Electrical resistance tomography experiments at the Oregon Graduate Institute

Three controlled experiments were conducted at the Oregon Graduate Institute (OGI) with the purpose of evaluating electrical resistance tomography for imaging underground processes associated with in-situ site assessment and remediation. The OGI facilities are unique: a double-wall tank 10 m square and 5 m deep, filled with river bottom sediments and instrumented for geophysical and hydrological studies. At this facility, liquid contaminants could be released into the confined soil at a scale sufficiently large to represent real-world physical phenomena.In the first test, images of electrical resistivity were made before and during a controlled spill of gasoline into a sandy soil. The primary purpose was to determine if electrical resistivity images could detect the hydrocarbon in either the vadose or saturated zone. Definite changes in electrical resistivity were observed in both the vadose and saturated soils. The effects were an increase in resistivity of as much as 10% above pre-release values. A single resistive anomaly was imaged, directly below the release point, principally within the vadose zone but extending below the phreatic surface. The anomaly remained identifiable in tomograms taken two days after the release ended with clear indications of lateral spreading along the water table.The second test involved electrical resistance measurements before, during, and after air sparging in a saturated soil. The primary purpose was to determine if the electrical images could be used to detect and delineate the extent of the zone influenced by sparging. The images showed an increase of about 20% in resistivity over background values within the sparged zone and the extent of the imaged zone agreed with that inferred from other information.Electrical resistivity tomography measurements were made under a simulated oil storage tank in the third test. Comparison of images taken before and during separate releases of brine and water showed effects of changes induced by the water or brine. The simulated leak and its location were imaged as a conductive anomaly centered near the point of origin and were observed to spread with time during the release.     

IGS Near Real-Time Products and Their Applications

The primary IGS products, including precise GPS orbits, Earth orientation parameters, and estimated and predicted GPS satellite clocks, are no longer used exclusively for essential geodetic support of scientic research. They are increasingly being used by a wide range of non-academic applications. In these applications, timeliness is extremely critical. To address the timeliness issue, the strengths and weaknesses of current IGS production processes are discussed, new ways to improve the timeliness and quality are explored, and recommendations are proposed to fulfill the application requirements. ? 2001 John Wiley & Sons, Inc.     

Seismological study of the central Ecuadorian margin: Evidence of upper plate deformation

A seismic study of a segment of the convergent margin of Ecuador is presented. During the SISTEUR campaign a network of 24 Ocean Bottom Seismometers (OBS) was deployed on the Carnegie Ridge, one line along the main axes of the ridge and two lines across the strike of the edge of the ridge, during one month. This marine network was complemented with a land network of 20 stations distributed in two lines: one parallel to the margin and the other perpendicular to it.The seismic event recorded by these networks, were located using different crustal models defined from the wide-angle seismic data modeling. Relative location techniques were used to improve earthquake locations. Seismogram waveform modeling allowed us to constrain hypocentral location for events farther than ～50 km from the network. This modeling also provided additional information to constrain the focal mechanisms of these events. The upper limit of the Interplate Seismogenic Zone (ISZ) is estimated to be at a 10 km depth in the region. The background seismic activity of the upper plate provided new insights:1) A seismic cluster that reaches the base of the overriding plate is linked to the Jipijapa-Portoviejo fault. The reactivation of this Quaternary fault is confirmed by focal mechanisms that provide rupture planes parallel to its superficial projection (N10°–N25°).2) The focal mechanisms presented in this study are compatible with a homogeneous regional stress field corresponding to an E–W to ESE–WNW compression and an NNE–SSW extension. The presence of strike-slip deformation, with a reverse component, corresponds to the NNE escape of the North Andean Block. Normal faulting accommodating this movement suggests that this part of the North Andean Block cannot be considered as a rigid block.     

Yosemite—the Incomparable Valley

Cut deep into the heart of California's Sierra Nevada, the Yosemite Valley has been dubbed the Incomparable Valley, because it is so seriously spectacular. And it is. It has also been called the finest example of a deep glaciated trough. But it's not. Yosemite is more than that, and the glaciation is only part of its splendid geological story.     

Deltaic plain development and environmental changes in the Petite Camargue, Rhone Delta, France, in the past 2000 years

The deltaic plain of the Petite Camargue which constitutes the western part of the Rhone Delta, began its main progradation around 2000 yr ago. Several delta lobes follow each other and have participated in the deltaic evolution. The deltaic lobes have distinct morphologies which reflect the dynamic fluvial and marine processes under the influence of climatic and human controls. Two delta lobe systems were built by the Daladel and Peccaïs channels, after which a deflected wave-influenced delta lobe was formed by the La Ville and Saint-Roman channels. The latest channel, the Rhone Vif channel, is skewed because this channel was completely canalized and engineered up to its mouth in the beginning of the 16th century. Since the avulsion of this channel about 1550 A.D., the coastline of the Petite Camargue has been especially affected by the influence of waves and currents. The spits replaced the beach ridges which juxtaposed themselves and have migrated westward since the 16th century. The formation of the western part of the delta in the last 2000 yr is affected by not only the fluvial sedimentary fluxes and the coastal dynamics to the mouth but also climatic change and human influence.     

Modelling of tidal hydrodynamics for a tropical ecosystem with implications for pollutant dispersion (Cohin Estuary, Southwest India)

Tidal circulation in the Cochin Estuary, a moderately polluted estuary along the southwest coast of India, was studied using a 2D hydrodynamic model. The predicted tides and currents showed very good agreement with measured tides. Particle trajectories and residual currents computed from the model have been used to classify the study region into three zones: northern estuary, central estuary, and southern estuary. The central estuary is dynamic, whereas the other two zones are relatively weak. An amplification of measured tides in the south estuary during March indicates the presence of standing waves caused by the hydraulic barrier at Thanneermukkom. Model results suggest that the northern and southern zones are sensitive to environmental pollution. The present level of pollution in the northern estuary is due to the direct release of industrial effluents into the river Periyar, which can be minimized if they are brought down to central estuary for disposal. The concept of different zones in the estuary will be useful to planners in protecting the vulnerable regions of this productive ecosystem from human interventions.     

Three-dimensional kinematic depth migration of converted waves: application to the 2002 Molise aftershock sequence (southern Italy)

Migration techniques, currently used in seismic exploration, are still scarcely applied in earthquake seismology due to the poor source knowledge and sparse, irregular acquisition geometries. At the crustal scale, classical seismological studies often perform inversions based on the arrival time of primary phases (P- and S-waves) but seldom exploit other information included in seismic records. Here we show how migration techniques can be adapted to earthquake seismology for converted wave analysis. As an example, we used data recorded by a dense local seismic network during the 2002 Molise aftershock sequence. In October and November 2002, two moderate magnitude earthquakes struck the Molise region (southern Italy), followed by an aftershock sequence lasting for about one month. Local earthquake tomography has provided earthquake hypocenter locations and three-dimensional models of P and S velocity fields. Strong secondary signals have been detected between first-arrivals of P- and S-waves and identified as SP transmitted waves. In order to analyse these waves, we apply a prestack depth migration scheme based on the Kirchhoff summation technique. Since source parameters are unknown, seismograms are equalized and only kinematic aspects of the migration process are considered. Converted wave traveltimes are calculated in the three-dimensional (3D) tomographic models using a finite-difference eikonal solver and back ray tracing. In the migrated images, the area of dominant energy conversion corresponds to a strong seismic horizon that we interpreted as the top of the Apulia Carbonate Platform and whose geometry and position at depth is consistent with current structural models from existing commercial seismic profiles, gravimetric and well data.