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S. Williams

Publications and source records attributed to S. Williams.

5 recordsLinked to original sources
USGS2018

Sedimentary basins reconnaissance using the magnetic Tilt-Depth method

We compute the depth to the top of magnetic basement using the Tilt-Depth method from the best available magnetic anomaly grids covering the continental USA and Australia. For the USA, the Tilt-Depth estimates were compared with sediment thicknesses based on drilling data and show a correlation of 0.86 between the datasets. If random data were used then the correlation value goes to virtually zero. There is little to no lateral offset of the depth of basinal features although there is a tendency for the Tilt-Depth results to be slightly shallower than the drill depths. We also applied the Tilt-Depth method to a local-scale, relatively high-resolution aeromagnetic survey over the Olympic Peninsula of Washington State. The Tilt-Depth method successfully identified a variety of important tectonic elements known from geological mapping. Of particular interest, the Tilt-Depth method illuminated deep (3km) contacts within the non-magnetic sedimentary core of the Olympic Mountains, where magnetic anomalies are subdued and low in amplitude. For Australia, the Tilt-Depth estimates also give a good correlation with known areas of shallow basement and sedimentary basins. Our estimates of basement depth are not restricted to regional analysis but work equally well at the micro scale (basin scale) with depth estimates agreeing well with drill hole and seismic data. We focus on the eastern Officer Basin as an example of basin scale studies and find a good level of agreement between previously-derived basin models. However, our study potentially reveals depocentres not previously mapped due to the sparse distribution of well data. This example thus shows the potential additional advantage of the method in geological interpretation. The success of this study suggests that the Tilt-Depth method is useful in estimating the depth to crystalline basement when appropriate quality aeromagnetic anomaly data are used (i.e. line spacing on the order of or less than the expected depth to basement). The method is especially valuable as a reconnaissance tool in regions where drillhole or seismic information are either scarce, lacking, or ambiguous.

Exploration Geophysics↗
USGS2003

GCIP water and energy budget synthesis (WEBS)

As part of the World Climate Research Program's (WCRPs) Global Energy and Water-Cycle Experiment (GEWEX) Continental-scale International Project (GCIP), a preliminary water and energy budget synthesis (WEBS) was developed for the period 1996-1999 fromthe "best available" observations and models. Besides this summary paper, a companion CD-ROM with more extensive discussion, figures, tables, and raw data is available to the interested researcher from the GEWEX project office, the GAPP project office, or the first author. An updated online version of the CD-ROM is also available at http://ecpc.ucsd.edu/gcip/webs.htm/. Observations cannot adequately characterize or "close" budgets since too many fundamental processes are missing. Models that properly represent the many complicated atmospheric and near-surface interactions are also required. This preliminary synthesis therefore included a representative global general circulation model, regional climate model, and a macroscale hydrologic model as well as a global reanalysis and a regional analysis. By the qualitative agreement among the models and available observations, it did appear that we now qualitatively understand water and energy budgets of the Mississippi River Basin. However, there is still much quantitative uncertainty. In that regard, there did appear to be a clear advantage to using a regional analysis over a global analysis or a regional simulation over a global simulation to describe the Mississippi River Basin water and energy budgets. There also appeared to be some advantage to using a macroscale hydrologic model for at least the surface water budgets. Copyright 2003 by the American Geophysical Union.

Journal of Geophysical Research D: Atmospheres↗
USGS1997

Southern California Permanent GPS Geodetic Array: Continuous measurements of regional crustal deformation between the 1992 Landers and 1994 Northridge earthquakes

The southern California Permanent GPS Geodetic Array (PGGA) was established in 1990 across the Pacific-North America plate boundary to continuously monitor crustal deformation. We describe the development of the array and the time series of daily positions estimated for its first 10 sites in the 19-month period between the June 28, 1992 ( M w =7.3), Landers and January 17, 1994 ( M w =6.7), Northridge earthquakes. We compare displacement rates at four site locations with those reported by Feigl et al. [1993], which were derived from an independent set of Global Positioning System (GPS) and very long baseline interferometry (VLBI) measurements collected over nearly a decade prior to the Landers earthquake. The velocity differences for three sites 65–100 km from the earthquake's epicenter are of order of 3–5 mm/yr and are systematically coupled with the corresponding directions of coseismic displacement. The fourth site, 300 km from the epicenter, shows no significant velocity difference. These observations suggest large-scale postseismic deformation with a relaxation time of at least 800 days. The statistical significance of our observations is complicated by our incomplete knowledge of the noise properties of the two data sets; two possible noise models fit the PGGA data equally well as described in the companion paper by Zhang et al . [this issue]; the pre-Landers data are too sparse and heterogeneous to derive a reliable noise model. Under a fractal white noise model for the PGGA data we find that the velocity differences for all three sites are statistically different at the 99% significance level. A white noise plus flicker noise model results in significance levels of only 94%, 43%, and 88%. Additional investigations of the pre-Landers data, and analysis of longer spans of PGGA data, could have an important effect on the significance of these results and will be addressed in future work.

Journal of Geophysical Research B: Solid Earth↗
USGS1991

Louisiana coastal GIS network: Graphical user interface for access to spatial data

Louisiana's coastal wetlands support a large percentage of the nation's seafood and fur industries, vast deposits of oil and natural gas, habitat for thousands of species of plants and animals, winter nesting grounds and migratory paths for numerous waterfowl, and many recreational resources enjoyed by residents and tourists. Louisiana's wetlands also have the highest rates of coastal erosion and wetland loss in the nation. While numerous studies across many disciplines have been conducted on both local and regional scales, no complete inventory exists for this information. The Louisiana Coastal Geographic Information System Network (LCGISN) is currently being developed to facilitate access to existing data for coastal zone planners, managers, and researchers. The Louisiana Geological Survey (LGS), in cooperation with the LSU Department of Geography and Anthropology, the Computer Aided Design and Geographic Information Systems Research Laboratory (CADGIS), and others, is pursuing this project under the terms of a cooperative agreement with the U.S. Geological Survey. LCGISN is an automated system for searching and retrieving geographic, cartographic, and bibliographic data. By linking original programming with an existing GIS software package and an industry standard relational database management system, LCGISN will provide the capability for users to search for data references by interactively defining the area of interest on a displayed map/image reference background. Several agencies will be networked to provide easy access to a wide variety of information. LCGISN, with its headquarters at LGS, will serve as the central node on the network, providing data format conversions, projection and datum transformations, and storage of several of the most commonly used data sets. Thematic mapper data, USGS 7.5-minute quadrangle map boundaries, political and legal boundaries, major transportation routes, and other digital data will provide a base map to aid the user in selecting the exact area of interest. Then, the user will set search criteria by proceeding through a series of menu-driven options. The system will then return any or all of the following: a list of digital maps or imagery that can be displayed immediately and visually overlayed, a list of maps/remotely sensed data and information on their availability, and a list of bibliographic references concerning the area and subject defined.

Conference Paper↗