MadSci Network: Earth Sciences
Query:

Re: what controls seismic wave velocity?

Date: Thu Mar 10 23:36:20 2005
Posted By: David Dunbar, Staff, Geophysics, GeoCenter, Inc.
Area of science: Earth Sciences
ID: 1110303251.Es
Message:

Merry,

The main factors that determine seismic wave velocity are the density and structural strength of the Earth. These vary with depth, location and composition. Other factors include the frequency of the seismic energy and the number, orientation and shape of cracks or other voids in the ground.

Energy in seismic waves travels in several modes. The compression or P wave travels through the body of the Earth. It is the fastest wave and is the same as sound waves in air or water. Particle motion for a P wave is a compression / expansion motion. The shear or S wave also travels through the body of the Earth. The S wave typically travels about half the speed of the P wave. Particle motion for an S wave is a twisting motion. S waves do not travel in air or water.

There are two waves that travel along the surface called Rayleigh and Love waves after the scientists who first described them. Both are slower than S waves. Particle motion in Rayleigh waves is a vertical ellipse. The slowest waves, Love waves, donít exist everywhere. They require a shear wave velocity profile with a low velocity layer at the surface and a much faster layer just under it. The particle motion is horizontal side to side. The equation describing the velocity of Love waves is quite complex. Both Rayleigh and Love waves can be dispersive, i.e. the velocity depends on frequency.

In oil and gas exploration the P waves are the most often used. Historically S waves were mostly ignored and Rayleigh and Love waves were considered a nuisance. In the last few years it has been shown that S waves can provide useful information to compliment the P waves. Surveys are being designed to record both P and S waves at the same time. This requires three axis detectors. Processing combined surveys requires a means of identifying the P and S wave reflections from the same structures. This is still something of a black art. Surveys are designed in an attempt to minimize Rayleigh and Love waves, collectively called ground roll. Ground roll frequently is much stronger than the P and S waves.

Earthquake studies try to use all four waves. The distances between earthquakes and recording stations are generally long enough that the waves are separated enough to cleanly identify each. The dispersive Rayleigh and Love waves can help constrain the velocity structure near the surface.

The frequency range of useful energy in earthquake studies is quite low. A frequency of 250 Hz is considered quite high. On the low end a frequency of 1 cycle per minute is fairly commonly recorded. In normal oil and gas exploration the frequency range is from 5 to 250 Hz. Engineering studies for construction projects and borehole studies for oil and gas wells may reach 2000 Hz.

Structural strength of rock is described in terms the elastic constants of the rock. The equations describing P, S and R velocities assume the modulii are constants over distances smaller than the wavelength of the seismic wave. This works fairly well for earthquake and oil and gas work. The frequencies are low enough that the wavelengths are tens of meters to many kilometers long. This is not true for engineering and borehole studies. The assumption also breaks down in regions where there is a regional orientation of cracks or voids in the ground. The S waves are especially sensitive to this.

David


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