The resonant acoustic signatures of lithic debitage

Abstract

Acoustic methods to search for submerged archaeological sites have shown that concentrations of knapped flint produce a visible acoustic response in chirp sonar profiles in a variety of geographical settings. Field tests and simulations have suggested that the submerged lithic signal is due to acoustic resonances of the flaked stone. We model and measure the resonant acoustic signatures of chert, obsidian, metavolcanic, and granitic lithic debitage. Struck lithics produce multiple resonant peaks under 30 kHz, with high quality factors that decrease with material coarseness. We use a combination of the finite element and boundary element methods to model the natural vibrations of lithic debitage in both air and water. Direct measurement of lithic material density and adjustment of the Young’s modulus and Poisson’s ratio provide excellent correspondence between measured and modeled resonances. Using a coupled finite element and boundary integral method, we model the acoustic scattering return of individual lithics in water as a function of frequency and incidence angle. We find the strongest resonant signal between 8 and 16 kHz for a collection of lithic debitage. Results indicate that the lithic resonance signal is highly directional, with target strength up to −20 dB when excited at optimal angles. For a flat-laying lithic, target strength at normal incidence is, on average, 10 dB lower than the strongest signal, typically found 55°±18° from normal incidence. We suggest that the best way to detect submerged lithics may not be through standard mono-static sub-bottom profiling with a direct downward pulse, but with a chirp pulse sent and received at an angle with respect to the sea bottom.