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In the absolute gravity instruments developed by the Joint Institute for Laboratory Astrophysics (JILA) (Zumberge et al., 1982; Niebauer et al., 1986), the release of the dropped object induces systematic vibrations in the floor‐gravimeter system. These vibrations can cause significant errors in the observed time‐distance values from which the gravitational acceleration is computed. Detailed study of the vibrations affecting the gravity observations shows them to contain random noise and site dependent systematic components, which can be modeled by decaying sinusoids in the range of 10 to 120 Hz. This paper discusses (1) a mathematical filtering method to correct the observed time‐distance array by identifying and removing all non‐random signals from each individual drop, (2) upgrades of the gravimeter controller, which allow the collection of the data required to implement the mathematical filtering, and (3) mechanical filtering experiments using shock dampening pads and braces to minimize the vibrations. The maximum correction to observed gravity has been 23 μGal using the mathematical filter; typical corrections are in the 2–7 μGal range. The use of the shock dampening devices alone resulted in a three‐fold reduction in the amplitudes and decay times of the systematic vibrations.

A marine geophysical study of the Andaman Sea has been conducted as part of the International Indian Ocean Expedition. A combination of magnetic, gravity, bathymetric, and sea‐floor heat‐flux measurements, seismic sparker reflection profiles, and bottom sediment samples has been used in a study of the seaward continuity of major subaerial tectonic trends. The data indicate positive continuity of the structural trend of the Barisan Range of northern Sumatra and the Burma Range. It was found that the central graben of the Barisan Range of northern Sumatra extends into the Andaman Sea north to latitude 10°N. A previously unreported interdeep has been observed between the outer sedimentary island arc and the inner igneous trend of the major primary arc which forms the western boundary of the Andaman Sea. Continental thickness of the crust is indicated under the sedimentary island platform. In the area of the backdeep, the north‐northeast trends of the Malaysian peninsula are prominent.

Geophysical data are presented and discussed for an area approximately 290 km long and 220 km wide in the Aleutian trench south of Kodiak Island. Variations in total magnetic intensity (residual) of more than 600 -• were found in the center of the trench and more than 1100 -• on the southern flank. The free-air gravity values varied by as much as 150 mgal. The residual magnetic and gravity anomalies suggest that a fissure zone, filled with basic igneous material, is located in the center of the trench. Free-air anomaly calculations indicate that the top of the mantle is 4 km below the trench floor. The magnetic anomaly trend o.n the southern flank of the trench is interpreted as a fracture zone. Both magnetic and gravity data suggest the existence of a ridge of igneous rock material underlying the northern trench slope. It is concluded that the geophysical results in this area are comparable to those obtained in many other continental margins of the earth. This conclusion supports the theory that continental margins and trenches are affected by tensional forces in the earth's crust. INTRODUCTION As part of the National Oceanographic Program, the U.S. Coast and Geodetic Survey in 1961 initiated a comprehensive survey of the area between the Aleutian and Hawaiian islands.The survey includes oceanographic, hydrographic, and geophysical measurements along N-S survey lines spaced 20 km apart and along widely spaced E--W lines. Navigation was controlled by Loran C over most of the area.The geophysical instruments used were a Varian V-4931 towed marine magnetometer (proton free precession) and the LaCoste and Romberg air-sea gravity meter, number S-11. Soundings were obtained with a precision depth recorder (PDR). The accuracy of the magnetic data is limited by the diurnal and shorter-period variations in the magnetic field. Records from the Sitka and Honolulu magnetic observatories were studied, and small parts of the data which were recorded during the times of large magnetic bays were excluded. The largest part of line 13 north of latitude 49øN (Figure 1) was excluded because of the magnetic storm of June 20 and 21, 1961.The average daily variation in total intensity was 5 7 at Sitka and 25 7 at Honolulu. Since these daily variations were small and their period much longer than the period of anomaly measurements made as the ship towed the magnetometer sensor over underlying magnetic rocks, no corrections for daily variations were made.The magnetometer sensor was towed 140 m astern, so that the influence of the ship's magnetic field on the data was insignificant.Accuracy of gravity data is dependent upon accurate knowledge of the ship's speed, course, and position and the period and magnitude of the ship's accelerations. Data were omitted when the horizontal accelerations were larger than 500 mgal. It was assumed that the data for the two E-W lines (16 and 17 in Figure 1) are accurate within -----5 mgal because the encountered accelerations were smaller than 100 mgal. (The major part of line 17 crosses the N-S lines s...

Echo-sounding, seismic-reflection, and magnetic measurements were made across the Chukchi cap in the Arctic Ocean from drifting station Charlie in 1959. The Chukchi cap is 130 km wide at the line of crossing and its minimum depth is 246 meters. A microrelief of 5 to 30 meters on the top is attributed to gouging by icebergs. Seismic reflection results indicate that the top of the Chukchi cap is thinly covered with unconsolidated sediment but that small basins at the foot of the western margin may be filled to depths of I kin. A magnetic anomaly with a maximum peak-to-trough amplitude of 1600 gammas, associated with the western margin, is attributed to a large ridge in a basement rock of high susceptibility. Magnetic diurnal variations show a characteristic morning maximum at 1800 GMT. It is concluded that the Chukchi cap is a dissociated section of continental shelf. Introduction. Drifting station Charlie was established and maintained in the Arctic Ocean during 1959 by the U.S. Air Force as a floating pack ice station for scientific research. It was a United States contribution to the International Geophysical Cooperation--1959 and was originally known as Alpha II. The drift of the ice station, under the influence of wind and current, carried it over the Chukchi cap, a submarine feature which rises from ocean depths greater than 2000 m to less than 300 m. The location The research programs were in operation from June 1959 until January 1960, when the camp was evacuated at 77ø05'N, 168ø38'W, after the floe had been heavily fractured during a storm. The programs were under the supervision of several different organizations. The U.S. Weather Bureau made surface and upper-air meteorological observations. Researchers from the University of Washington studied micrometeorology, physical oceanography, and ice petrofabrics. Underwater sound transmission was investigated by the U.S. Navy Underwater Sound Laboratory. Lamont Geological Observatory of Columbia University conducted studies in marine geophysics, marine biology, sedimentation, and underwater sound propagation. Cromie [1961_] reported on the preliminary results of the 235