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.

On land seismic data are obtained from explosive or vibrating sources and long lines of geophones. Each vertical line is one “seismogram”.

At sea, the ship tows sound sources and long seismic streamers containing hydrophones to record data as the ship moves.

Seismic boat shooting large air gun arrays.

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).

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).

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:

The formula for the wave above is: 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; 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?