VELOCITY OF ELASTIC WAVES IN GRANITE

Abstract

The velocity of elastic waves in granite was determined at Quincy and Rockport, Massachusetts, and Westerly, Rhode Island. The waves measured were generated by dynamite explosions. They were recorded by portable seismographs at distances ranging from fifty feet to four thousand six hundred feet. The observed velocities for longitudinal waves were: $$\begin{array}{c}\mathrm{Quincy\; \dots \; \dots }\text{16,260±\hspace{0.17em}70\hspace{0.17em}}\mathrm{ft./sec.\; or}\text{\hspace{0.17em}4.96±0.02\hspace{0.17em}}{\mathrm{km/sec.}}^{*}\\ \mathrm{Westerly\; \dots \; .}\text{\hspace{0.17em}16,400±120\hspace{0.17em}}\mathrm{ft./sec.\; or}\text{\hspace{0.17em}5.00±0.04\hspace{0.17em}}\mathrm{km/sec.}\\ \mathrm{Rockport\; \dots \; .}\text{16,670±\hspace{0.17em}40\hspace{0.17em}}\mathrm{ft./sec.\; or}\text{\hspace{0.17em}5.08±0.01\hspace{0.17em}}\mathrm{km/sec.}\\ \mathrm{Average}\mathrm{\dots \; .\; .}\text{16,530±\hspace{0.17em}90\hspace{0.17em}}\mathrm{ft./sec.\; or}\text{\hspace{0.17em}5.04±0.03\hspace{0.17em}}\mathrm{km/sec.}\end{array}$$ A three‐component seismograph, used only at Quincy, recorded transverse waves, the velocity of which was 8150±90 ft/sec., or 2.48±0.03 km/sec. From the two velocities determined at Quincy and the density of specimens taken from the shooting location, 2.65 grams/cm³, values for the bulk modulus, k, compressibility, β, rigidity,μ, Poisson's Ratio, σ and Young's Modulus E, were obtained as follows: k=44±1×10¹⁰ dynes/cm²; β=2.28±0.05×10⁻¹² cm²/dynes; μ=16.3±0.4×10¹⁰ dynes/cm²; σ=0.333±0.005; E=43±1×10¹⁰ dynes/cm². The form of the time‐distance curves, straight lines through the origin, indicated that the waves did not penetrate deeply. Accordingly, the values obtained are for pressures of only a few atmospheres. The bearings of these results upon earlier investigations of the elastic constants of granite are discussed. Although direct comparisons between laboratory and field results are not conclusive, they indicate that the Adams and Williamson curve is incorrect for pressures below 2,000 megabars, and that there is no marked difference between dynamically and statically determined compressibilities of granite.