José Antonio Cuchí

Accurately Locating a Vertical Magnetic Dipole Buried in a Conducting Earth

The use of a buried vertical magnetic dipole to determine the position of a point inside the Earth is based on the geometric structure of the measured field. A new systematic technique that improves the location accuracy of the surface point in the axis of the buried dipole is developed and tested. It is based on the use of state-of-the-art topographic equipment for the null-field direction measurements, that are further processed by two estimation algorithms, namely, the extended information filter and the extended Kalman filter. In addition, emphasis is placed on the study of the sources of error in order to establish the accuracy of the method. This analysis is conducted in a half-space model in which electromagnetic propagation is studied by numerical integration of analytical solutions. The results are confirmed by a direct solution of the problem by the finite-element method. From the computations, simple equations are provided for practical use. Then, an experimental study shows that the uncertainty of the null-field direction measurements can be modeled by a Gaussian probability density function. In addition, the data of the field experiment are used to validate the proposed subsurface location technique.

Hydrogeochemical characteristics at Cape Lamb, Vega Island, Antarctic Peninsula

Abstract Environmental changes in the northern Antarctic Peninsula provide a sensitive local indicator of climate warming. A consequence of these changes is the activation of surface and subsurface hydrological cycles in areas where water, in colder conditions, would remain frozen. This paper analyses the effects of hydrological cycle activation at Cape Lamb, Vega Island. The conclusions are based on hydrochemistry and isotope interpretation of 51 representative water samples from precipitation, streams, lakes, ice, snow and groundwater. Based on these results relationships between the different components of the hydrological cycle are proposed. This paper highlights the important contribution of groundwater to surface water chemistry, the disconnection of the lakes from the overall flow, the lack of an ocean spray signature in surface water and groundwater and the significant influence of windblown dust in the composition of the analysed waters.

Improving Medium Access in Through-The-Earth VLF-LF Communications

In order to improve the communication range of Through-The-Earth (TTE) radio using electrodes, the impedance load seen by power stage load must be minimized. This impedance depends on the wires, the electrode contact and the path between the electrodes (earth). Of the three elements, we cannot influence the earth impedance. The wire impedance can be minimized employing short cables and avoiding coiling them. This paper presents a method for characterizing the electrode contact impedance and provides suggestions to minimize it. Therefore some impedance measurements with several electrodes and a variety of contact conditions have been performed in order to improve our knowledge of medium access. To further prove the results measurements have been made with a voice radio application.

Avalanche beacon magnetic field calculations for rescue techniques improvement

Nowadays, many of the deaths on account of snow avalanches are related to an excessive delay during the rescue. The best way to locate an avalanche victim, when there is no sign, is by an avalanche beacon. The current techniques based on this device assume the condition of near field under the free-space propagation model. This work shows that this model is not adequate during the first steps of the search, and a half-space model, with air and soil layers, has to be used. In the region close to the buried victim, the classical propagation model performs well enough but depending on the orientation of the emitter avalanche beacon and the type of ARVA used to detect it, some errors are unavoidable.

Linear system techniques applied to the Fuenmayor Karst Spring, Huesca (Spain)

Black box identification techniques are very useful tools to study the relationship between rainfall and the discharge of karst aquifers. Autocorrelation, cross-correlation, cross-spectral, impulse response identification (Wiener–Hopf and error minimization methods) and parametric identification of transfer functions techniques have been used by different authors. A review of these techniques has been realized to assess the possibilities and limitations. The reviewed techniques are applied to a time rainfall–discharge series of Fuenmayor karst spring, (southern central Pyrenees, Spain) to establish the relative quality of the obtained models. The quality is evaluated with the Nash–Sutcliffe model efficiency coefficient. In addition, effective rainfall has been used to improve the performance of every model. The whitening technique has been used in the cross-correlation technique with good results. The studied spring shows a Fontestorbes type behavior. The linearity of the system has been analyzed by the coherence function. Fuenmayor has an acceptable linear response, except for high frequency events. The parametric identification provides the best efficiency value E = 0.8164 and provides a proper transfer function, with five parameters (one zero, two poles, the gain and the delay). The prediction power of this transfer function is validated with another time series, with an efficiency value of E = 0.7904.

Application of Wavelet Correlation Analysis to the Karst Spring of Fuenmayor. San Julian de Banzo, Huesca, Spain

The Fuenmayor spring has continuously been monitored for identification purposes, to study the behavior, and optimize the management of a karst system. Previous studies have examined the model through classical spectral analysis and system identification techniques under a linearity hypothesis. Since a karst area is generally accepted as a non-linear and dynamic system, different approaches, from wavelet paradigmto high order statistically stable wavelet techniques, are discussed. The results obtained from spectral and polyspectral wavelet analysis shed light on karst system non-linearity, to better understand its behavior.

Nonlinear System Engineering Techniques Applied to the Fuenmayor Karst Spring, Huesca (Spain)

Fuenmayor is a modest karst spring that tapes a small limestone aquifer near Huesca, Spain. A previous paper proposes a transfer function between effective rainfall and discharge concluding that Fuenmayor has an acceptable linear response. However, the linear model does not estimate adequately the response to some events where the nonlinearities are evidenced. To deal with the nonlinear characteristics of Fuenmayor, it is proposed a black-box model based on the Hammerstein-Wiener block-oriented structure. It is composed by a linear dynamic system surrounded by two static nonlinearities at its input and output. Seven different configurations of blocks are presented. Their efficiency has been evaluated by the Nash–Sutcliffe model efficiency coefficient. A good result is obtained with a configuration where the linear block is a second-order transfer function, with a zero and seven unit delays. The first nonlinear block is a piecewise polynomial and the second block has been suppressed. The running test draws out a maximum Nash-Sutcliffe efficiency of E = 0.9383.

Through-the-earth magnetic field propagation: Modelling for underground applications

A Finite-Element Method (FEM) model of a Through-the-Earth (TTE) Magnetic Field propagation field experiment is developed using COMSOL Multiphysics. The geometry of the model is obtained from the topographic data of the test site. The soil description is set from the literature concerning the region interpreted by means of a vertical electric sounding and the geophysical analysis of the electromagnetic measurements and computations. A frequency range of 1 to 100 kHz, for seven emitter station points along 200 m was covered. In this paper, the coefficient of variation of the root of the mean squared error (CVRMSE) is studied. The results are compared with the computations of analytical propagation models. Free-space, infinite dissipative, semi-infinite and three-layered models for linear, homogeneous and isotropic horizontal mediums are considered. As a result, the accuracy of the analytical models is highly improved by the FEM model. Therefore, FEM modelling can successfully be used for improving TTE propagation predictions for environments of complex geological structure.