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Geophysics > Geophysics > Flashcards

Geophysics Flashcards

1
Q

broad definition of geophysics

A

the application of physics to understanding the Earth

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2
Q

narrow definition of geophysics

A

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

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3
Q

divisions in geophysics

A

global (solid-earth), exploration (applied)

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4
Q

environmental geophysics

A

subset of applied geophysics oriented to the shallow subsurface

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5
Q

Why use geophysics?

A

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

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6
Q

principal methods in applied geophysics

A

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

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7
Q

caveats of geophysics

A

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

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8
Q

We address the issues of geophysics by:

A

integrating multiple methods

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9
Q

target

A

the feature or object to be located by geophysical means

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10
Q

resolution

A

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

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11
Q

sensitivity

A

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

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12
Q

model

A

a description of a the spatial distribution of relevant physical properties

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13
Q

forward modelling

A

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

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14
Q

inversion

A

automated recovery of a model from geophysical data

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15
Q

targets in applied geophysics

A

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

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16
Q

method used for minerals

A

magnetics

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17
Q

method used for near surface

A

electromagnetics

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18
Q

method used for hydrocarbon exploration

A

seismic reflection

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19
Q

profile

A

measurement of a laterally varying quantity

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20
Q

mapping

A

profiling in two dimensions

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21
Q

sounding

A

measurement of a quantity varying with depth below a surface point

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22
Q

section

A

combination of a sounding and a profile

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23
Q

3D survey

A

multiple sections over a large area

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24
Q

signal

A

desired quantity to be measured

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25
Q

noise

A

spurious quantity superimposed on the signal

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26
Q

signal to noise ratio

A

characterizes the quality of the measurement

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27
Q

environmental noise

A

external sources of noise

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28
Q

cultural noise

A

environmental noise produced by humans

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29
Q

geological noise

A

non-target variations in the subsurface which interfere with observation

30
Q

coherent noise

A

the same in repeated measurement

31
Q

incoherent noise

A

differs for repeated measurement

32
Q

stacking

A

averaging together repeated measurement

33
Q

filtering

A

removing a range of frequencies from the data

34
Q

muting

A

deleting data points in a certain time or space range

35
Q

deterministic signals

A

rule based and may be predicted

36
Q

stochastic signals

A

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

37
Q

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

A

deterministic, stochastic

38
Q

periodic signal

A

repeats at precise intervals

39
Q

transient signal

A

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

40
Q

analog data

A

continuously varying

41
Q

digital data

A

measured at discrete intervals

42
Q

equivalent quantity to frequency in space

A

wave number, spectral frequency, k

43
Q

units of angular frequency

A

rad/s

44
Q

reasons for spectral representation

A

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

45
Q

Fourier’s theorem

A

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

46
Q

three parameters to describe a sinusoid

A

amplitude, frequency, phase

47
Q

phase difference of 180degrees

A

polarity reversal

48
Q

spectral analysis

A

decomposition of a signal into its constituent sinusoids

49
Q

wave

A

signal which varies in space and time

50
Q

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

A

shorten

51
Q

geophysical resolution is better in ______ velocity media

A

lower

52
Q

digitization

A

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

53
Q

limitations imposed by digitization

A

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

54
Q

longest period limit

A

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

55
Q

shortest period limit

A

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

56
Q

Nyquist period

A

the shortest period recordable at a given sample interval

57
Q

Nyquist frequency

A

the highest frequency recordable at a given sample interval

58
Q

Nyquist frequency formula

A

fN= 1/2?

59
Q

aliasing

A

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

60
Q

how to avoid aliasing range

A

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

61
Q

error intoduced by digitization

A

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

62
Q

dynamic range

A

the ratio of the largest to the smallest recordable response

63
Q

seismology

A

the study of elastic waves within the earth

64
Q

seismogram

A

recording of elastic waves

65
Q

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

A

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

66
Q

properties of seismic refraction

A

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

67
Q

properties of seismic reflection

A

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

68
Q

properties of downhole tomography

A

useful for structures that are far from horizontal

69
Q

bulk modulus

A

?

70
Q

Lame coefficients

A

? and ?

71
Q

Young’s modulus

A

E

72
Q

Poisson’s ratio

A

?

Geophysics (1 decks)

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