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207 Cards in this Set
- Front
- Back
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ferrimagnetic
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a mineral in which the magnetic moments of the atoms are opposed. However, the opposing moments are unequal and a net magnetization remains.
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Ferromagnetic
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Ferromagnetic minerals exhibit a net magnetic moment in the absence of an external magnetic field due to the alignment of all of the magnetic moments of its constituent particles.
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Magnetic Susceptibility
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The degree of magnetization of a material in response to an applied magnetic field.
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Diamagnetic:
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Materials with negative magnetic susceptibility.
Diamagnetic objects will create a magnetic field in opposition of an externally applied magnetic field, thus causing a repulsive effect. Example: superconductor |
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paramagnetic
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Paramagnetic materials are attracted to magnetic fields.
Their magnetism only manifests in the presence of an externally applied magnetic field. |
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list ferrimagnetic minerals:
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magnetite, titanomagnetite, ilmenite, pyrhottite
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stress
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a force per unit area
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strain
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deformation, change in size or shape
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types of strain
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elastic (recoverable)
plastic (permanent) |
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Hooks Law: stress and strain
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linear relationship between stress and strain for linear elastic solids
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shear modulus
("mu") |
rigidity, resistance to shear
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young's modulus
("E") |
elasticity
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Bulk modulus
("K") |
incompressibility
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Poisson's ratio
("sigma") |
ratio of latitudinal contraction to longitudinal extension
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seismic wave equation
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a partial differential equation that relates the second time and space derivatives of propagating wave disturbances.
Describes particle motions as a function of x,y,z, and t Predicts displacement at the surface as a function of x,y,z, and t |
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Solutions to the seismic wave equation
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Are harmonic
Two types: P (compressional) waves and S (shear) waves Divergence of the seismic wave equation -> P-wave Curl of seismic wave equation -> S-wave |
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P-wave velocity
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alpha=sqrt((lambda + 2 mu)/rho))
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S-wave velocity
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beta = sqrt (mu/rho)
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body waves
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p-waves and s-waves
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In a continuous velocity gradient (increasing downward), waves turn . . .
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upwards
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in a stepped velocity gradient (increasing downward), waves turn . . .
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the angle of incidence increases until the ray is horizontal
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snell's law
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V1 * sin(P2) = V2 * sin(P1)
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when do triplications occur?
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when the ray experiences a rapid velocity increase
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How are low velocity zones identified?
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As gaps in the record: shadow zones on plots of x (distance or depth) vs t
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What happens to waves originating in a shadow zone?
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The zone acts as a wave guide sending waves along the zone to large distances.
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What is a passive seismic source?
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One over which we have no control. Ex: earthquakes. Need location & time info for inversion.
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What is an active seismic source
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One which we control. We control the number of sources, the locations, amplitude, and the timing.
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Forward Problem:
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Given model parameters, compute the result (theory >> prediction).
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Inverse Problem:
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Given the data, determine the model parameters (observations >> theory).
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What is the triangulation method of seismic location based on?
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Travel time difference between P and S waves.
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What factors contribute to the decay of seismic energy with distance from the source?
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geometric spreading, attenuation, reflection and transmission.
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Describe geometric spreading
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Energy decreases with distance from the source. Assumes conservation of energy.
Amplitude proportional to sqrt (Energy) |
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What is attenuation?
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Energy loss due to internal friction / anelasticity.
Internal Friction: conversion of energy into heat Higher frequencies attenuate more. |
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Reflection angle =
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angle of incidence
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How do we calculate transmission angles?
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snell's law
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Seismic boundary conditions:
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Continuity of displacement across a boundary.
Continuity of traction (stress or force) across a boundary. |
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Phase conversion:
P-waves generate: S-waves generate: |
non-vertical p-waves hitting a boundary generate 4 different scattered waves: reflected and transmitted P and S waves.
A shear wave will generate reflected and transmitted shear waves.??? |
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What is the origin of surface waves?
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Consist of energy brought to the surface by body waves
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Describe Love waves
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"surface shear" waves, related to S-waves.
Origin: constructive interference of higher order S-wave multiples. Require velocity increase with depth or spherical geometry to exist. High amplitude, low frequency. Amplitude decreases with depth. Amplitude decays as 1/sqrt(r) = can travel far from the event. |
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Describe Rayleigh waves
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"ground roll"
Origin: coupled P and SV waves. Particle motions are retrograde at surface and prograde at depth. Amplitude decays as 1/sqrt(r) Dispersive in velocity gradients: lower frequencies go faster. |
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What is dispersion?
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velocity depends on frequency. Dispersion is caused by velocity variations.
The effect: interference. (constructive or destructive) Wave packets result from constructive interference. |
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What is signal aliasing and when does it occur?
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Aliasing occurs when the sampling frequency is too low. The result is that high frequency motions manifest themselves as low frequency events.
Nyquest theorem: sampling rate must be higher than or equal to twice the highest frequency of the signal. |
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What is an example of an anti-aliasing filter?
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A low-pass filter may be applied before analog to digital conversion to restrict the signal bandwidth.
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What is a prediction?
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A definite statement of when and where an event will happen.
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What is a forecast?
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A non-absolute, probabilistic statement about an event within a given time frame.
Components include: likelihood of the event, time window, location (depth), and magnitude. |
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Strike-slip behavior: what does strain accumulation / failure threshold depend on?
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velocity of fault movement, coefficient of static friction
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Strike-slip behavior: What does the magnitude / amount of slip depend on?
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coefficient of dynamic friction
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Describe EQ activity at ocean ridges
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swarms of small magnitude earthquakes (magmatic)
magnitude of events rarely exceeds 6.5 max depth: 5-10km (dependent on spreading rate). Note: fast spreading centers will have EQs only on transform boundaries. |
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Describe EQ activity at transform faults
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maximum depths ~ 10km (decreases with spreading velocity)
Note: slow systems have EQs on ridges and transforms, fast systems have EQs only on transform faults. |
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Describe EQ activity at continental collision zones
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The size and strength of the collision zone depends on the size of the indenting block and the forces driving the block (its velocity).
Bigger, fast blocks produce larger EQs |
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Describe EQ activity at continental extension zones
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Large (M = 7.5) EQs occur around 6-15km depth - higher stress drops than plate boundary EQs
Rifts follow zones of weakness and avoid cratons. |
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Describe intraplate EQs.
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Poorly understood.
High stress drops - strong faults. May reflect plate tectonic driving forces. May localize at sites of prior deformation. Triggered seismicity suggests that continental midplates are generally near failure. Intraplate EQs are rare in oceanic crust. |
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What are the differences in deformation and seismicity in the oceanic and continental crust due to?
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Rheology - continental lithosphere is weaker
Density - continental crust is not subducted, older structural heterogeneity - continental crust is assembled from different terranes and old zones of weakness are preserved. |
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Describe EQ activity on continental strike-slip boundaries
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Max magnitude ~ 7.5 - 8
Max depth of seismogenic zone: 15 - 20km complete seismic coupling (deformation matches energy release). exception: creeping sections. |
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Describe EQ activity on subduction zones
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Majority of seismicity occurs here.
Larger EQs occur here (M ~ 9.5) Interplate (between) and intraplate (within) EQs, and deep events. |
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Describe Interplate events at subduction zones
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Recurring (similar size and location).
The nature of the subducting slab explains variations in EQ size along a plate (sediments, seamounts, horst/graben, etc.) |
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Describe intraplate events at subduction zones
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Outer rise - bending before subduction: magnitudes up to 8.5, depths of 25-40km, normal or thrust faulting.
Fore-arc: weak overriding plate, thrust or strike slip faulting, magnitudes up to 7 - 7.5, depths up to 20 - 30km. Back-arc: thrust on faults dipping opposite from subduction zone, magnitudes up to 8.0 |
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Describe deep subduction zone events
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(Wadati-Benioff Zone)
Magnitudes up to ~ 8.5 Too deep for brittle failure. Concentrated at depths of: 100-300km and 500-700km. Perhaps due to phase transitions? dehydrating reactions? unbending of the slab? |
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What are EQ swarms associated with?
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volcanic or fluid flow processes.
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What does this mean?
sigma subC = tau subBeta - mu (sigma subN - P) |
Coulomb stress = shear stress - internal friction (normal stress - pore pressure)
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What is an asperity?
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An area of the fault that is stuck.
EQ ruptures begin at asperities. |
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Explain B values
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B-values are the slope of the frequency / magnitude distribution.
Mainshock/aftershock sequences: b~1 high b: more small EQs than large EQs. Thought indicative of volcanic/geothermal systems. b<1 at asperities and on creeping faults |
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What does a high b value indicate?
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More small EQs than large EQs. Thought indicative of volcanic/geothermal systems.
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What is the Richtor magnitude based on?
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The largest s-wave amplitude relative to a reference event, recorded on a Wood-Anderson seismograph.
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What is the body magnitude based on?
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The maximum amplitude of the P-wave
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What are surface wave magnitudes based on?
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Rayleigh wave magnitudes
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What are coda magnitudes based on?
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The length of the seismic coda.
(Length of wave from event until return to baseline) Useful for shallow events. |
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What is the moment magnitude based on?
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Seismic moment: depends on the length of rupture, displacement, and shear modulus.
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What is a fault plane solution?
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A graphical representation of a double couple focal mechanism.
Limitation: cannot distinguish between nodal planes. |
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What does Green's function do?
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Relates surface displacement due to a force acting on a point.
Relates displacement to force. Accounts for the effects of the Earth's structure: Displacement = G*(Source Force Vector) Must be computed empirically. |
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What does seismic moment depend on?
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seismic moment = shear modulus x avg. fault displacement x area of the fault
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Hook's Law
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stress is linearly proportional to strain
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Newton's first law
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F=ma
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p-wave velocity
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sqrt((K+4/3 mu) / rho)
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s-wave velocity
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sqrt(mu/rho)
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radar wave velocity
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c/sqrt(epsilon_r * mu_r)
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relative permitivity(epsilon_r) of:
rock air water |
rock: 4
air: 1 water: 80 |
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The amount of energy reflected at an interface depends on:
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the density of the material
the velocity difference of the two mediums the angle of incidence |
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Attenuation of radar waves is due to:
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geometrical spreading, loss of energy to heat, conductivity (losing energy to free electrons)
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If we convolve the ground with our input signal, we retrieve the resulting signal. How do we back out information about the ground?
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We measured the output, we know the input, so get back the ground signal we perform a deconvolution
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When filtering:
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Fourrier transfer to the frequency domain. Cut out high or low frequency energy.
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Aliasing occurs when:
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sampling frequency is too low to capture the true frequency of the wave.
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Nyquist frequency
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= 1/2 the sampling frequency.
This is the highest frequency signal that you can accurately capture, Higher frequencies will fold about the nyquist frequency. |
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NMO stacking does what?
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Converts traces to appear as if source and receiver were at the same location.
(Transforms hyperbolas made of many traces to horizontal lines and then adds all traces) This is done to amplify reflections. |
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Diffraction hyperbolas: How will the shape of the hyperbola change in a fast medium as compared to a slow medium?
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Fast medium: less curvature
Slow medium: more curvature |
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Diffraction hyperbolas: If an object is buried some distance and an identical object is buried at twice that distance, how will the shape of the hyperbola change for the deeper object?
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The hyperbola due to the deeper object will be wider and have less curvature.
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What can static corrections correct for?
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topography
lateral velocity variations due to weathering (things we know about before hand) |
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What does a migration attempt to do?
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Return all energy to its source position
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Types of migration:
Diffraction Stack |
Pretend there is a hyperbola everywhere, look at neighboring traces and, given a velocity, stack the energy to a point (hyperbolas become points if data is coherent)
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Types of migration:
Kirchoff Diffraction Stack |
As one proceeds out the wings of a diffraction, the amplitude and phase of the wave change. Kirchoff takes this into account and performs a correction before stacking. (Performs a phase shift).
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Why do we stack data?
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Reflections add coherently, noise does not.
Stacking brings out the amplitudes of the reflectors. |
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What are the boundary conditions for electromagnetic waves?
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The normal component of the B field is continuous across the boundary.
The tangenital component of the E-field must be continuous across the boundary. There are no signal sources: del * B = 0 Time varying electric fields generate magnetic fields: del x E = - dB/dt |
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Phase velocity
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velocity of component waves
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group velocity
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velocity of the pulse
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How are waves propagated?
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Displacements of rock particles.
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Poisson's Ratio
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The ratios of the eolongations:
(change in length/initial length)1 / (change in length/initial length)2 |
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Bulk modulus:
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A measure of incompressibility
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Common assumptions made to apply seismic wave equation:
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The material in which the disturbance occurs is assumed to be isotropic, homogeneous, and infinite in extent.
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Rayleigh waves
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Propagate by particle motion that is confined to a vertical plane, is retrograde elliptical, and is in the direction of wave travel.
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Love waves
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Particle motion is in the horizontal plane and decreases downward.
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Seismic P-wave velocity
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sqrt((K+4/3G)/rho)
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Seismic S-wave velocity
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sqrt(G/rho)
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Unsaturated sediments have (higher / lower) seismic velocities than saturated sediments)
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Lower
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Unconsolidated sediments have (higher / lower) seismic velocities than consolidated sediments
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lower
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Average P-wave velocities for
a) dry unconsolidated materials b) wet unconsolidated materials c) sedimentary rocks d) unweathered igneous and metamorphic rocks |
a) dry unconsolidated materials - 500m/s
b) wet unconsolidated materials - 1500m/s c) sedimentary rocks 4000m/s d) unweathered igneous and metamorphic rocks - 6000m/s |
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The range of frequencies generally used as seismic sources:
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10 - 100 Hz
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Huygens Principle
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All points on a wave front can be considered as point sources for the generation of spherical secondary wavelets; after a time t the new position of the wavefront is the surface of tangency to these wavelets.
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Fermat's Principle
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Principle of least time: The wave path between any two fixed points is the one along which travel time is the least of all possible paths.
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Waves created when a P-wave is incident on a surface
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Reflected P and S, refracted P and S
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Critical Refraction
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The angle of incidence beyond which no refraction occurs and the ray is totally reflected.
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Head Wave
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The critically refracted wave.
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When is diffraction important?
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Most important when sharp changes in curvature have radii that are similar in dimension to seismic wavelengths.
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Seismic exploration methods concentrate almost exclusively on (P-waves / S-waves)
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P-waves
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Ground Roll
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Rayleigh waves
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Travel time curve
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Graph of distance from the shot point vs arrival time.
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What is wave amplitude proportional to?
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sqrt (wave energy)
Thus amplitude decreases as 1/r (E decreases as 1/r^2) |
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Absorption
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Loss of wave energy to heat.
Much greater at high than low frequencies. |
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Why is Earth occasionally referred to as a low-pass filter?
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Lower frequency waves are transmitted with less energy loss, and higher frequencies are progressively attenuated.
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Waves traveling at lower velocities will lose energy (more / less) quickly than waves traveling at higher velocities.
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Waves traveling at lower velocities will lose energy more quickly than waves traveling at higher velocities.
Thus S-waves lose energy faster than P-waves. |
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At low angles of incidence, most of the wave energy is in the (reflected / refracted) P-wave
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At low angles of incidence, most of the wave energy is in the refracted P-wave
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At high angles of incidence, most energy is in the (reflected / refracted) P-wave
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At high angles of incidence, most of the wave energy is in the reflected P-wave
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Geophone design essentially consists of:
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A cylindrical coil of fine wire suspended in a cylindrical cavity in a magnet.
When ground motion occurs, the geophone moves, inducing relative motion between the magnet and the coil due to the inertia of the coil. This motion generates a voltage proportional to the amount of ground displacement. |
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What is the natural frequency of a geophone?
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The frequency for which the output of the geophone has the greatest value.
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Why are geophones damped?
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To control the instruments oscillations about the natural frequency.
Damping reduces the sensitivity of the system, so the amount of damping is a compromise between controlling oscillations and maintaining sensitivity. |
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In what frequency range do refraction surveys usually operate?
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Refraction surveys operate in the low frequency range.
This will provide the largest possible signal for waves that arrive first at the geophone. A typical geophone has a natural frequency of 14 Hz. |
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In what frequency range are most reflection surveys conducted?
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In shallow reflection surveys we are usually interested in as much resolution as possible.
Shorter wavelengths have higher frequencies and therefore higher resolution. Most shallow reflection work seeks to isolate the high frequency component of the seismic pulse. We might choose geophones with natural frequencies of 50 or 100 Hz. |
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What constraint is placed on a digital system?
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Available memory in the digital recorder. More memory is required to store larger numbers and to sample at higher rates.
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What is the most frequently applied technique for shallow subsurface investigations?
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Seismic refraction
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Why is the time - distance plot a straight line?
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The time-distance plot is a straight line because geophones are equally spaced and the wave travels at a constant velocity.
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How do we determine the velocity of a direct or refracted seismic wave?
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Construct a travel-time plot. The wave velocity = 1/slope.
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The travel time equation for a refracted wave is an equation of a straight line with what slope?
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Refracted slope = 1/velocity of the second layer.
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What is the intercept time and how can it be used?
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The intercept time is found by following the refracted ray back until it intersects the time axis.
The thickness of the first layer can be calculated if the intersect time, v1 and v2 are all known. |
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When do reflections arrive relative to the refracted wave?
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Reflections arrive later than the critically refracted wave. The only exception is at the critical distance where they arrive at the same time.
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What is reciprocity?
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The fact that the time it takes the final refracted wave to traverse the entire path (the latest time on the travel time plot) will be the same for a forward and reverse traverse regardless of subsurface geometry.
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Why do we always collect data for both forward and reverse traverses (seismics)?
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This is the only way to determine if an inclined interface is present.
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How do we determine, with seismics, that an inclined interface is present?
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Collect forward and reverse traverses. If the curves are not symmetric, the interface is inclined.
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How do we determine the direction of dip of an interface (seismics)?
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The intercept time is less for the traverse with its energy source in the up-dip portion of the interface.
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When does critical refraction not occur?
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If a low velocity zone lies below a high velocity zone. In this case, no refracted energy is returned to the surface. We see no evidence of a second layer.
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If a low velocity seismic zone is present, will the calculated (incorrect) depth be greater than or less than the actual depth?
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The incorrect depth is always greater than the actual depth.
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The presence of a thin layer will yield a depth less than or greater than the actual depth? (assuming v1<v2<v3)
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The hidden-layer problem due to a thin layer yields a depth value less than the true depth.
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How do we determine the presence of a laterally varying velocity in reflection work?
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The first segment of either the forward or reverse traverse will have a slope less than the second segment.
The location of the boundary may be located by determining the crossover distance of the forward and reverse curves. |
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How are steps or fault scarps recognized on a travel time plot?
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The 1/v2 segments will display offsets with the offset sense switched for forward and reverse traverses.
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What is the delay time?
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The delay time is the time spent by a wave to travel up or down through the v1 layer compared to the time the wave would have spend if traveling the distance entirely in the second layer along the boundary.
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What are the steps to follow if the interface has some 'topography' to it?
Method: reflection seismology |
Use geometric relations to compute the depth (h) under each geophone. This is possible by determining the delay time for each geophone.
Determine velocities by plotting travel time differences (delay times) vs distance. The slope of this plot = 2/v2. |
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What should the spread length (total length of geophone array) of a given seismic refraction survey be, approximately?
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Use spread lengths of at least three times the target's depth.
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A hammer source is generally adequate for refraction survey if target depths are what?
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A hammer source is generally adequate for refraction surveys if the target depth does not exceed 50m and saturated sediments are close to the surface.
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In refraction surveys, line lengths are long. What are some ramifications of this?
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As line lengths of refraction surveys are long, ray paths are long. This means that much attenuation occurs, first break amplitudes are likely to be small, and the higher frequency component of the signal is likely to 'disappear'.
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Reflection wave energy will occur at an interface if (v1<v2 / v2<v1)
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Reflection wave energy will occur at an interface for both v1<v2 and v1>v2.
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What is acoustic impedance?
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Acoustic impedance = (velocity)x(density)
Factor controlling the partitioning of energy at an interface. |
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For reflection travel time curves, is the degree of curvature greatest for shallow or deep reflections?
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The degree of curvature is greatest for shallow reflections, with curvature decreasing with increasing depth.
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How does increasing velocity affect the curvature of reflection travel-time curves?
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Increasing velocity reduces the curvature of reflection travel time curves for constant h.
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What is normal move-out (NMO)?
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The difference in reflection travel times from a horizontal reflecting surface due to variations in the source-geophone distance.
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NMO (decreases / increases) with increasing depth to the reflecting surface.
NMO (decreases / increases) with increasing velocity. NMO (decreases / increases) with increasing source-geophone distance. |
NMO decreases with increasing depth to the reflecting surface.
NMO decreases with increasing velocity. NMO increases with increasing source-geophone distance. |
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One of the ultimate goals in reflection surveying is to produce a seismic section such that all reflections on each geophone trace are presented how?
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One of the ultimate goals in reflection surveying is to produce a seismic section such that all reflections on each geophone trace are presented as if the trace was recorded at x=0.
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How does one plot the distance, time information from a reflection survey?
How does one determine the velocity? |
Reflection survey information is plotted on an x^2 - t^2 graph.
The data then plots in a straight line with the slope = 1/v1^2 |
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How are reflections recognized on a seismogram?
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Reflections can be recognized by NMO and waveform similarity.
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What is the Green method?
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In the Green method, we create an x^2-t^2 plot for each reflection on a seismogram.
The values for the velocity and thickness of the first layer are correct. Values for all deeper interfaces are only approximations because the method does not take into account the curving of the rays at interfaces. |
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What is the Dix Equation and how is it used?
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The Dix equation provides a method for calculating vrms.
The slope on the x^2 - t^2 graph = 1/vrms^2. The Dix equation allows us to calculate the velocity for any layer for which we have reflections from the interfaces bounding the unit on its upper and lower surfaces. |
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For reflection surveying, how long should the geophone spread be?
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For reflection surveys, geophone spreads should not exceed target depths.
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In reflection surveys, what should the source-receiver offset be relative to the target depth?
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For reflection surveying, the source-receiver offsets should be small relative to target depth.
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What is the only geometry that will allow determination of true dip from a seismic line?
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Only if the seismic line is oriented perpendicular to the strike of the plane will true dip be evident.
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What is the optimum window?
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The optimum window is the range of source-receiver distances over which reflections are most clearly seen.
The near side (shorter distance) limit is defined by the coincidence of the low velocity (air) wave with reflections. The far side (upper distance limit) is much less obvious. At larger distances, shallow reflections will interfere with deeper reflections. |
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What are short path multiples?
Are they a concern? Why or why not? |
Short path multiples occur when the wave reflects multiple times off of a thinner layer either before or after traversing a thicker layer.
Short path multiples arrive shortly after a corresponding primary and, as such, are not discerned as separate events. Thus they are not a huge concern. |
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What are long path multiples?
How can they be recognized? |
Long path multiples traverse the entire distance from surface to depth multiple times and are likely to appear as distinct events.
Multiple times are approximately (but not exactly) twice the primary times. Their amplitude is less than that of the primary event. |
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Why is reflection work concerned with enhancing the high frequency component of the seismic signal?
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Reflection work seeks to enhance the high frequency signal in order to remove lower frequency noise to improve the chance of observing reflections and to improve resolution in order to observe detailed structures.
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What is the approximate limit of vertical resolution? (seismics)
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vertical resolution = (wavelength) / 4.
However, in shallow seismic work the more likely limit is (wavelength) / 2 |
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What factors affect horizontal resolution of reflection seismics and how?
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Horizontal resolution is increased with higher frequencies. Geophone spacing also determines in what detail we sample the subsurface.
For horizontal reflecting surfaces, the area of reflection is located halfway between receiver and source. |
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Briefly describe electrical resistivity methods.
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In electrical resistivity methods, current is applied at the ground surface and the potential difference is measured between two points.
Variations in resistance to current flow at depth cause distinctive variations in the potential difference measurements, which provide information on subsurface structure and materials. |
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The flow of charged particles in the ionosphere due to solar emissions is responsible for alternating currents that flow through the upper regions of the Earth. What is this natural current flow called and how can it be used to explore the subsurface?
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These are referred to as telluric currents.
Telluric currents are altered by the varying conductivity properties of rocks. The telluric method takes advantage of these natural current variations by measuring potential differences at the surface. |
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What is current density?
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Current density is defined as the current divided by the cross sectional area of the material through which it is flowing.
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How much is one ohm of resistance?
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One ohm of resistance allows a current of one ampere to flow when one volt of emf is applied.
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Resistance depends on what?
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Resistance depends on the material and its dimensions.
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What is resistivity?
What are its units? |
Resistivity is the fundamental property of the material on which resistance depends.
The units of resistivity are ohm meters. |
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What is conductance?
Conductivity? |
Conductance is the inverse of resistance.
Conductivity is the inverse of resistivity. |
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What is the resistivity of air?
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Air has infinite resistivity.
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How does current flow in relation to equipotential surfaces?
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Current flows perpendicular to equipotential surfaces.
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How do we determine the locations of equipotential surfaces?
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If we calculate the potential at many points, we can draw contours through points of equipotential.
We can then draw current flow lines perpendicular to these surfaces. |
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For a homogeneous, isotropic medium, at what depth is 50% of the current confined?
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50% of the current is confined above a horizontal plane with a depth = 1/2 the current electrode spacing.
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In the resistivity method, what three main actions occur?
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In the resistivity method, current is entered into the ground, potential difference is measured, and resistivity is determined.
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Current flow tends to avoid a poor conductor in favor of a good conductor. Does this mean that more or less current will flow above an interface with a poor conductor?
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A greater percentage of current will flow above the interface with a poor conductor compared to the homogeneous case.
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As current electrode spacing is increased, the current flow at depth ____?
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As current electrode spacing is increased, the current flow at depth is increased.
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The percentage of current penetrating below an interface is controlled by what two factors?
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The percentage of current penetrating below an interface is controlled by the relative magnitude of the resistivities of the two layers and the electrode spacing.
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If the resistivity of the second layer is greater, the flow lines bend (toward / away) the normal and as a consequence are (more / less) widely spaced.
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If the resistivity of the second layer is greater, the flow lines bend toward the normal and as a consequence are more widely spaced.
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When we measure the potential difference between the electrodes, what quantity are the values proportional to?
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When we measure potential difference between the potential electrodes, the values are proportional to the current density.
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Variations in current density near the surface will result in variations in _____
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Variations in current density near the surface will result in variations in apparent resistivity.
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For the measured apparent resistivity values to approach the resistivity of the second layer, how much of the subsurface must be sampled?
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A great volume of the subsurface must be sampled in order for the measured apparent resistivity values to approach the resistivity of the second layer.
This seriously limits the resistivity method because it requires sufficient space in which to place electrodes. |
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What are the axis of an apparent resistivity curve?
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x-axis: electrode spacing
y- axis: apparent resistivity. |
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The effect of the shallow interface is felt. . .
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The effect of the shallow interace is felt even at small electrode spacing.
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What resistivity curves are so similar that they may be impossible to differentiate with field data.
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p 1< p2 and p1 < p2 < p3
or p1 > p2 and p1 > p2 > p3 |
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Which three layer resistivity curves are easiest to identify?
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Those due to the presence of a low resistivity layer between two higher resistivity layers and those due to the presence of a high resistivity layer between two lower resistivity layers.
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An electrical resistivity traverse oriented parallel and close to a vertical contact appears remarkably similar to what?
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A traverse oriented parallel and close to a vertical contact appears remarkably similar to a two layer curve.
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What type of traverse should be used if the goal is to seek information about apparent resistivity variations with depth to define the location of a highly resistive contact?
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Expanding spread traverses
You should always run another traverse at right angles to determine if lateral variations are present. |
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What type of traverse should be used if the goal is to map lateral variations in resistivity?
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Constant spread traversing.
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What is the rule of thumb regarding the depth of current flow in the resistivity method?
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The depth of current flow ~= the electrode spacing.
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What should be done if dry surface materials are present in an electrical resistivity survey?
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If dry surface materials are present, wet the ground around the electrode.
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In the shallow subsurface, current is conducted almost entirely by what?
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In the shallow subsurface, current is conducted almost entirely by the fluid present.
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What controls the resistivity of sediments and rocks in the shallow subsurface?
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The resistivities of sediments and rocks in the shallow subsurface are controlled by the amount of water present and its salinity.
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Increasing silt or clay contents in poorly sorted rocks or sediments will (reduce / increase) resistivities.
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Increasing silt or clay contents in poorly sorted rocks or sediments will reduce resistivities.
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In saturated materials, increasing porosity will (reduce / increase) resistivity.
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In saturated materials, increasing porosity will reduce resistivity.
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Well sorted materials have (poorer / greater) porocities than poorly sorted materials.
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Well sorted materials have greater porocities than poorly sorted materials.
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In general, finer-grained sediments will have (lower / higher) porosities, and (lower / higher) resistivities than coarse-grained sediments.
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In general, finer-grained sediments will have higher porosities, and lower resistivities than coarse-grained sediments.
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What factors reduce resistivity?
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The following reduce resistivity:
increasing water content increasing water salinity increasing clay content decreasing grain size assuming that water is available to fill the voids: increasing porosity increasing fracturing increasing weathering |
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What factors increase resistivity?
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Resistivities are raised by compaction and lithification.
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Bedrock almost always has (higher / lower) resistivities than overlying sediments.
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Bedrock almost always has higher resistivities than overlying sediments.
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Generally, unsaturated sediments above the water table will have (higher / lower) resistivities than saturated sediments below the water table.
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Generally, unsaturated sediments above the water table will have higher resistivities than saturated sediments below the water table.
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What are the two specific problems that arise when trying to interpret ER field data based on comparisons with theoretical curves representing three or more layers?
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Equivalence and Suppression
Equivalence - various models generate very similar curves (non-uniqueness) Suppression - Occurs when the presence of a thin layer on a multilayer sequence cannot be recognized on an apparent resistivity curve |
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Why are geothermal waters better conductors than normal waters?
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Geothermal waters are better conductors than normal waters due to their greater content of dissolved salts.
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