Rainfall intensity patterns derived from the urban network of Barcelona (NE Spain)

Theoretical and Applied Climatology - An analysis of the normalised rainfall intensity curves in Barcelona (NE Spain) has been undertaken from 41 selected rain rate episodes recorded by an urban...     

Prediction of a mesoscale convective system over catalonia (Northeastern Spain) with a nested numerical model

Summary A mesoscale convective system that affected Northeastern Spain on October 10, 1994, with rainfall amounts up to 400 mm, is simulated reasonably well by a nested 3-dimensional hydrostatic mesoscale model. Previous studies carried out in this region had already portrayed the main synoptic patterns that give rise to these devastating episodes. The present contribution takes a further step since it goes down to the mesoscale by means of a numerical model providing a more detailed representation not otherwise achieved by earlier analysis methods. Although the model was unable to forecast accurately the precipitation fields, it captured satisfactorily the framework in which the convective system originated and evolved.With 16 Figures     

Saturated permeability measurements on pumice and welded-tuffaceous materials

Fluid flows in consolidated porous media of volcanic origin are being investigated to support such diverse efforts as the modeling of thermal/outgassing phenomena at Mount St. Helens and the hydrological modeling of tuffaceous rocks in support of the Department of Energy’s (DOE) Nevada Nuclear Waste Storage Investigations Project An experimental apparatus was designed and built to allow water-saturated permeabilities as low as 10⁻¹⁸ m² to be measured on cores of diameter 5 cm and length 10 cm under steady-state flow conditions. This same apparatus can also be utilized in a transient (pressure-decay) mode in order to measure permeabilities several orders of magnitude lower than the steady-state limit. Tests were conducted on samples of pumice, fractured welded tuff, and welded tuff, representing a permeability range of seven orders of magnitude Pumice was found to have a permeability of ∼3×10⁻¹² m², sufficiently high to allow the complete Darcy-to-Ergun regime to be investigated Welded (unfractured) tuff was tested in the transient mode, yielding a permeability of ∼5×10⁻¹⁹ m². Two, long-time-scale, steady-flow experiments were conducted on a core of welded tuff containing a single, through-going fracture. For the first experiment, the core was an integral cylinder containing a naturally occurring fracture. For the second experiment, the core was separated into two pieces along the existing fracture plane, then rejoined. Effects of essentially constant, as well as rapidly varied, circumferential stress were studied in both tests. Results showed core permeability to decay to 2×10⁻¹⁸ m² in both cases, independent of the initial fracture state (closed versus open). With a naturally occurring fracture, core permeability decreased by a factor of 2 over a 200-h test period. With an initially open fracture, core permeability decreased by a factor of 4 under the influence of a comparable 200-h load-time history, after 700 h of testing, core permeability was reduced by an order of magnitude from its initial level. Final effective hydraulic fracture aperture was calculated to be 10⁻⁶ m, corresponding to a calculated effective fracture permeability of 10⁻¹³ m² Fracture flow was thus estimated to account for 80% of the total flow rate through this core under final test conditions.     

Impact of desalination plants brine injection wells on coastal aquifers

A new methodology is developed in assessing environmental impacts of desalination plants discharging brine into the ground. The main environmental problem of the desalination of seawater is the brine disposal. The brine is commonly discharged into the sea or injected into a saline aquifer. In the case of injection into the ground, it is necessary to design a disposal system in a way that respects the environment and is sustainable. Laboratory and computational methods have been utilized to simulate the unsteady three-dimensional (3D) phenomena of subsurface brine disposal. The computational software used is SEAWAT, which is a 3D unsteady variable-density flow simulation model. The model is first used to simulate the laboratory results, and good agreement is achieved. Then, hypothetical problems are designed and simulated of groundwater extraction and brine disposal by desalination stations. The major purpose of these hypothetical problems is to delineate a methodology and to create design charts for design and management of production and injection well fields for coastal desalination plants. Several design charts have been developed with 36 scenarios for two well configurations created by four design parameters: relative salt concentration (RSC), production and injection rates (Q _d , Q _r ), well spacing (S), and simulation period (T).     

Main features of the sea-breeze in Barcelona

Summary From a data set of sea-breeze observations corresponding to cases of no synoptic-scale flow in Barcelona during the period 1970–89, some features of this wind have been deduced. Maximum velocities of between 6–14 m/s generally occur during 12–16 SLT. Diurnal evolution gives a clockwise rotation of sea breeze so that this wind blows roughly parallel to the shoreline in late afternoon. The rate of the change of direction is in agreement with numerical results from a simple nonlinear sea breeze model.With 7 FiguresThis work has been supported by the DGICYT, Project No. PB87-0718.     

Non-parametric Data Analysis of Low-latitude Auroras and Naked-eye Sunspots in the Medieval Epoch

We have studied solar activity by analyzing naked-eye sunspot observations and aurorae borealis observed at latitudes below \(45^{\circ}\). We focused on the medieval epoch by considering the non-telescopic observations of sunspots from AD 974 to 1278 and aurorae borealis from AD 965 to 1273 that are reported in several Far East historical sources, primarily in China and Korea. After setting selection rules, we analyzed the distribution of these individual events following the months of the Gregorian calendar. In December, an unusual peak is observed with data recorded in both China and Japan, but not within Korean data.In extreme conditions, where the collection of events is reduced and discontinuous in some temporal intervals, we used the non-parametric kernel method. We opted for the plug-in approach of Sheather and Jones instead of cross-validation techniques to estimate the probability density functions (pdf) of the events. We obtained optimized bandwidths of 13.29 years for sunspots and 9.06 years for auroras, and 95% confidence intervals. The pdf curves exhibit multiple peaks occurring at quasi-periodic times with a very high positive correlation, \(r_{\mathrm{tt}} = 0.9958\), between the dates of occurrence of the nine extrema of sunspots and auroras. Furthermore, these extrema enabled us to evaluate mean periods at two standard deviations, \(66.77 \pm 7.25~\mbox{years}\) for sunspots and \(65.06 \pm 9.36~\mbox{years}\) for auroras. The accuracy of the average periods, 62.00 years for sunspots and 61.80 years for auroras, was improved by the use of the power spectrum method. The percentage of the total number of non-observed sunspots, using redundant data, from AD 1151 to 1275 was estimated to be greater than or equal to 78%.     

A Study of Permeability Changes Due to Cold Fluid Circulation in Fractured Geothermal Reservoirs

Reservoir behavior due to injection and circulation of cold fluid is studied with a shear displacement model based on the distributed dislocation technique, in a poro‐thermoelastic environment. The approach is applied to a selected volume of Soultz geothermal reservoir at a depth range of 3600 to 3700 m. Permeability enhancement and geothermal potential of Soultz geothermal reservoir are assessed over a stimulation period of 3 months and a fluid circulation period of 14 years. This study—by shedding light onto another source of uncertainty—points toward a special role for the fracture surface asperities in predicting the shear dilation of fractures. It was also observed that thermal stress has a significant impact on changing the reservoir stress field. The effect of thermal stresses on reservoir behavior is more evident over longer circulation term as the rock matrix temperature is significantly lowered. Change in the fracture permeability due to the thermal stresses can also lead to the short circuiting between the injection and production wells which in turn decreases the produced fluid temperature significantly. The effect of thermal stress persists during the whole circulation period as it has significant impact on the continuous increase in the flow rate due to improved permeability over the circulation period. In the current study, taking into account the thermal stress resulted in a decrease of about 7 °C in predicted produced fluid temperature after 14 years of cold fluid circulation; a difference which notably influences the potential prediction of an enhanced geothermal system.     

Poststack diffraction imaging using reverse‐time migration

We propose a method for imaging small‐scale diffraction objects in complex environments in which Kirchhoff‐based approaches may fail. The proposed method is based on a separation between the specular reflection and diffraction components of the total wavefield in the migrated surface angle domain. Reverse‐time migration was utilized to produce the common image gathers. This approach provides stable and robust results in cases of complex velocity models. The separation is based on the fact that, in surface angle common image gathers, reflection events are focused at positions that correspond to the apparent dip angle of the reflectors, whereas diffracted events are distributed over a wide range of angles. The high‐resolution radon‐based procedure is used to efficiently separate the reflection and diffraction wavefields. In this study, we consider poststack diffraction imaging. The advantages of working in the poststack domain are its numerical efficiency and the reduced computational time. The numerical results show that the proposed method is able to image diffraction objects in complex environments. The application of the method to a real seismic dataset illustrates the capability of the approach to extract diffractions.