Iterative resolution estimation in least‐squares Kirchhoff migration

We apply iterative resolution estimation to least‐squares Kirchhoff migration. Reviewing the theory of iterative optimization uncovers the common origin of different optimization methods. This allows us to reformulate the pseudo‐inverse, model resolution and data resolution operators in terms of effective iterative estimates. When applied to Kirchhoff migration, plots of the diagonal of the model resolution matrix reveal low illumination areas on seismic images and provide information about image uncertainties. Synthetic and real data examples illustrate the proposed technique and confirm the theoretical expectations.     

The use of modern statistical theories in the assessment of earthquake hazard, with application to quiet regions of eastern North America

It is generally accepted that both deterministic and statistical approaches are useful for the characterization of earthquake hazard. Although the most reliable estimates of seismic hazard can only be based on an improved understanding of the earthquake mechanism, efficient utilization of the appropriate methods provided by recent statistical theories is also important in seismic risk analysis. This is especially true in regions where the connection between seismicity and geologic structure is tenuous at best. We are particularly interested in developing better statistical treatments of data for regions with little known seismic activity. To this end, we have applied three statistical methods to the historical record of seismicity in relatively quiet regions of eastern North America. These are: (1) the threshold method for tail inference, a new theory for modeling earthquakes with sizes above a given threshold, (2) the ‘bootstrap’ technique in which the characteristics of an unknown population are simulated by replacing the true population by an estimated one, and (3) a technique to estimate the number of earthquakes below a given size, in order to compensate for the under-reporting of small earthquakes in most catalogs. A combination of these techniques has been used to estimate the probabilities of future large earthquakes for the regions studied. Because of limitations imposed by existing catalogs, the size estimate used has been maximum intensity.     

Relationships between tissue concentrations of chlorinated hydrocarbons (polychlorinated biphenyls and chlorinated pesticides) and antioxidative responses of marine mussels, Perna viridis

Marine mussels, Perna viridis, were transplanted from a reference site to various polluted sites around Hong Kong. After 30 d of exposure, antioxidative responses in the gills and hepatopancreas and tissue concentrations of chlorinated hydrocarbons [polychlorinated biphenyls (PCBs) and chlorinated pesticides (CPs)] were determined for individual mussels. Glutathione S transferase (GST) and glutathione (GSH) were positively correlated with tissue PCB concentrations. Only one of the enzymatic antioxidants, glutathione peroxidase (GPx), showed significant response to tissue PCB. No significant correlation was found between tissue concentrations of chlorinated hydrocarbons and other enzymatic antioxidants (superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR) and NADPH DT-diaphorase (DT-d). Oxidative stress, measured as thiobarbituric acid reactive substances, was correlated with chlorinated pesticide concentrations in tissues. This study demonstrated a correlation between GST/GSH and chlorinated hydrocarbons. The apparent lack of correlation between trace organic pollutants and some of the enzymatic antioxidants may be due to the inhibitory effects caused by these chemicals. The above results suggest that more investigations are needed before these enzymes can be used as biomarkers.     

Solar–terrestrial effects possibly stronger in biblical times

Paleomagnetic studies have shown that, moving backwards in time, the geomagnetic dipole moment increased to a peak nearly 50% greater than at present ca. 2500 years ago. Attempts to model how changes in dipole moment affect solar–terrestrial relations have hitherto invoked a scaling relation for the size of the magnetosphere based on finding where the magnetic pressure of the dipole field balances the ram pressure of the solar wind. This approach predicts that, following a solar storm, the strength of the terrestrial response represented by the electrical potential across the auroral zones in the ionosphere should vary as the 1/3 power of dipole moment. Such a weak dependence suggests that a 50% increase in dipole moment would minimally effect (14%) terrestrial manifestations of solar storms. Recent work, however, based on a feedback mechanism involving electrical currents coupling the magnetosphere and ionosphere has identified a stronger 4/3, power scaling relation applicable to storm conditions. Here we use a global MHD simulation to calculate for a 50% increased dipole moment the correspondingly increased auroral-zone potential and its extension to low latitudes.     

Modelling of meteorological conditions at an urban scale for the PUMA winter campaign

For the winter 2000 campaign of the Pollution of Urban Midlands Atmosphere project, observation and numerical modelling of meteorological conditions over the West Midlands conurbation, UK, was undertaken. Modelling was performed using the regional atmospheric meteorological system (RAMS). This paper presents a comparison of modelled and observed wind and temperature for 25 and 26 January 2000. The RAMS model uses two nested grids with a mesh size of 2 km for the inner grid which is embedded in the outer grid with a mesh size of 8 km. Statistical evaluation of the model results against the observational data of wind speed, direction and temperature at 10 m was conducted. In general, the modelling results are in a reasonable agreement with observation. The statistical evaluation suggests that model performance is poorer for the inner grid than the outer grid as the model uncertainties (mainly mean bias) transfer from the outer to inner one. The low indices of agreement of temperature and wind are mainly associated with the systematic root-mean-square-difference values. For temperature, the systematic bias may also be affected by representation of cloud amount by the model. For wind, the model tends to have a poor performance for calm conditions, as under a stable anti-cyclonic situation local wind patterns associated with topography may develop, although the topography of the region is relatively flat. The results for the inner grid reveal some subtle spatial patterns at a scale smaller than 10 km near hills and valleys with differences in elevation of a few hundred metres.     

Predicting average annual groundwater levels from climatic variables: an empirical model

On the basis of one-dimensional theoretical water flow model, we demonstrate that the groundwater level variation follows a pattern similar to recharge fluctuation, with a time delay that depends on the characteristics of aquifer, recharge pattern as well as the distance between the recharge and observation locations. On the basis of a water budget model and the groundwater flow model, we propose an empirical model that links climatic variables to groundwater level. The empirical model is tested using a partial data set from historical records of water levels from more than 80 wells in a monitoring network for the carbonate rock aquifer, southern Manitoba, Canada. The testing results show that the predicted groundwater levels are very close to the observed ones in most cases. The overall average correlation coefficient between the predicted and observed water levels is 0.92. This proposed empirical statistical model could be used to predict variations in groundwater level in response to different climate scenarios in a climate change impact assessment.     

Combined GPS and InSAR Models of Postseismic Deformation from the Northridge Earthquake

--Models of combined Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data collected in the region of the Northridge earthquake indicate that significant afterslip on the main fault occurred following the earthquake. Additional shallow deformation occurred to the west of the main rupture plane. Both data sets are consistent with logarithmic time-dependent behavior following the earthquake indicative of afterslip rather than postseismic relaxation. Aftershocks account for only about 10% of the postseismic motion. The two data sets are complimentary in determining the postseismic processes. Fault afterslip and shallow deformation dominate the deformation field in the two years following the earthquake. Lower crustal deformation may play an important role later in the earthquake cycle.     

On the Generation of ULF Magnetic Variations by Conductivity Fluctuations in a Fault Zone

v--vPrior to the October 18, 1989 Loma Prieta M_s 7.1 earthquake, Fraser-Smith et al. (1990) recorded a 10-100 fold increase in ultra-low frequency (ULF) magnetic fields near the earthquake epicenter. Several mechanisms for generation of these ULF fields by fluid flow in the earth have been advanced, but all appear to require unrealistic fluid velocities or hydraulic permeabilities to match the observations. As an alternative explanation, Merzer and Klemperer (1998) proposed that the increase in ULF magnetic fields could result from induced electric currents flowing in a fault-zone made temporarily much more electrically conductive by stress-induced reorganization of pore geometry. Using a numerical model we show that while this mechanism could produce a significant increase in ULF variations, mutual induction between the fault zone and the surrounding crust would probably limit the amplitude increase to levels well below those observed at Loma Prieta. We consider a variant on this quasi-static conductive fault zone model in which low frequency telluric currents are modulated by small higher frequency variations of bulk fault zone conductivity. We show that because the spectrum of natural EM variations is red, substantially larger relative increases in ULF magnetic fields could be produced by this mechanism with even small conductivity fluctuations at these frequencies. These variations would be easy to detect with a well-designed experiment, if they occurred. In principle this mechanism could explain the Loma Prieta ULF observations, however the magnitude of conductivity fluctuations that would be required to match the very large reported amplification factors still appears to be too large to be physically plausible.     

DNAPL Characterization Methods and Approaches, Part 2: Cost Comparisons

Contamination from the use of chlorinated solvents, often classified as dense nonaqueous phase liquids (DNAPLs) when in an undissolved state, pose environmental threats to ground water resources worldwide. DNAPL site characterization method performance comparisons are presented in a companion paper (Kram et al. 2001). This study compares the costs for implementing various characterization approaches using synthetic unit model scenarios (UMSs), each with particular physical characteristics. Unit costs and assumptions related to labor, equipment, and consumables are applied to determine costs associated with each approach for various UMSs. In general, the direct-push sensor systems provide cost-effective characterization information in soils that are penetrable with relatively shallow (less than 10 to 15 m) water tables. For sites with impenetrable lithology using direct-push techniques, the Ribbon NAPL Sampler Flexible Liner Underground Technologies Everting (FLUTe) membrane appears to be the most cost-effective approach. For all scenarios studied, partitioning interwell tracer tests (PITTs) are the most expensive approach due to the extensive pre-and post-PITT requirements. However, the PITT is capable of providing useful additional information, such as approximate DNAPL saturation, which is not generally available from any of the other approaches included in this comparison.     

Surface vibration due to a sequence of high speed moving harmonic rectangular loads

The transmission of vibrations over the surface of the ground, due to high-speed moving, vertical harmonic rectangular loads, is investigated theoretically. The problem is three-dimensional and the interior of the ground is modelled as an elastic half-space or a multilayered ground. The transformed solutions are obtained using the Fourier transform on the space variable. A new damping model in the spatial wavenumber domain, presented in Lefeuve-Mesgouez et al. [J. Sound. Vibr. 231 (2000) 1289] is used. Numerical results for the displacements on the surface are presented for loads moving with speeds up to and beyond the Rayleigh wave speed of the half-space.