gg450 march 20 2008 l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
GG450 March 20, 2008 PowerPoint Presentation
Download Presentation
GG450 March 20, 2008

Loading in 2 Seconds...

play fullscreen
1 / 20

GG450 March 20, 2008 - PowerPoint PPT Presentation


  • 114 Views
  • Uploaded on

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.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'GG450 March 20, 2008' - ulf


Download Now An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
gg450 march 20 2008

GG450 March 20, 2008

Introduction to

SEISMIC EXPLORATION

slide2

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.

slide3

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.

slide4

Refraction utilizes the fact that seismic waves bend as they encounter materials with different velocities.

slide5

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.

slide6

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.

slide9

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

slide10

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

slide12

Seismic reflection profiles provide pictures that reflect the structures below the profile when conditions are good.

The unconformity in these data is very obvious.

slide13

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:

slide14

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).

slide15

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).

slide16

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.
slide17

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:

slide18

The formula for the wave above is:

Write this equation using velocity instead of wavelength.

slide19

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')
slide20

What is the period of the wave in the above plot? What is it’s wavelength? What is it’s velocity?