Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
109 Cards in this Set
- Front
- Back
|
What Physical property of the subsurface is the gravity method sensitive to?
|
Density (ρ) or, more specifically, density contrasts (Δρ).
|
|
|
What are the range of values for gravity measurements?
|
1.8 - 3.0 g/cc
|
|
|
What processes on Earth cause variations in this property in the subsurface?
|
tides and large bodies
|
|
|
Where is the gravity method commonly used?
|
Useful for complex bodies; Basins, cavities, salt-domes, faults
|
|
|
What processing steps need to be done in order to use gravity data?
|
I. Drift Correction
II. Tie to Absolute Base III. Latitude IV. FA V. BC VI. Terrain |
|
|
Why are density variations significant in gravity measurements?
|
Density varies in the subsurface, therefore, so does gravitational acceleration. Sediments are less dense than basalts. Voids are easy to find because of the huge density contrast.
|
|
|
Why do we tie to a base station when taking gravity measurements?
|
Absolute gravimeters must are large and difficult to calibrate so we use Relative gravimeters and tie to these Absolute Base Stations. Must tie to the absolute base station at the beginning and at the end to close the loop. Must return to absolute base station often enough to assume a linear relationship. The mass and bulk of the Earth change over the day because of tides, so should tie in every hour.
|
|
|
If we want to know our gravity anomaly to a precision of 0.1mGals how well must we know our vertical position?
|
Need to use GPS to get within 10cm
|
|
|
What is the universal gravitational constant?
|
G=6.67×10-11 Nm2kg
|
|
|
How to relative gravimeters work?
|
based on the Zero Length Spring, by holding K constant in Hooke's Law; temp causes instrument drift – so must measure change in time to correct.
|
|
|
Why are simple shapes important for calculating gravity?
|
Simple shapes have analytical solutions for the anomaly they produce, helps with survey design
|
|
|
What is drift correction?
|
Drift correction is used to adjust for instrument drift during measurements.
|
|
|
What is a latitude correction?
|
Since the earth is actually a geoid rather then an perfect sphere latitude correction is used to correct for measurements based on the latitudinal location on the earth.
|
|
|
What is terrain correction?
|
Terrain correction is based on unevenness of the terrain and gravitation pulls that are not in the vertical component.
|
|
|
What is the free air anomaly and correction?
|
Normal gravity (gn) assumes your at sea level as defined by reference ellipsoid, however this is usually not the case, so must account for the vertical difference.
∆Fg=-.3086mGalm Derive Free Air FG=Gm1m2r2 → dFGr=-2Gm1m2r3=-2gr |
|
|
What is the bouguer anomaly(simple and complex)?
|
Bouguer Anomaly - potential error is introduced by assuming density;
Bouguer Correction=gBG=2πGρ∆z=.04193 ρ∆z Simple Bouguer Anomaly= ∆gB= gobs-gn+FAcorrBcorr - does not include the terrain correction Complete Bouguer Anomaly= ∆gB=gobs-gn+FAcorr-Bcorr+ T.C. |
|
|
What is isostatic correction used for?
|
There are often roots beneath mountains that account for large gravity anamolies
|
|
|
What Physical property of the subsurface is the magnetic method sensitive to?
|
Magnetic susceptibility (K), or more specifically, differences in magnetic susceptibility; typically looking at magnetite
|
|
|
What is the range in common values for the magnetic method?
|
Magnetic susceptibility can span several orders of magnitude for 1 rock type (Ex-basalt)
Susceptibility (K) Sed rx .00005 Met rx .0003 Granite .0005 Gabbro .006 Ultrabasic .012 |
|
|
Where is the magnetic method commonly used?
|
Used in recon settings; measurements happen on the go in nanoseconds; huge scale range from deep crustal to buried pottery
|
|
|
What are the magnetic measuring devices and how do they work?
|
Flux Gate Magnetotmeter- measure vertical and horizontal components of Earths field; 1nT precision
Alkali Vapor/Proton Precession – measures earths total field (FE); 0.1nT precision |
|
|
How do we acquire magnetic data?
|
When modeling magnetic data, need latitude and strike of survey
|
|
|
What processing steps need to be done in order to use magnetic data?
|
Goal is to separate Earths total field from anomalous field so we can measure just the anomalous field.
Magnetic Elevation Correction = -.025 nT/m Latitude Correction = .00578 nT/m Magnetic Reduction I. Drift Correction II. Subtract Main Field (IGRF) |
|
|
What are some pros and cons of using the magnetic method?
|
Quantitative data hard to get
Magnetics is one of the most versatile methods; readings happen on the go in nanoseconds; cheap; More complicated than gravity modeling |
|
|
Why do we have a magnetic field to passively measure?
|
nner Core made of ferromagnetic minerals (Fe,Ni); current flow in outer core creates magnetic field; flow can switch directions and cause pole reversals
|
|
|
What are the 3 components of the magnetic field?
|
Earths Main Field, External Field, Crustal Field
Poles=70,000 nT Equator=30,000 nT |
|
|
How do we get temporal changes in the earths magnetic field?
|
Secular Variatons
Diurnal Variations – similar to drift Magnetic Storms -IGRF (International Geomagnetic Reference Field) – stations around the world constantly measure Earths Main Field |
|
|
What is magnetic susceptibility and the equation associated with it?
|
Intensity=I=KH
H=Earths Main Field K=proportionality constant-can vary by orders of magnitude |
|
|
What is diamagnetic, paramagnetic, and ferromagnetic? Give examples.
|
Diamagnetic – low negative susceptibility; Ex- qtz
Paramagnetic – low positive susceptibility; Ex- Pyroxene, Olivine, Amphibole Ferromagnetic – the core; Ex- Fe,Ni,Co |
|
|
What are the advantages of using the refraction method?
|
1. Observations generally employ fewer source and receiver location and are thus relatively cheap to acquire
2. Little processing Is done on refraction observations with the exception of trace scaling or filtering to help in the process of picking the arrival times of the initial ground motion 3. Because such a small portion of the recorded ground motion is used, developing models and interpretations is not difficult |
|
|
What are the disadvantages of using the refraction method?
|
1. Refraction seismic observations require relatively large source-receiver offsets
2. Refraction seismic only works if the speed at which motions propagate through the Earth increases with depth 3. Refraction seismic observations for near-surface structure are generally interpreted in terms of layers. These layers can have dip and topography 4. Refraction seismic observations use the arrival time of the initial ground motion at different distances from the source (offset) 5. A model for the subsurface is constructed by attempting to reproduce the observed arrival times. |
|
|
What are the advantages of using the reflection method?
|
1. eflection seismic observations are collected at small source-receiver offsets
2. Reflection seismic methods can work no matter how the speed at which motions propagate through the Earth varies with depth 3. Reflection seismic observations can be more readily interpreted in terms of complex geology 4. Reflection seismic observations us the entire reflected wavefield (i.e., the time-history of ground motion at different distances between the source and the receiver 5. The subsurface is directly imaged from the acquired observations |
|
|
What are the disadvantages using the reflection method?
|
1. Because many source and receiver locations must be used to produce meaningful images of the Earth’s subsurface, reflection seismic observations can be expensive to acquire
2. Reflection seismic processing can be very computer intensive, requiring sophisticated computer hardware and a relatively high-level of expertise. Thus, the processing of reflection seismic observations is relatively expensive. 3. Because of the overwhelming amount of data collected, the possible complications imposed by the propagation of ground motion through a complex Earth, and the complications imposed by some of the necessary simplifications required by the data processing schemes, interpretations of the reflection seismic observations require more sophistication and knowledge of the process. |
|
|
What is the dielectric constant?
|
measures the ability of a material to store a charge
-Unitless - Ratio of dielectric permittivity of the medium to the dielectric permittivity of free space εr=ε/ε0 -the more electrically conductive a material is, the less dielectric it is Dry Sediments 4-10 Geologic Materials 3-40 Water ~80 |
|
|
What is magnetic permeability?
|
-Measure of the ability of a medium to become magnetized when an electromagnetic field is imposed upon it
-assumed to be 1 for most geologic materials -also the ratio between value for material and value for free space -Magnetite, iron-oxide cement, iron-rich soils all have high magnetic permeability |
|
|
What is the difference between GPR and EM methods?
|
For GPR, wavelengths were very short (often less than one meter)
-For other EM methods, wavelengths are very long: -consider 10kHz waves, V=3x108 m/s -Wavelength= 30km -Wavelength is so much larger than the survey area that we consider it a constant source spatially and only consider time-varying properties of the field -Very different from GPR |
|
|
What is conduction and example of values?
|
- conduct electricity by flow of electrons
-ranges over ~5 orders of magnitude -currents largely carried by ions in pore waters -Generally, Conductivity↑ with porosity↑ -Good Conductors -Seawater, metallic ores -Shales, clays, contaminated water, sulfide -Poor Conductors -Dry qtz sand, granite -Limestone, dolomite, evaporates |
|
|
What fundamental properties are important to EM methods?
|
-EM properties change with Frequency (f) of applied alternating current
-Low frequency inputs = constraints on conductivity -High frequency inputs = info on dielectric properties (10-1000MHz) |
|
|
What is the frequency range for GPR?
|
GPR: 100MHz – 1 GHz
|
|
|
What is the radar velocity equation?
|
V=c/sqrt(μrεr)
|
|
|
What is fundamental about geophysics?
|
takes advantage of variable physical properties, physical property contrasts
|
|
|
Gravity measures:
|
density
|
|
|
Magnetics measures:
|
magnetic susceptibility
|
|
|
seismology measures:
|
velocity and density (ie. acoustic impedence)
|
|
|
radar measures:
|
dielectric constant
|
|
|
Em measure:
|
conductivity and is therefor good for measuring fluids
|
|
|
What is the radius of the earth?
|
6731 km
|
|
|
What are the average crustal thicknesses of the oceanic and continental crust?
|
Continental=40km
Oceanic=5km |
|
|
What is the compositional break down of the crust and mantle?
|
Crust=heterogeneous
Mantle=homogeneous |
|
|
What is the depth of the moho?
|
30-40 km
|
|
|
What is maximum age of oceanic crust?
|
100 ma
|
|
|
What does the lithosphere signify?
|
where convection stops and conduction starts
|
|
|
Why does one use geophysics?
|
to know something about geology or structure of the Earth in subsurface and cant use regular geologic mapping.
|
|
|
What do we use to make geophysical observations?
|
variations in physical properties of the subsurface; measure the physical fields they produce; infer the structure from that rather than physically mapping.
|
|
|
Why do we use different methods in geophysics?
|
range of scales – cm scale to score; almost scale independent; some methods work better at some scales.
|
|
|
Seismic works on what scale?
|
meter to core scale
|
|
|
How can the subsurface vary in terms of physical properties?
|
Deformation in response to a stress; magnetic properties (susceptibility- ability to become magnetized); conductivity; other electrical properties of materials (ability to store a charge)
|
|
|
What observations can we use to infer porosity and why?
|
Conductivity because more water increases conductivity and density because the more porous something is the less dense it is.
|
|
|
How is temperature related to stress and strain, as well as magnetics?
|
Temperature relates to stress and strain; mag susceptibility ( no magnetization above the Currie point);
|
|
|
What are we trying to determine with geophysical measurements?
|
lithology/porosity/permeability.
|
|
|
How is gravity measured?
|
measure changes in acceleration
|
|
|
How are magnetics measured?
|
measure frequency of proton precession which is related to the intensity of the magnetic field
|
|
|
How is seismic measured?
|
measure voltage generated from geophone motion and relate to ground motion, get arrival times.
|
|
|
How is EM measured?
|
measure voltage; get conductivity; environmental
|
|
|
How is GPR measured?
|
speed of EM wave
|
|
|
What are measurements used for?
|
First we measure
Then we estimate Then we interpret |
|
|
What applications is geophysics used for?
|
Subsurface Geologic Mapping – crustal scale or basin scale
EM?- Fluid flow – hydrocarbons/contaminants |
|
|
What is a passive source?
|
measure existing field to tell us about properties; no source provided; passive fields not constant and need to be accounted for laterally; both fields have temporal and spatial changes which need to be accounted for accurately to produce accurate measurements of desired field.
|
|
|
Is gravity a passive or active source and why?
|
Gravity is a passive source: because– gravitational acceleration results from attraction between two point mass’s; small mass on our gravimeter is attracted to the main mass (mass of the Earth; gravity field changes spatially on the surface of the Earth; Use latitude correction to account for this; gravity @ poles is greater than equator as a result of radius. Acceleration @ poles is greater than at equators for 3 reasons but changes nicely as a function of latitude; Temporal changes in the gravity field from tides related to the moon
|
|
|
Is magnetics a passive or active source and why?
|
magnetics is passive. because- changes spatially on the surface of the Earth; International Geomagnetic Reference Field (IGRF) has worldwide stations to calculate a reference field which you subtract out to get rid of main field; Changes temporally
|
|
|
What is an active source and what are some examples?
|
An active source is one where we put energy in to get measurements out.
-Reflection -Refraction -GPR -EM |
|
|
Why do we use a base station in gravity and magnetics?
|
to account for diurnal variations. Magnetic storms out of luck. Secular variations accounted for by modeling.
Gravity- relative base station moved backed to in the field to account for drift absolute base station to tie to Earths main field and other surveys; collecting data without tying to a base station make data useless. Other Corrections include – Gravity – terrain; large mass; mountain roots |
|
|
What units is gravity measured in?
|
mGals
|
|
|
What units is magnetics measured in?
|
nT
|
|
|
What instruments do we use taking gravity measurements?
|
Absolute Gravimeter- not used for surveys
Relative Gravimeter- pretty much always use |
|
|
What instruments do we use taking magnetic measurements?
|
Flux Gate- measures vertical and horizontal components; iron core with coils=EM induction & more data but time consuming. Slow – additional info not worth extra time.
Cesium vapor/ Proton Precession – measure total field |
|
|
Why would one want to use gravity measurements?
|
longer wavelengths for deeper structures. Use wavelength filtering to get rid of longer or shorter wavelengths.
Combined with magnetics one can generate a psuedo-geological map based on lithological ties. |
|
|
What are the depth limitations of gravity?
|
no real depth limitation; if trying to measure things that are really deep you need a large survey area.
|
|
|
What are the depth limitations of magnetics?
|
relates to the Currie Isotherm, below which materials will not be magnetic
|
|
|
Why do we use regional and residual fields in magnetics?
|
separating long and short wavelengths, doesn’t have to be a large regional feature like Moho; for subtracting regional, assume longer wavelength is linear
Density of crustal materials: 2-3g/cc Density of mantle 2.9-3.5 g/cc |
|
|
What is the depth limitation of seismic and why?
|
increasing depth, source attenuates, lose high frequencies, and lose resolution
|
|
|
What are the relative changes like with reference to the magnetic method?
|
not as many corrections, theoretical change in magnetic intensity with distance from the source, much smaller nT/m than mGal/m change. So doesn’t change very much as a function of radius.
range of densities: 2-3g/cc range of susceptibilities: large |
|
|
What is the difference in gravity and magnetic fields as far as a measurement approach?
|
fields changing as a function of radius, latitude, time; Since you remove the main field, the anomaly for the same object in a gravity survey will be the same wherever you are on the Earth. For Magnetic surveys, anomaly will be different at different locations
|
|
|
What does gravity produce as far as models go?
|
almost always make a 2d x-sectional model or maps of basins
|
|
|
What does magnetics produce as far as a model goes?
|
make contour maps of magnetic intensity.
|
|
|
What considerations should one make when using active source methods?
|
Depth of penetration
Frequency of sources |
|
|
What are the different source types for active source methods?
|
Seismic
Electromagnetic Drift Current (resistivity) |
|
|
What is the definition of seismic?
|
study of how the Earth deforms to an applied stress. Strain wave – deformation moving through the Earth. “Propagation of deformation.” Induced deformation at point A moves to point B. Elastic moduli control propagation of deformation wave by relating stress to strain.
|
|
|
What do we measure for all active sources?
|
resistivity – voltage directly related through Ohm’s Law to resistance
EM – voltage is related to conductivity structure of the subsurface; induced secondary magnetic field generates a secondary voltage. Seismic- measure original source itself. |
|
|
What are seismic source parameters related to frequency? ie What source frequencies for what depth?
|
Basin Scale: 10-100Hz
Shallow: 250-300Hz |
|
|
What are the 3 main causes for energy loss in the seismic method?
|
Spherical spreading
Absorption – conversion to heat energy Energy partitioning - @ boundaries |
|
|
What are the 3 main seismic propagation principles?
|
Snells Law- get angles of ray propagation
Fermats Principle- pathway of a wave ray is that of the traverse of least time Huygens Principle- any point along a wave front acts as a source for new waves. |
|
|
What is the seismic wave equation?
|
Combine Newtons Second Law of Motion with description of material deformation (constitutive equations for elastic isotropic materials with two paramaters lambda and mu).
|
|
|
What is a air wave and how does it travel?
|
slower in seismic than ground waves, faster than ground waves in GPR b/c travels @ Speed of light (C).
|
|
|
What is reflectivity?
|
impedance contrasts at boundaries control how much energy goes up/down, controls the amplitude
|
|
|
What is acoustic impedance?
|
density * velocity
|
|
|
What is the average velocity of granite?
|
6000 m/s
|
|
|
What is the average velocity of unconsolidated wet sediments?
|
2000 m/s
|
|
|
What is the average velocity of water?
|
1500 m/s
|
|
|
How do we collect seismic data and how about when you have dipping beds?
|
-When collecting data we always get info on Refracted and Reflected waves.
-why cant we use refraction data to do reflection? Stacking-CMP-100’s of traces reflect from same subsurface points and add their energy to get rid of noise so we can see the reflector. Cant do this over large areas because of time/cost. -reflections are extremely low amplitude(hard to see); -refraction in general is cheaper -select refraction vs reflection based on scale For dipping beds you shoot forward and backwards in refraction. |
|
|
How is vertical resolution determined in reflection?
|
lambda/4~25m; applicable to seismic reflection
|
|
|
How do we get velocity in reflection?
|
velocity analysis (Ex – x2-t2); NMO-time difference as a function of offset in CMP
|
|
|
How do we get velocity in refraction?
|
Same as reflection, but better constraint on velocity than reflection b/c of long distances. Larger offsets= more velocity sensitivity.
|
|
|
How is GPR similar to seismic?
|
similar to seismic reflection in terms of processing; Biggest differences are EM waves, and Frequency – huge effect on resolution so we can see cm’s; but quicker attenuation limits to a meters; outcrop scale.
Applications – similar to seismic but at smaller scale Use common offset approach Use CMP to get velocity so can convert time to depth. |
|
|
How does conductivity effect the depth of penetration in GPR?
|
high conductivites at the surface will eat up your signal, EX- clay; controls where you can use the method; sand good
|
|
|
How is GPR measured?
|
change in dielectric constant (ability of material to store a charge) but not mapping, make an image; unitless;
water-81 – so water table very easy to see; very sensitive to water Stack offsets like in seismic; units of nanoseconds; quick trace |
|
|
What is frequency domain electromagnetics?
|
two coils, initial coil that you pass a current through generates a primary magnetic field, goes into subsurface, if conductor…creates voltage which induces a secondary magnetic field which produces a secondary voltage which you measure at your second coil. Look at ratio of primary to secondary voltage in terms of apparent conductivity, which can make a map of.
|
|
|
Difference in resistivity and conductivity measurements?
|
instead of measuring apparent conductivity, estimate resistivity; Measuring voltage, inject a direct current in direct contact with ground (unlike EM which is an induction method)
|
|
|
What is seismic refraction commonly used for?
|
Determine layer thickness and seismic velocity
|
