Geophysics

Question 1

broad definition of geophysics

Answer 1

the application of physics to understanding the Earth

Question 2

narrow definition of geophysics

Answer 2

the use of physics-based techniques to obtain information about the subsurface

Question 3

divisions in geophysics

Answer 3

global (solid-earth), exploration (applied)

Question 4

environmental geophysics

Answer 4

subset of applied geophysics oriented to the shallow subsurface

Question 5

Why use geophysics?

Answer 5

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

Question 6

principal methods in applied geophysics

Answer 6

gravity, magnetics, seismics, electrical, self-potential, induced polarization, ground-penetrating radar, radiometrics, remote sensing, heatflow, borehole geophyics

Question 7

caveats of geophysics

Answer 7

measures physical properties, not lithology, depth can be ambiguous, measurements are non-unique

Question 8

We address the issues of geophysics by:

Answer 8

integrating multiple methods

Question 9

target

Answer 9

the feature or object to be located by geophysical means

Question 10

resolution

Answer 10

a measure of a method’s ability to locate spatial anomalies

Question 11

sensitivity

Answer 11

a measure of a method’s ability to detect variations in a property

Question 12

model

Answer 12

a description of a the spatial distribution of relevant physical properties

Question 13

forward modelling

Answer 13

prediction of geophysical data for a given geological area from a model

Question 14

inversion

Answer 14

automated recovery of a model from geophysical data

Question 15

targets in applied geophysics

Answer 15

hydrocarbons, metallic minerals, non-metallic minerals, industrial minerals, groundwater, hydrothermal, geotechnical, environmental, archaeological and forensic, geophysical risk assesment

Question 16

method used for minerals

Answer 16

magnetics

Question 17

method used for near surface

Answer 17

electromagnetics

Question 18

method used for hydrocarbon exploration

Answer 18

seismic reflection

Question 19

profile

Answer 19

measurement of a laterally varying quantity

Question 20

mapping

Answer 20

profiling in two dimensions

Question 21

sounding

Answer 21

measurement of a quantity varying with depth below a surface point

Question 22

section

Answer 22

combination of a sounding and a profile

Question 23

3D survey

Answer 23

multiple sections over a large area

Question 24

signal

Answer 24

desired quantity to be measured

Question 25

noise

Answer 25

spurious quantity superimposed on the signal

Question 26

signal to noise ratio

Answer 26

characterizes the quality of the measurement

Question 27

environmental noise

Answer 27

external sources of noise

Question 28

cultural noise

Answer 28

environmental noise produced by humans

Question 29

geological noise

Answer 29

non-target variations in the subsurface which interfere with observation

Question 30

coherent noise

Answer 30

the same in repeated measurement

Question 31

incoherent noise

Answer 31

differs for repeated measurement

Question 32

stacking

Answer 32

averaging together repeated measurement

Question 33

filtering

Answer 33

removing a range of frequencies from the data

Question 34

muting

Answer 34

deleting data points in a certain time or space range

Question 35

deterministic signals

Answer 35

rule based and may be predicted

Question 36

stochastic signals

Answer 36

behave in statistical ways according to probability but cannot be predicted in advance

Question 37

For most geophysical measurements, we assume that our data will consist of ____ signal with ____ noise.

Answer 37

deterministic, stochastic

Question 38

periodic signal

Answer 38

repeats at precise intervals

Question 39

transient signal

Answer 39

restricted to a limited length of time and doesn't repeat

Question 40

analog data

Answer 40

continuously varying

Question 41

digital data

Answer 41

measured at discrete intervals

Question 42

equivalent quantity to frequency in space

Answer 42

wave number, spectral frequency, k

Question 43

units of angular frequency

Answer 43

rad/s

Question 44

reasons for spectral representation

Answer 44

many natural phenomena are oscillatory, earth's effect on signals is often linear, spectral representation is useful for manipulation, sinusoids are pure frequencies

Question 45

Fourier's theorem

Answer 45

any periodic signal may be represented as a sum of sinusoids at fixed frequencies

Question 46

three parameters to describe a sinusoid

Answer 46

amplitude, frequency, phase

Question 47

phase difference of 180degrees

Answer 47

polarity reversal

Question 48

spectral analysis

Answer 48

decomposition of a signal into its constituent sinusoids

Question 49

wave

Answer 49

signal which varies in space and time

Question 50

if a wave of constant frequency travels into a medium with decreasing velocity, its wavelength will ____

Answer 50

shorten

Question 51

geophysical resolution is better in ______ velocity media

Answer 51

lower

Question 52

digitization

Answer 52

conversion of a signal from a continuous measurement to a discrete representation

Question 53

limitations imposed by digitization

Answer 53

length of data, finite sampling interval, finite sampling accuracy, finite dynamic range

Question 54

longest period limit

Answer 54

to observe a given period or wavelength, it must fit within the recording interval

Question 55

shortest period limit

Answer 55

sampling interval must be 1/2 of the wavelength or less

Question 56

Nyquist period

Answer 56

the shortest period recordable at a given sample interval

Question 57

Nyquist frequency

Answer 57

the highest frequency recordable at a given sample interval

Question 58

Nyquist frequency formula

Answer 58

fN= 1/2?

Question 59

aliasing

Answer 59

a fictious lower frequency, must be lower than the Nyquist limit

Question 60

how to avoid aliasing range

Answer 60

select fN to be greater than the maximum frequency in the analog data, use an analog filter

Question 61

error intoduced by digitization

Answer 61

uniformly distributed random noise in the range +-?/2

Question 62

dynamic range

Answer 62

the ratio of the largest to the smallest recordable response

Question 63

seismology

Answer 63

the study of elastic waves within the earth

Question 64

seismogram

Answer 64

recording of elastic waves

Question 65

seismology has the widest range of applications of all geophysical methods because

Answer 65

it is scale independent and allows for a wide range of geometries

Question 66

properties of seismic refraction

Answer 66

large offset compared to target depth, energy travels mostly horizontally, large sources, measure depth and velocity independently, poor lateral resolution

Question 67

properties of seismic reflection

Answer 67

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

Question 68

properties of downhole tomography

Answer 68

useful for structures that are far from horizontal

Question 69

bulk modulus

Answer 69

?

Question 70

Lame coefficients

Answer 70

? and ?

Question 71

Young's modulus

Answer 71

E

Question 72

Poisson's ratio

Answer 72

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