Geophysical Exploration Methods

(notes from Colorado School of Mines web site)

Geophysical MethodsPassive methods: measure natural properties or fields of the Earth, e.g., gravity, magnetic, radioactive, etc.Active methods: perturb the natural system and measures the response. Artificial signals are introduced into the earth and subsequently recorded after being modified by the earth materials

Geophysical MethodsGravity - Isostasy, density variationsMagnetic - magnetic intensitySeismic - Reflection, RefractionElectric - Conductance/ Resistance

LimitationsLack of sufficient contrast in physical propertiesInverse modeling: the process of observing the effects (geophys. measurements) and solving for the causeWarning: Various causes could produce the same effect; no unique solution/ interpretationBetter to use more than one method or combine data from one method with some existing info (e.g. well logs, outcrops)

LimitationsResolution: the ability to separate two features that very close together or, the smallest change in input that will produce a detectable change in outputNoise: any unwanted signal, can be from the instrument, uncontrollable conditions, operator; noise can mask the contrast or manifest as an uncertainty in the inversion process

Shallow Subsurface InvestigationsPreferred method: depends on the specific objective of the investigationExamples: Magnetic method to locate a corroded steel drum containing toxic material; seismic refraction for regional mapping of shallow structures; Electrical resistivity for hydrogeological investigations

Steps in Geophysical InvestigationMethod selection: technical, cost considerationsDesigning the survey: choosing the configuration that works best to serve the objectiveAcquisition: conducting the survey, data collectionProcessing: processing of the geophysical data; curve fitting, modeling, signal processing using soft wares (geophysicists are better at this)Iinterpretation: physical meaning of the data, choosing between the alternatives, integrating with info from other sources,

Seismic MethodsReflection: standard in oil exploration (deep)Refraction: for shallow features like depth to bedrock or thickness of the unconsolidated material, Also used in deciphering the internal structure of the earth (very deep, need a strong source of vibration like an earthquake)Wave/ ray behavior similar to light

Limitations- Seismic RefractionSeismic refraction method: density of the layers must increase with depthV3 > V2 >V1 ( if V2 < V1 and V3 > V1 then V2 will not be seen by seis. refractionThe ray bends towards the normal when going from denser (V1) to lighter (V2) medium; refracted into the earthWhen going from a lighter to denser medium the ray is refracted back to the surface

Geophone

Seismic RefractionEnergy source: vibration created by a sledge hammer blow or explosive in a drill holeWave propagation: spherical waves in a homogeneous medium, wave frontsRays: perpendicular to wave fronts, shown on diagramsGeophone: device that detects vibrationsSeismograph: device that records the arrival times

Seismic WavesSurface wave: considered noiseBody waves: P (compressional) and S (shear)Velocity depends on the density of the layers; increases with increasing densityIncident, reflected, and refracted raysRay gets deflected away from the normal (lighter to a denser medium)Critical angle: refracted wave travels along the interface

Snells Law

MaterialP wave Velocity (m/s)S wave Velocity (m/s)Air332Water1400-1500Petroleum1300-1400Steel61003500Concrete36002000Granite5500-59002800-3000Basalt64003200Sandstone1400-4300700-2800Limestone5900-61002800-3000Sand (Unsaturated)200-100080-400Sand (Saturated)800-2200320-880Clay1000-2500400-1000Glacial Till (Saturated)1500-2500600-1000

Shot Record

Shot Record(travel-time curves superimposed)

Travel-Time Curves

Arrival SequenceGeophones record the ground vibrations (vertical motion) caused by wave propagationDirect wave reaches first at the geophones closer to the sourceHead wave (refracted wave) reaches the the geophones located farther away from the sourceReflected waves are not the first in any situation

Important Info for RefractionTime of arrival of first wave at the geophone is the most important for refraction

Velocity DeterminationVelocity = distance/ timeSlope = time/ distance = 1/VVelocity = reciprocal of slopeSlope is a function of the velocity of the layer alone (not thickness)Changing the thickness will change the distance at which head wave arrives first

Calculating the ThicknessIncreasing the thickness of V1 from 50m to 100 increases the distance at which head wave reaches the geophone firstTakes longer because it has to travel at a velocity of V1 (slow) for a longer time (100 m) compared to the original situation where it travels only a short time (50 m)Or, the distance at which it travels faster (V2) is reduced

Calculating the ThicknessEither t0 or xc could be used along with the velocities to calculate the thickness, hConventionally t0 is used