Introduction to Geophysics and Geophysical Data

Lecture N. 1: Introduction to
Geophysics and Geophysical
Data Analysis
H. SAIBI
October 8, 2015
Department of Earth Resources Engineering
Program
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What is Geophysics?
密度
抵抗
磁性
放射能
コンダクタンス
探査機＋コンピュータ＋フィールド
＋地質学＋その他
重力探査
電気探査
磁気探査
放射能探査
電磁探査
Georadar: dielectric constant;
Seismics: elastic constants, density;
Spontaneous Polarization (SP): oxidation potential, hydrogen concentration;
Induced Polarization (IP): electrochemical properties of electronically conducting particles in
rock pores.
Each rocks has its specific physical properties
それぞれの岩石は特有の物性地を持っている。
To study the physical properties of rocks we use
specific geophysical methods helped by
computers and tools.
岩石の物理的性質を研究するために、コンピュータや探査
機によって特別な地球物理学的手法を用います。
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地震探査で石油を探す
Source: SEG
Doctor uses X-Ray to see what’s going on in your
body.
Geophysicists use special tools to look inside the
Earth.
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石油堆積層の厚さの測定
建造物のための地下の浅い部分の探査
地下水の探査
鉱物の探査
考古学的探査
地下空洞の探査
地熱貯留層のモニタリング
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氷河地帯の物理探査
Aims of geophysical
measurements
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Required physical property
contrast for various methods
All geophysical methods depend on
physical property contrasts between the
target and the background geology.
● The physical property determines the
geophysical technique.
● Interpretation of geophysical results together
with geologists, hydrogeologists, civil
engineers, ...
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Table: Geophysical methods and their main applications.
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Number of times that the use of a specific technique is
mentioned in articles published in Near Surface Geophysics
2002-2010
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Unexploded Ordnance (UXO)
(a) EM induction response of metal pipes buried at a test site.
(b) EM induction response of UXO at a live site with highly
magnetic soil (Hawaii) from Huang and Won (2003)
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GPR stratigraphy
GPR stratigraphy of an active dune rich in high quality Quartz in New
Zealand from van Dam et al. (2003)
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Mars radar sounding
Orbiting 85 ms chirp radar image of stratigraphy at the Mars north
polar ice cap. From Phillips et al. (2008)
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High-resolution urban seismic reflection (ex:
Barcelona, Spain)
Seismic depth section from Marti et al. (2008)
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Planning a geophysical survey
Figure: Schematic flow diagram to illustrate the decisionmaking process leading to the selection of geophysical
and utility software.
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Geophysical Survey Design:
Target identification
Figure: Examples of (A) a gravity anomaly over a
buried sphere, and (B) a magnetic anomaly over
an inclined magnetic sheet.
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Figure: Contrasts in physical properties from different geological
targets give rise to a geophysical target. When there is no contrast,
the target in undetectable geophysically.
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Arrangements of
measurements (Optimum line configuration)
(A)
(C)
(B)
Figures: Geophysical anomaly
plots: (A) profile, (B) map, (C)
isometric projection, (D)
shaded relief.
２Ｄ比抵抗マップ
(D)
A) Profiling and mapping: Measurements are
carried out along a profile or in an area:
f(x)
f(x,y)
Detection of lateral variation of the
physical parameter at a certain depth.
B) Sounding: Measurements are carried
out along a profile. Several data per
location are measured
f(x,z)
Determination of the variation of the
physical parameter as a function of 17
depth.
Seismic Image of Ancient Reef in Alberta
[400 million years old]
３Ｄイメージング
Selection of
station intervals
Correct choice of station
interval is necessary
➔ Waste of time and money to
record too many data
➔ Also wasteful if too few are
collected
Figure: Examples of various
degrees of spatial aliasing using
different sampling intervals.
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Noise
Figure: Schematic illustrating some
common sources of geophysical noise
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Figure: Signal-to-noise ratio. In (A) the signal has a much larger
amplitude than that of the background noise, so the signal can be
resolved. In (B) the signal amplitude is less than, or about the same
as, that of the noise and thus the signal is lost in the noise.
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Data
analysis
Figure: Schematic to
show the relationship
between various input
devices, through data-file
formats to the computer,
and subsequently to some
form of hardcopy device.
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Data Analysis
Lindley (1956): “if the state of nature
is known, then no experiment can be
informative”
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• Geophysicists are looking to extract information
about the Earth.
• Information is a quantum, bit of new knowledge
(a) The unit comb function consisting of regularly spaced unit impulses. This signal
captures the regular movement of a pathological ant. (b) A signal which captures an
unexpected movement of the ant.
Electrical
conductivity
Clay content
Porosity
(Left) Scatter in data based on one explanatory variable. (Right)
partition of data into three discernible subsets based on the
value of a second explanatory variable.
SENSORS
• A sensor is a device that converts a physical input
signal into a voltage.
• Dynamic range (g) = 20 log10 amax/amin [dB]
(Left) Geophones. (Right) earthquake seismogram showing P-, S-,
and surface waves.
FREQUENCY RESPONSE
Measured and ideal frequency of a loudspeaker.
Modeled frequency response of a dual-mass accelerometer for
landmine detection (MARTIN ET AL., 2006).
DISCRETE FOURIER
TRANSFORM
(Left) A continuous function f(t) and (Right) its discretely
sampled representation {a(kDt)}
(Top) Impulse signal; (middle) amplitude spectrum Rn and;
(bottom) phase spectrum F(n) of the discrete Fourier series.
 The boxcar function is important in time-series analysis for
removal of bad data or isolating of a long data sequence.
(Top) Boxcar signal; (bottom) amplitude spectrum Rn of the
discrete Fourier.
FILTERING
Amplitude spectra of Butterworth n-pole low-pass filters and a
bandpass filter, where fc = wc/2p and fb = wb/2p
Original signal
Low-pass filter with cut-off
frequency w = p/30
Low-pass filter with cut-off
frequency w = p/100
Low-pass filter with cut-off
frequency w = p/300
 Higher-frequency signal energy is removed as the cut-off frequency is
reduced
Effect of low-pass filtering
CONVOLUTION
(a) Unit impulse function d (t-t0); (b) impulse response h(t); (c) source
function s(t); (d) actual Earth response r(t).
Effect of sampling
DATA WINDOWS AND SPECTRAL ANALYSIS
Power spectra of rectangle and Hann windows
Effect of Hann windowing on geomagnetic time series: (left)
original and Hann-windowed time series; (right) corresponding
power spectra.
De-spiking a magnetics data set (N=256) using l = 0.005. The
inset shows the number of spikes as a function of parameter l
Scaled and translated Mexican hat wavelet functions, using
p=0.5
Continuous wavelet transform and 39-point moving-average
filter applied to the original geophysical time series shown at
top.
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