Browsing by Author "Williams, Richard T."

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    A geophysical characterization of New River terrace deposits in Giles County, Virginia
    Cyrnak, Jennifer Sue (Virginia Tech, 1996-06-05)
    A series of geologically recent faults was discovered within a folded succession of stratified alluvial sediments, commonly referred to as New River terrace deposits, near the town of Pembroke in Giles County, southwest Virginia. Geological and geophysical investigations were utilized to characterize the terrace deposits and investigate the nature of the observed faults. Geologic mapping of the underlying carbonate bedrock provided orientation measurements of fault, bedding, and joint planes; however, none of the features mapped within the bedrock could be directly correlated with the observed faults. The results of geophysical studies suggest significant variations in lithology within the alluvial sediments. Seismic velocities (P-wave) obtained from reversed seismic refraction profiles range from 900-1700 meters/second, while apparent electrical resistivity values vary from 300-2000 ohm-meters. A 75 meter wide, east-west trending low resistivity zone extending across the center of the study area is juxtaposed against an extremely high resistivity zone which is present to the north. The results of seismic reflection and electrical resistivity data analysis are interpreted to suggest that several small and large scale extensional faults may exist throughout the sediments, possibly reaching depths of up to 30 meters. An isopach map of preserved terrace thickness indicates that the faults lie within an east-west trending zone of maximum terrace thickness (35-40 meters thick), which corresponds to a topographic rise in land surface. A structure contour map of the bedrock surface reveals an area of lowest bedrock elevation beneath this zone, implying inverted topography. Results indicate that the evolution of the terrace might be related to ongoing karst processes within the bedrock.
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    The ocean tide and waves beneath the Ross ice shelf, Antarctica
    Williams, Richard T. (Virginia Tech, 1979)
    Widely spaced tidal gravity records have been used to determine the spatial and temporal variation of the ocean tide beneath the Ross Ice Shelf. Cotidal-coamplitude maps have been drawn for the six greatest harmonic constituents of the tide. These are K₁, P₁, O₁, M₂, S₂, and N₂. The tide is principally diurnal, the diurnal amplitudes being roughly 3 times longer than the semidiurnals. The range of the tropic tide is about 1 m at the northern extremity of the ice shelf, and can be as great as 2 m in the southeastern part of the region. The diurnal constituents can each be viewed as a wave that propagates towards the southwest across the sea, having an amplitude that is closely related to the thickness of the water-layer beneath the ice. For each of the semidiurnal constituents there is an amphidromic region located within the Ross 5ea near 80° S latitude, 1900 W longitude, and having a clockwise sense of rotation. Theoretical calculations of the tidal current indicate that the semidiurnal and diurnal current constituents have roughly the same amplitude. The semidiurnal current is magnified by near resonance with the inertia current due to the high latitude of the sea. Because of the resonance, calculations of the semidiurnal components of the tidal current are sensitive to the treatment of the retarding effects of the ice shelf and sea floor. Waves having periods shorter than 20 min were observed in the ice shelf. These have been identified as flexural waves that are generated by the action of the ocean swell on the northern edge of the shelf. The observed speed of these waves was predicted within the uncertainty of the measurement by the classical flexural wave theory.
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    The ocean tide beneath the Ross Ice Shelf
    Williams, Richard T. (Virginia Tech, 1976)
    Measurements of the ocean tide beneath the Ross Ice Shelf by a gravimetric method at five locations (79.3°S, 189.7°W; 79.8°S, 169.1°W; 80.2°S, 161.6°W; 82.5°S, 166.0°W; 78.2°S, 162.3°W), and conventional sea level measurements at one location (77.9°S, 193.4°W), show that the diurnal constituents of the tide account for approximately 75% of the tidal fluctuation of the thickness of the water layer beneath the shelf, at times of spring tide. In the gravimetric method, fluctuations in the gravity on the surface of the ice shelf are attributed to changes in elevation and water mass beneath the gravimeter, due to changes in the thickness of the water layer. The amplitudes of the diurnal constituents P₁K₁ and 0₁ increase from approximately 30 cm and 20 cm, respectively, near Ross Island at the northwestern extremity of the ice shelf, to twice those values at the southern extremity. The amplitudes of the semidiurnal constituents M₂, N₂, and S₂ are generally less than 10 cm, with the largest amplitudes occurring beneath the southern portion of the shelf. Cotidal lines for the diurnal constituents trend north to northwest. The phase, relative to Greenwich, is between 150° and 210° for P₁K₁, and 140° and 200° for 0₁. The phases of the diurnal constituents of the tide in the southern Ross Sea are consistent with the phases of corresponding constituents in the southern Pacific Ocean. Tidal currents, inferred from the shape of the sea-surface by means of the Laplace Tidal Equations, were found to be. generally less than 20 cm/sec at times of spring tide. Maximum currents occur in a northwest trending zone in the southern part of the area where the water layer is most thin.