The Harvard seismograph‐station, after 25 years of operation in the Geological Museum, Cambridge, is now housed in a new vault at Oak Ridge, Massachusetts, near Harvard and about 25 miles northwest of Cambridge. The coordinates of the station, determined astronomically, are 42° 30′ 26″±1″ north latitude and 71° 33′ 45″±15″ west longitude. The new vault has been constructed as a unit of the research program directed by the Harvard Committee for Geophysical Research, and financed jointly by the Rockefeller Foundation and by friends of the Division of Geological Sciences at Harvard. The opening for the main room was excavated from solid rock, a Paleozoic micaceous schist, to a depth of over 15 feet. The room is 22 feet square inside. Two corners are occupied by small piers, and the main pier is an octagonal strip five feet wide. Its open center makes all parts readily accessible. The piers and floor were poured as a unit with the underlying rock. An eight‐foot covering of dirt renders the temperature in the vault practically constant; thorough waterproofing has prevented any infiltration of water; and calcium‐chloride driers are proving entirely satisfactory for the control of humidity. A small work‐shop adjoins the vault. A special dark‐room is located in a separate building. A standard B. Howard pendulum‐clock is used for timing. It is checked with NAA time‐signals by means of a short‐wave radio receiver. The clock is mounted in the main vault under optimum conditions. In the first three weeks of service its rate was reduced to one second per week, and it is expected this will be further improved.
As is pretty generally known by now, there has developed in recent years a commercial activity which involves the setting up of elastic vibrations in the Earth by means of dynamite blasts. This work, generally known as “Seismic prospecting,” has for its aim the study of subsurface geologic structure by means of these vibrations, following the theory by which seismology offers means to a study of the Earth's deeper interior. The greater part of such commercial investigations, however, has had for its more limited objective the location of salt‐domes as favorable structures for the accumulation of oil.
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: Quincy … …16,260± 70 ft./sec. or 4.96±0.02 km/sec.*Westerly … . 16,400±120 ft./sec. or 5.00±0.04 km/sec.Rockport … .16,670± 40 ft./sec. or 5.08±0.01 km/sec.Average… . .16,530± 90 ft./sec. or 5.04±0.03 km/sec.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.
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