Download GG450 Lec 20 March 6, 2006

GG450 March 20, 2008
Introduction to
SEISMIC EXPLORATION
Introduction
As more than 90% of geophysical exploration utilizes
seismic methods, it’s appropriate to spend at least
half of this course on seismic methods.
Seismology utilizes variations in elastic waves to
determine structures inside the earth. Important
variables include elastic constants and density.
For example, the shear modulus of liquids is zero,
and they cannot propagate shear waves. The lack of
shear waves traveling through the outer core is how
we know that the earth’s outer core is liquid.
There are two principle methods of seismic
exploration, seismic refraction and seismic
reflection. Both are important, but reflection is by far
the most important.
Reflection is used extensively in oil exploration and
marine exploration, while refraction is used in
engineering applications and crustal studies. In both
cases, the energy is supplied by the experimenter.
About 90% of what we know about the earth’s interior
is based in seismic data. For very deep studies below the crust, we need to use earthquakes (or
nuclear explosions) for sources.
Refraction utilizes the fact that seismic waves bend as they
encounter materials with different velocities.
The primary data in
refraction are the times
it takes for the seismic
waves to get back to
the surface. When the
waves pass through
materials with higher
velocities, the travel
times are less than if
the material was
slower.
This figure shows a cross section showing seismic
velocities and a resistivity profile (top). Both show the
presence of a basin. With low velocity and high resistivity
overlying high velocity and low resistivity.
Explosive sources are used to get deep-crustal data. Often
delineating the depth to mantle.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
On land seismic data are obtained from explosive or
vibrating sources and long lines of geophones. Each
vertical line is one “seismogram”.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
At sea, the ship tows sound sources and long seismic
streamers containing hydrophones to record data as the
ship moves.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Seismic boat shooting large air gun arrays.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Seismic reflection
profiles provide
pictures that
reflect the
structures below
the profile when
conditions are
good.
The unconformity
in these data is
very obvious.
Before getting into the methods of exploration, we
need to understand some of the theory and jargon
of seismology.
Since we’re dealing with waves, it would be a good
idea to understand wave terminology:
The figure below shows a wave as seen on an
instrument. This wave has an amplitude of 0.5 –
the height from the flat (zero) to the peak of the
wave. This wave has a frequency of 2 Hz, the
number of cycles there are in one second. This is the
inverse of period, which is the number of seconds
per cycle (0.5 s).
1
Amplitude
0.5
0
-0.5
-1
0
0.5
1
Tim e, sec
1.5
2
The figure below shows the same wave, but now we
look at how it looks along a line on the ground in the
direction that the wave is traveling in. The
wavelength of this wave is given by the distance
traveled in one cycle, (0.8 km).
0.6
Amplitude
0.4
0.2
0
-0.2
0
0.2
0.4
0.6
0.8
-0.4
-0.6
Distance along the ground, km
1
1.2
How fast is this wave moving along the ground?
We can figure this out using some very simple
relationships:
Frequency = cycles/ time
Wavelength = distance/ cycle
Velocity = distance/time
Notice that if we multiply frequency by wavelength, we
get:
Frequency * wavelength = cycles/time * distance / cycle
= distance / time = velocity
So the velocity of the wave above is 0.5*0.8= 0.4 km/s.
Looking at this wave in 3-D, you can see that the velocity is how
fast the peak of the wave sweeps over the ground:
5
0
-5
50
45
40
50
35
30
40
25
30
20
15
20
10
Time
5
10
Distance

The formula for the wave above is:
y  A  Sin2  ft x / 
where A is the amplitude
f is the frequency
t is the time
x is the distance along the path
and  is the wavelength.
Write this equation using velocity instead of
wavelength.
In MatLab:
% 3-d plot of sin wave
clear all;
bign=50; % points in the series
period=25; % period of wave
lambda=20; % wavelength
amp=5; %amplitude
for k=[1:bign];
for l=[1:bign];
x(k,l)=k;
t(k,l)=l;
a(k,l)=amp*sin(2*pi*(t(k,l)/period-x(k,l)/lambda));
end; end;
plot3(x,t,a,'r');
grid on
axis equal
xlabel ('Distance')
ylabel('Time')
What is the period of the wave in the above plot? What
is it’s wavelength? What is it’s velocity?
5
0
-5
50
45
40
50
35
30
40
25
30
20
15
20
10
Time
5
10
Distance