Physical modeling I : reflections off a low-Q material

Joe Wong and Laurence R. Lines

ABSTRACT

Theoretical considerations predict that reflections with significant amplitude should occur on boundaries between media with different attenuation characteristics even if the velocity-density contrasts are small. We have acquired near-zero offset ultrasonic seismograms over aluminum and Crisco blocks immersed in water that give experimental verification of this prediction. Aluminum is a high-Q material whose acoustic impedance is large compared to that of water. Unaltered Crisco (lard) is a low-Q material whose impedance is very similar to that of water. In our data, reflections off the aluminum and unaltered Crisco (straight out of its store-shelf packaging) have large and almost equal amplitudes, even though one would expect the water-unaltered Crisco reflection to be weak because of the small impedance contrast. This observation is consistent with previous physical modeling results from Carl Sondergeld (personal communication).

After melting and re-solidification, Crisco changed from a low-Q material to a higher-Q material, but its density and P-wave velocity remained practically the same. Reflections from the altered Crisco in water were weaker than those for the unaltered Crisco, but were still much stronger than predicted by the almost negligible impedance contrast with water. Numerical seismograms produced from a reflectivity code that is able to simulate the effects of Q in stratified layers also show strong reflections caused only by large contrasts in Q on the reflecting interface.

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