Geology Reports⌕ Search

Geology topics

J. J. Miller

Publications and source records attributed to J. J. Miller.

At least 19 recordsLinked to original sources
USGS2001

Geophysical constraints on the location and geometry of the Las Vegas Shear Zone, Nevada

We model the basin configuration beneath Las Vegas Valley, Nevada, on the basis of gravity, drillhole, and seismic reflection data. We locate and characterize the various strands of the Las Vegas Valley Shear Zone (LVVSZ) by variations in basin thickness beneath the valley. The pre-Tertiary bedrock surface is complex, with subbasins buried beneath the flat alluvial surface of the valley. We suggest that these basins are formed from transtensional strain. Subbasins elongated N70??W and N50??W are interpreted as strike-slip basins. The deepest subbasin is 5 km west of Frenchman Mountain and strikes N40??E. This basin probably formed by combined movement on nonparallel strands of the LVVZ and an earlier episode of normal faulting. The basin thickness map constrains the minimum depth of the inferred detachment fault beneath Las Vegas Valley to at least 4 km. Seismic reflection data do not image a detachment fault in the upper 10 km beneath Las Vegas Valley. Our results also illustrate the utility of gravity in determining basinal structures and providing a three-dimensional perspective in areas with limited seismic reflection control.

Tectonics↗
USGS2001

Thickness and geometry of Cenozoic deposits in California Wash area, Nevada, based on gravity and seismic-reflection data

Gravity and seismic-reflection data provide insights into the subsurface stratigraphy and structure of the California Wash area of southern Nevada. This area is part of the Lower Colorado flow system and stratigraphic and structural data are important inputs into developing the hydrogeologic framework. These data indicate that the basin beneath California Wash reaches depths of 2-3 km. The eastern margin of the basin coincides with a system of young (Quaternary and late Tertiary) faults, although both seismic and gravity data indicate that the major basin-bounding fault is 2-3 km west of the mapped young faults. Dry Lake Valley, the adjacent valley to the west, is characterized by thinner basin fill. The basin configuration beneath both California Wash and Dry Lake Valleys based on the inversion of gravity data is unconstrained because of the lack of gravity stations north of 36030?. Broad aeromagnetic anomalies beneath pre-Cenozoic basement in the Muddy Mountains and Arrow Canyon Range reflect Precambrian basement at depths of ~ 5 km. These rocks are probably barriers to ground-water flow,except where fractured.

Open-File Report↗
USGS1996

The southern Whidbey Island fault: An active structure in the Puget Lowland, Washington

Information from seismic-reflection profiles, outcrops, boreholes, and potential field surveys is used to interpret the structure and history of the southern Whidbey Island fault in the Puget Lowland of western Washington. This northwest-trending fault comprises a broad (as wide as 6–11 km), steep, northeast-dipping zone that includes several splays with inferred strike-slip, reverse, and thrust displacement. Transpressional deformation along the southern Whidbey Island fault is indicated by along-strike variations in structural style and geometry, positive flower structure, local unconformities, out-of-plane displacements, and juxtaposition of correlative sedimentary units with different histories. The southern Whidbey Island fault represents a segment of a boundary between two major crustal blocks. The Cascade block to the northeast is floored by diverse assemblages of pre-Tertiary rocks; the Coast Range block to the southwest is floored by lower Eocene marine basaltic rocks of the Crescent Formation. The fault probably originated during the early Eocene as a dextral strike-slip fault along the eastern side of a continental-margin rift. Bending of the fault and transpressional deformation began during the late middle Eocene and continues to the present. Oblique convergence and clockwise rotation along the continental margin are the inferred driving forces for ongoing deformation. Evidence for Quaternary movement on the southern Whidbey Island fault includes (1) offset and disrupted upper Quaternary strata imaged on seismic-reflection profiles; (2) borehole data that suggests as much as 420 m of structural relief on the Tertiary-Quaternary boundary in the fault zone; (3) several meters of displacement along exposed faults in upper Quaternary sediments; (4) late Quaternary folds with limb dips of as much as ≈9°; (5) large-scale liquefaction features in upper Quaternary sediments within the fault zone; and (6) minor historical seismicity. The southern Whidbey Island fault should be considered capable of generating large earthquakes (M s ≥7) and represents a potential seismic hazard to residents of the Puget Lowland.

Washington↗
USGS1994

Seismic character of gas hydrates on the Southeastern U.S. continental margin

Gas hydrates are stable at relatively low temperature and high pressure conditions; thus large amounts of hydrates can exist in sediments within the upper several hundred meters below the sea floor. The existence of gas hydrates has been recognized and mapped mostly on the basis of high amplitude Bottom Simulating Reflections (BSRs) which indicate only that an acoustic contrast exists at the lower boundary of the region of gas hydrate stability. Other factors such as amplitude blanking and change in reflection characteristics in sediments where a BSR would be expected, which have not been investigated in detail, are also associated with hydrated sediments and potentially disclose more information about the nature of hydratecemented sediments and the amount of hydrate present. Our research effort has focused on a detailed analysis of multichannel seismic profiles in terms of reflection character, inferred distribution of free gas underneath the BSR, estimation of elastic parameters, and spatial variation of blanking. This study indicates that continuous-looking BSRs in seismic profiles are highly segmented in detail and that the free gas underneath the hydrated sediment probably occurs as patches of gas-filled sediment having variable thickness. We also present an elastic model for various types of sediments based on seismic inversion results. The BSR from sediments of high ratio of shear to compressional velocity, estimated as about 0.52, encased in sediments whose ratios are less than 0.35 is consistent with the interpretation of gasfilled sediments underneath hydrated sediments. This model contrasts with recent results in which the BSR is explained by increased concentrations of hydrate near the base of the hydrate stability field and no underlying free gas is required.

Marine Geophysical Research↗
USGS1993

Method of estimating the amount of in situ gas hydrates in deep marine sediments

The bulk volume of gas hydrates in marine sediments can be estimated by measuring interval velocities and amplitude blanking of hydrated zones from true amplitude processed multichannel seismic reflection data. In general, neither velocity nor amplitude information is adequate to independently estimate hydrate concentration. A method is proposed that uses amplitude blanking calibrated by interval velocity information to quantify hydrate concentrations in the Blake Ridge area of the US Atlantic continental margin. On the Blake Ridge, blanking occurs in conjunction with relatively low interval velocities. The model that best explains this relation linearly mixes two end-member sediments: hydrated and unhydrated sediment. Hydrate concentration in the hydrate end-member can be calculated from a weighted equation that uses velocity estimated from the seismic data, known properties of the pure hydrate, and porosity inferred from a velocity-porosity relationship. Amplitude blanking can be predicted as the proportions of hydrated and unhydrated sediment change across a reflection boundary. Our analysis of a small area near DSDP 533 indicates that the amount of gas hydrates is about 6% in total volume when the interval velocity is used as a criterion and about 9.5% when amplitude information is used. This compares with a calculated value of about 8% derived from the only available measurement in DSDP 533.

North Carolina, South Carolina↗
USGS1992

A method of estimating the amount of in-situ gas hydrates in deep marine sediments

The bulk volume of gas hydrate in marine sediment can be estimated by measuring interval velocities and amplitude blanking of hydrated zones from true-amplitude processed multichannel seismic reflection data. In general, neither velocity nor amplitude information is adequate to independently estimate hydrate concentration. We propose a method that uses amplitude blanking calibrated by interval-velocity information to quantify hydrate concentrations in the Blake Ridge area of the U.S. Atlantic continental margin. On the Blake Ridge, blanking occurs in conjunction with relatively low interval velocities. The model that best explains this relation linearly mixes two end-member sediments, hydrated and unhydrated sediment. Hydrate concentration in the hydrate end-member can be calculated from a weighted equation that uses velocity estimated from the seismic data, known properties of pure hydrate, and porosity inferred from a velocity/porosity relationship. Amplitude blanking can be predicted as the proportions of hydrated and unhydrated sediment change across a reflection boundary. Our analysis of a small area near DSDP 533 indicates that the amount of gas hydrates is about 6% in total volume when interval velocity is used as a criterion and about 9.5% when amplitude information is used. This compares with a calculated value of about 8% derived from the only available measurement in DSDP 533.

Open-File Report↗
USGS1992

Program TracePlot, version 1.1; seismic trace plotting program for the Macintosh

Program TracePlot, Version 1.1, is a Macintosh software application to access a SEG-Y format seismic data file and display the seismic traces on the CRT screen of a Macintosh microcomputer. Both color and monochrome monitors are supported. Display modes are Wiggle-trace, Variable Area, or Wiggle plus Variable Area. Horizontal and vertical resolution is dependant upon the size of the monitor that is attached to the computer. The user controls the following parameters: 1) number of traces to be displayed on the screen at any one time, 2) the time window displayed, 3) the seismic trace in the file at which to begin plotting and trace increment to display thereafter, 4) plot direction 5) timing-line annotation increment, and 6) type of gain to apply (either constant gain factor, or trace-by-trace normalization). A pull-down menu provides interactive help during program execution. A utility program is also included which provides the function of copying a portion of a SEG-Y format data file to another file.

Open-File Report↗
USGS1991

An analysis of a seismic reflection from the base of a gas hydrate zone, offshore Peru

Seismic reflection data recorded near ODP Site 688, offshore Peru, exhibit a persistent bottom-simulating reflector (BSR) from a depth corresponding to the theoretical base of the gas hydrate stability field. Although gas hydrate has been recovered in cores from scientific drilling in both the Pacific and the Atlantic, drilling in areas exhibiting a BSR has usually been avoided because of the perceived risk of releasing free gas trapped beneath the hydrated sediments. To carry out a quantitative analysis of the BSR, the seismic data were reprocessed using signature deconvolution and true amplitude recovery techniques. Synthetic seismograms that were created using acoustic parameters extracted from the seismic data and physical properties from Site 688 borehole measurement were compared to the observed seismic data to estimate the thickness of the free-gas zone. Results indicate the BSR is discontinuous laterally. Where the BSR is of high amplitude, free gas in a zone 5.5-17 m thick beneath the hydrated sediments provides the observed waveform; where the BSR amplitude is low, the free-gas zone is much thinner than 5.5 m or is entirely absent.

American Association of Petroleum Geologists Bulle↗