Jennifer L. Hare

Active displacement transfer and differential block motion within the central Walker Lane, western Great Basin

Velocities determined for 50 global positioning system sites within the central Walker Lane indicate differential motion among tectonic blocks forming a boundary zone between the Great Basin extensional province and the Sierra Nevada. The velocity field is related to displacement transfer from the Owens Valley and Furnace Creek fault systems of east- ern California to transtensional structures of the Walker Lane and extensional faults of the central Nevada seismic belt. Block boundaries are sharp and appear to be inherited from pre-Tertiary crustal structure. The block geometries exert strong influence on dif- ferential displacements concentrated along boundaries as belts of divergent, transcurrent, and convergent motion. The aggregate velocity accounts for about 25% of the relative motion between the Pacific and North American plates. About 5 mm/yr of the motion is localized along the eastern margin of the Sierra Nevada, whereas about 10 mm/yr is stepped 100 km east along a belt of east-northeast-trending transtensional faults that merge with northwest-trending transcurrent structures of the Walker Lane. About 6 mm/ yr of the velocity field is transferred to north-northeast-trending extensional faults of the central Nevada seismic belt. The heterogeneous distribution of motions is consistent with partitioning of a regional velocity field formed by westward extension and N408W-directed shear.

The 4-D microgravity method for waterflood surveillance; a model study for the Prudhoe Bay reservoir, Alaska

Forward and inverse gravity modeling is carried out on a suite of reservoir simulations of a proposed water injection in the Prudhoe Bay reservoir, Alaska. A novel surveillance technique is developed in which surface gravity observations are used to monitor the progress of a gas cap waterflood in the reservoir at 8200-ft (2500-m) depth. This cost‐effective method requires that high‐precision gravity surveys be repeated over periods of years. Differences in the gravity field with time reflect changes in the reservoir fluid densities. Preliminary field tests at Prudhoe Bay indicates survey accuracy of 5–10 μGal can be achieved for gravity data using a modified Lacoste & Romberg “G” type meter or Scintrex CG-3M combined with the NAVSTAR Global Positioning System (GPS). Forward gravity modeling predicts variations in surface measurements of 100 μGal after 5 years of water injection, and 180–250 μGal after 15 years. We use a constrained least‐squares method to invert synthetic gravity data for subsurface densi...

The 4D microgravity method for waterflood surveillance: Part 3 - 4D absolute microgravity surveys at Prudhoe Bay, Alaska

The 4D microgravity method is becoming a mature technology.Aprojecttodeveloppracticalmeasurementandinterpretation techniques was conducted at Prudhoe Bay,Alaska, from1994through2002.Beginningin2003thesetechniques have been systematically applied to monitor a waterflood in the gas cap of the Prudhoe Bay reservoir. Approximately 300 stations in a 150 km 2 area are reoccupied in each survey year with sub-5 Gal precision absolute gravity and centimeter precision Global Positioning System GPS geodetic measurements. The 4D gravity measured over epochs 2005‐2003, 2006‐2003, and 2007‐2003 has been successfullymodeledtotrackthemassofwaterinjectedsincelatein 2002.AnewandimprovedversionoftheA-10field-portable absolute gravity meter was developed in conjunction with this project and has proven to be a key element in the success of the 4D methodology.The use of an absolute gravity meter in a field survey of this magnitude is unprecedented. There aresubstantialdifferencesbetweena4Dabsolutemicrogravity survey and a conventional gravity survey in terms of station occupation procedures, GPS techniques, and the 4D elevation correction. We estimate that the overall precision of the4Dgravitysignalineachepochislessthan10 Gal.

The 4D microgravity method for waterflood surveillance: Part IV — Modeling and interpretation of early epoch 4D gravity surveys at Prudhoe Bay, Alaska

Between March 2003 and March 2007, four high-precision 4D absolute microgravity surveys were performed at Prudhoe Bay, Alaska. These surveys are part of an ongoing effort to monitor the progress of a very large water-injection project in the gas cap of the Prudhoe Bay reservoir at a depth of 2.5 km . These carefully acquired gravity data must be modeled and interpreted in terms of water movement within the reservoir. A constrained linear inversion scheme was tested on reservoir simulations during the planning and development phase of this project (preinjection). The inver-sion methodology has been applied to data for three epochs (2005–2003, 2006–2003, and 2007–2003), and mass-distribution models have been produced for the reservoir. The time evolution of the water-mass distribution in the reservoir is visualized from these three snapshot models. The waterflood is expanding into the gas cap at the expected rate but is exhibiting nonsymmetric behavior that is consistent with a greater degree of structural ...

Quantitative characterization and elevation estimation of Lake Lahontan shoreline terraces from high‐resolution digital elevation models

The precision with which the elevation of a feature, such as a terrace, can be measured depends on the characterization of the noise contaminating the measurement. A method for identification and extraction of terrace feature elevations is presented and the topographic noise, due to erosion, as well as measurement error, is quantified. High-resolution digital elevation models (DEM) are acquired at six wave-cut, volcanic bedrock terrace sites from around the highstand of paleo-Lake Lahontan in the western Great Basin. Local DEMs, which are tied to regional geodetic control, were acquired using conventional total station, rapid postprocessed and real-time kinematic Global Positioning System methods. The topographic signal is processed with derivative filters for geomorphic feature recognition and averaging for noise reduction. Results indicate that noise levels for identifiable features such as riser crest, knickpoint, and slope inflection point are statistically equivalent and on the order of 0.5 m standard deviation. Averaging within topographic bins spanning ∼50 m along terrace strike yields feature elevation estimates with standard errors on the order of 0.12 m. The mean bench window elevation (between the riser crest and knickpoint) has the lowest standard error and is systematically related to water level. Propagation of surveying, geoid estimation, and terrace feature elevation estimation errors indicates that displacements on the order of 0.5 m may be resolved using these methods. Elevation estimate interpretation involves terrace development, degradation, and neotectonics, but this new methodology has significant advantages in studies of neotectonic or geomorphic processes using local terrace elevation measurements.

The Value of Multi‐Component TEM Data for the Estimation of UXO Target Parameters

Two multi-component multi-gate data sets from the Naval Research Laboratory’s Baseline Ordnance Classification Test Site at Blossom Point, one acquired statically with a Geonics EM61-3D3C and the other acquired dynamically with a Zonge NanoTEM system (DNT), are analyzed to determine the relative classification performance of the two systems. Not surprisingly, our classification performance is better with 3-component static data than it is with the 3-component dynamic data. Confirming published work by Grimm [4], classification is significantly improved when it is applied to the 3-component static data than when it is applied to a decimated data set consisting of only a single (z) component. However, early analyses of the dynamic data indicated that horizontal components provide marginal, if any, improvements in classification. Noise analyses of data from the two systems show that noise levels in the EM61-3D data set are approximately 40dB lower than those in the DNT system and that noise levels in the horizontal components at late times are 2-5 times higher in the vertical component. Noise reduction in statically acquired data can be attributed to stacking (~20dB) and the elimination of microphonic noise from antenna cart movements. With dynamically acquired data, the higher noise levels in the horizontal components together with uncertainties in antenna position and attitude are most likely the reason that the horizontal components do not unequivocally improve classification performance in the dynamic data.

The Ecuadoran Geodetic/gravity Observations (EGO) Project: GPS/Gravity Acquisition From the Pacific Coast

A gravity project was carried out in Ecuador in which GPS (Global Positioning Systems) provided navigation as well as rapid ( five minutes per station) position control with a height accuracy of a few centimeters. Fifteen hundred gravity stations were observed to 0.01 mGal accuracy, acquired along roads by vehicle, jungle trails on foot and river valleys by helicopter during thirty days in January-February, 1994. Seven geodetic P-code and four C/A code GPS receivers, and five gravity meters were used. The main effort was in the Oriente Basin area of eastern Ecuador but data were also observed in the Guayaquil area and the Andes Mountains . One hundred and ten airstrips were used in the Amazon. .The position and gravity data were processed to completion (terrain corrected Bouguer anomalies) within a day of the observations on site. The geophysical data have been integrated with digital geological GIS ( Geographical Information Systems) data for integration and analysis with interesting geologic interpretations related to thick skin tectonics. The OSU 91A geoid model can be significantly improved. Rapid GPS under the latest satellite coverage and with the most recent hardware and software proved almost totally successful with survey parameters less restrictive than manufacturers’ recommendations.