law of Mutual Attraction= Force per unit pole strength exerted by magnetic monopole p2 H is magnetic analog of gUnit measure - N/Amp.m = tesla (T)Use nanotesla (nT) = 10-9 TAverage strength of the Earth’s field is ~50,000 nTMagnetic Field strengthCoulomb’s LawMagnetic Susceptibility Intensity of induced field is proportional to the strength of the applied external field.Magnetic InductionMagnetic InductionMaterial Material Susceptibility x 10^3 (SI)*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 WOWWOW!!!!!!Geology of Geology of Northeastern Northeastern Minnesota