Geophysics Flashcards
broad definition of geophysics
the application of physics to understanding the Earth
narrow definition of geophysics
the use of physics-based techniques to obtain information about the subsurface
divisions in geophysics
global (solid-earth), exploration (applied)
environmental geophysics
subset of applied geophysics oriented to the shallow subsurface
Why use geophysics?
images the subsurface directly, non-intrusive and does not disturb the medium, can give 2D or 3D information, measures properties in situ, cheaper than drilling
principal methods in applied geophysics
gravity, magnetics, seismics, electrical, self-potential, induced polarization, ground-penetrating radar, radiometrics, remote sensing, heatflow, borehole geophyics
caveats of geophysics
measures physical properties, not lithology, depth can be ambiguous, measurements are non-unique
We address the issues of geophysics by:
integrating multiple methods
target
the feature or object to be located by geophysical means
resolution
a measure of a method’s ability to locate spatial anomalies
sensitivity
a measure of a method’s ability to detect variations in a property
model
a description of a the spatial distribution of relevant physical properties
forward modelling
prediction of geophysical data for a given geological area from a model
inversion
automated recovery of a model from geophysical data
targets in applied geophysics
hydrocarbons, metallic minerals, non-metallic minerals, industrial minerals, groundwater, hydrothermal, geotechnical, environmental, archaeological and forensic, geophysical risk assesment
method used for minerals
magnetics
method used for near surface
electromagnetics
method used for hydrocarbon exploration
seismic reflection
profile
measurement of a laterally varying quantity
mapping
profiling in two dimensions
sounding
measurement of a quantity varying with depth below a surface point
section
combination of a sounding and a profile
3D survey
multiple sections over a large area
signal
desired quantity to be measured
noise
spurious quantity superimposed on the signal
signal to noise ratio
characterizes the quality of the measurement
environmental noise
external sources of noise
cultural noise
environmental noise produced by humans
geological noise
non-target variations in the subsurface which interfere with observation
coherent noise
the same in repeated measurement
incoherent noise
differs for repeated measurement
stacking
averaging together repeated measurement
filtering
removing a range of frequencies from the data
muting
deleting data points in a certain time or space range
deterministic signals
rule based and may be predicted
stochastic signals
behave in statistical ways according to probability but cannot be predicted in advance
For most geophysical measurements, we assume that our data will consist of ____ signal with ____ noise.
deterministic, stochastic
periodic signal
repeats at precise intervals
transient signal
restricted to a limited length of time and doesn’t repeat
analog data
continuously varying
digital data
measured at discrete intervals
equivalent quantity to frequency in space
wave number, spectral frequency, k
units of angular frequency
rad/s
reasons for spectral representation
many natural phenomena are oscillatory, earth’s effect on signals is often linear, spectral representation is useful for manipulation, sinusoids are pure frequencies
Fourier’s theorem
any periodic signal may be represented as a sum of sinusoids at fixed frequencies
three parameters to describe a sinusoid
amplitude, frequency, phase
phase difference of 180degrees
polarity reversal
spectral analysis
decomposition of a signal into its constituent sinusoids
wave
signal which varies in space and time
if a wave of constant frequency travels into a medium with decreasing velocity, its wavelength will ____
shorten
geophysical resolution is better in ______ velocity media
lower
digitization
conversion of a signal from a continuous measurement to a discrete representation
limitations imposed by digitization
length of data, finite sampling interval, finite sampling accuracy, finite dynamic range
longest period limit
to observe a given period or wavelength, it must fit within the recording interval
shortest period limit
sampling interval must be 1/2 of the wavelength or less
Nyquist period
the shortest period recordable at a given sample interval
Nyquist frequency
the highest frequency recordable at a given sample interval
Nyquist frequency formula
fN= 1/2?
aliasing
a fictious lower frequency, must be lower than the Nyquist limit
how to avoid aliasing range
select fN to be greater than the maximum frequency in the analog data, use an analog filter
error intoduced by digitization
uniformly distributed random noise in the range +-?/2
dynamic range
the ratio of the largest to the smallest recordable response
seismology
the study of elastic waves within the earth
seismogram
recording of elastic waves
seismology has the widest range of applications of all geophysical methods because
it is scale independent and allows for a wide range of geometries
properties of seismic refraction
large offset compared to target depth, energy travels mostly horizontally, large sources, measure depth and velocity independently, poor lateral resolution
properties of seismic reflection
short offsets compared to target depth, energy travels near-vertically, smaller and more numerous sources, data are stacked to reduce noise, velocity/depth trade off, gives information on seismic impedance, lateral and vertical resolution are high
properties of downhole tomography
useful for structures that are far from horizontal
bulk modulus
?
Lame coefficients
? and ?
Young’s modulus
E
Poisson’s ratio
?
