*Box 2.* *Dynamic range*

*Effects of absorption on a seismic impulse. Absorption changes the shape of an ideal seismic shot pulse as distance from the shot increases. The sketches represent particle displacement and particle velocity for a P-wave (After Ricker, Geophysics 1953).*

**T**he dynamic range of a seismic recording system is the ratio of the largest to smallest signals that it can faithfully detect and record. Seismic reflections have a wide range of amplitudes. The large-amplitude part is not hard to understand—anything that begins with a dynamite blast is obviously pretty loud—but what about the small-amplitude part? There are three phenomena that attenuate seismic reflections: spherical spreading, transmission loss, and intrinsic absorption. Together these effects mean that a seismic recording system needs a dynamic range of 100 decibels (dB) or more.

*Spherical spreading.* The seismic wave from a localized impulsive source spreads out equally in all directions. After it has traveled some distance, *r*, the energy in the original impulse is spread out over a spherical area of radius *r*. Since energy is conserved and the area of a sphere is proportional to *r*^{2}, the energy per unit area must be proportional to 1/*r*^{2}. The amplitude of a seismic wave is proportional to the square root of its energy. Thus, the amplitude of a seismic reflection is inversely proportional to the distance it has traveled.

*Transmission loss.* As explained in Box 1, a pressure wave that encounters an impedance contrast has its incident energy partitioned into several events. Only one of these is a P-wave that continues onward. Since the signal from a deep reflector may traverse many impedance contrasts on its way down to the reflector and back up to the surface, even a small energy loss per contrast can quickly add up to a significant effect.

*Intrinsic absorption.* As a seismic wave travels through a material, the particles that make up the material vibrate, generating heat. The energy that goes into that heat—intrinsic absorption—is supplied by the seismic wave. Intrinsic absorption attenuates higher frequencies more rapidly than lower frequencies, thereby changing the shape of the seismic pulse. The rate of intrinsic absorption varies greatly from one type of material to another.