Xavier Garcia

Atmospheric sources for audio-magnetotelluric (AMT) sounding

The energy sources for natural-source magnetotelluric (MT) frequencies > 1 Hz are electromagnetic (EM) waves caused by distant lightning storms and which propagate within the Earth–ionosphere waveguide. The properties of this waveguide display diurnal, seasonal, and 11-year solar-cycle fluctuations, and these temporal fluctuations cause significant signal amplitude attenuation variations—especially at frequencies in the 1- to 5-kHz so-called audiomagnetotelluric (AMT) dead band. In the northern hemisphere these variations increase in amplitude during the nighttime and the summer months, and they correspondingly decrease during the daytime and the winter months. Thus, one problem associated with applying the AMT method for shallow (<3 km) exploration can be the lack of signal in certain frequency bands during the desired acquisition interval. In this paper we analyze the time variations of high-frequency EM fields to assess the limitations of the efficient applicability of the AMT method. We demonstrate th...

Robust processing of magnetotelluric data in the AMT dead band using the continuous wavelet transform

The energy sources for magnetotellurics (MT) at frequencies above 8 Hz are electromagnetic waves generated by distant lightning storms propagating globally within the earth-ionosphere waveguide. The nature of the sources and properties of this waveguide display diurnal and seasonal variations that can cause significant signal amplitude attenuation, especially at 1–5 kHz frequencies — the so-called audiomagnetotelluric (AMT) dead band. This lack of energy results in unreliable MT response estimates; and, given that in crystalline environments ore bodies located at some 500–1000-m depth are sensed initially by AMT data within the dead band, this leads to poor inherent geometric resolution of target structures. We propose a new time-series processing technique that uses localization properties of the wavelet transform to select the most energetic events. Subsequently, two coherence thresholds and a series of robust weights are implemented to obtain the most reliable MT response estimates. Finally, errors are...

2D inversion of 3D magnetotelluric data: The Kayabe dataset

In the last two Magnetotelluric Data Interpretation Workshops (MT-DIW) the participants were asked to model the Kayabe magnetotelluric dataset, a dense (100 m) grid of thirteen lines, with thirteen stations in each line. Bahr’s phase-sensitive skew and the Groom and Bailey decomposition were used to select those lines for which the data could be considered two-dimensional. For these lines we used a 2D inversion algorithm to obtain a series of resistivity models for the earth. Finally, we constructed a 3D model using the 2D models and critically examined the validity and practicality of this approach based on 3D model study. We found that in the Kayabe dataset case the common practice of using 2D models to depict 3D models, can only be used to create a starting model for 3D interpretation. The sequential 2D models as a representation of a 3D body is unacceptable in terms of fit to the observed data. We question the validity of some of the conductivity structures in the 2D models, as they can be mere artifacts created by the algorithm to match 3D effects.

Okak Bay AMT data-set case study: Lessons in dimensionality and scale

Electromagnetic (EM) exploration for base metals using the natural‐source audio‐magnetotelluric (AMT) technique has increased significantly during the last five years due to enhancements in all aspects of AMT and to the demand for imaging deeper than conventional controlled‐source EM methods. However, regional currents induced by natural sources can be problematic in certain situations, and the appropriate interpretational dimensionality must be known. Herein we demonstrate that a two‐dimensional (2D) interpretation is valid for a defined frequency band, but that the effects of large‐scale three‐dimensional (3D) structures must be considered at lower frequencies. Using an AMT dataset from an area located north of Voiseys Bay, Labrador, Canada, we analyse the responses to determine the appropriate dimensionality and to test them for internal consistency. Maps of the distortion‐corrected data identify the lateral extent of connected conducting mineralization intersected by a drilling program. One‐dimension...

Hysteresis effects studied by numerical simulations : Cyclic loading-unloading of a realistic sand model

When Hertz-Mindlin force laws are considered in the context of the effective-medium theory, the predictions yield a constant Poisson coefficient and bulk/shear elastic moduli that scale with pressure with a 1/3 power law exponent (P1/3). This prediction contradicts early and recent experimental findings that conclude moduli grow faster with a 1/2 power law exponent (P1/2). Such a conclusion is also reached by recent second-order corrections to linear elastic theory. In this work we use a discrete-particle method to study the elastic response of a model of sand that is unconsolidated because of cyclic loading. We use a detailed molecular dynamics simulation that accounts for Hertz-type grain interactions and history-dependent shear forces. The porous sand model is constructed from spherical particles whose size distribution mimics well-sorted unconsolidated sands. The geometry of the model is obtained by simulating critical processes in sedimentary rock formations. Hysteretic behavior and relations between...

A new methodology for the acquisition and processing of audio-magnetotelluric (AMT) data in the AMT dead band

Distant lightning activity, the natural energy source for the audio-magnetotelluric (AMT) method, has a signal minimum between 1 and 5 kHz, the so-called AMT dead band. The energy in this band exhibits both diurnal and annual variation; magnetic-field amplitudes during the daytime are often well below the noise levels of existing sensors (coil magnetometers), thus reducing the effectiveness of the method for quantitative high-resolution studies of near-surface targets. To overcome this deficiency, we propose a hybrid acquisition and processing methodology based on combining the telluric-telluric (T-T) and telluric-magnetotelluric (T-MT) methods in this frequency range. Our method records the telluric channels at several sites and at base and remote reference stations during the day and records the full magnetotelluric (MT) components at the base and remote stations only during the night. Applying a tensor multiplicative relationship between these responses, we obtain the T-MT AMT transfer functions for the sites; these transfer functions can represent a reasonable approximation of the real AMT impedance tensors. To test the approach, a T-MT experiment was carried out in Sudbury, northern Ontario, during summer 2000. We compare the processed daytime data using the conventional MT approach to those obtained from our T-MT approach. The results demonstrate that our method can determine high-quality estimates in the dead band, although the estimates can be severely affected by noise.

Wound healing in the fetal period: The resistance of the scar to rupture

Wound healing in the fetal period is fundamentally different from that of the adult. In order to better understand this difference, we have studied wound healing in three types of wounds which are the most common in surgical practice, and have paid special attention to tensile strength in the scar. A sutured wound, a nonsutured wound, and an electrocautery burn were performed on a group of 30 rabbit fetuses with a gestational age of 23 days. Seven days later, the resulting scars were examined using histological, mechanical, and biochemical studies. The results were compared with those obtained in a group of 30 newborn rabbits and in a group of 30 adult rabbits who had had the same type of wounds performed on them. The body weight of operated fetuses was smaller than their control siblings. The sutured wound in the fetuses showed a more perfect macroscopic repair than in older subjects. However, the nonsutured wounds and the burns of the fetal group did not close. In the fetuses, the tensile strength of the sutured wound was 20% of the tensile strength of healthy skin. A similar relationship occurred in the group of adult rabbits. In the sutured wound of the fetal group, the synthesis of collagen was significantly less than in the adult group. The ratio between hydroxyproline (HYP) concentration in the sutured wound and HYP concentration in healthy skin was also significantly less in the fetal group than in the adult group.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonstationary magnetotelluric data processing with instantaneous parameter

Nonstationarity in electromagnetic data affects the computation of Fourier spectra and therefore the traditional estimation of the magnetotelluric (MT) transfer functions (TF). We provide a TF estimation scheme based on an emerging nonlinear, nonstationary time series analysis tool, called empirical mode decomposition (EMD) and show that this technique can handle nonstationary effects with which traditional methods encounter difficulties. In contrast to previous works that employ EMD for MT data processing, we argue the advantages of a multivariate decomposition, highlight the possibility to use instantaneous parameters, and define the homogenization of frequency discrepancies between data channels. Our scheme uses the robust statistical estimation of transfer functions based on robust principal component analysis and a robust iteratively reweighted least squares regression with a Huber weight function. The scheme can be applied with and without aid of any number of available remote reference stations. Uncertainties are estimated by iterating the complete robust regression, including the robust weight computation, with a bootstrap routine. We apply our scheme to synthetic and real data (Southern Africa) with and without nonstationary character and compare different processing techniques to the one presented here. As a conclusion, nonstationary noise can heavily affect Fourier-based MT data processing but the presented nonstationary approach is nonetheless able to extract the impedances.

Internal structure of the western flank of the Cumbre Vieja volcano, La Palma, Canary Islands, from land magnetotelluric imaging

[1] Large-scale mass wasting is a natural part of the evolution of volcanic islands, where deformation and indications of flank instability, such as large-scale faulting and seismic and aseismic slip are common. The Cumbre Vieja volcano on the island of La Palma (Canary Islands) provides an ideal setting to address fundamental questions about the structure, evolution and stability of island volcanoes. The island of La Palma is still in a shield-building stage, and it has been postulated that the western side of the island lies over a pre-existing zone of weakness that can nucleate ruptures. We undertook an audiomagnetotelluric (AMT) survey over the proposed unstable western flank to try to image structures that may be associated with the zone of weakness. The magnetotelluric method (MT) is a geophysical technique used to map the presence of fluids or image important structural contrasts. The goals of this study were (1) to delineate the unstable flank, (2) to map the structures underneath and (3) to determine the presence and geometry of fluids. The results show a 1 km thick top resistive layer overlaying an area of reduced resistivity (enhanced conductivity), interpreted as a layer consisting of an alteration zone and also fluids. Our results confirm previous studies that suggested the existence of a western flank lying over collapse debris material and hyaloclastites, and also they allow us to map part of the subaerial southern extent of the Cumbre Nueva units that lie beneath the more recent Cumbre Vieja rocks. In addition, dimensionality analysis maps the rotation of the dike emplacement off ridge, along the western flank in an en echelon fashion.

Analgesic and Antiallodynic Effects of Antidepressants after Infiltration into the Rat

Tricyclic antidepressants (TCA) have potent local anesthetic properties and may produce a long-lasting pain blockade that could be of interest in relieving chronic pain states such as neuropathic pain, but there are only few data comparing their dose-response curves of analgesic activity under the same experimental conditions. This study examines the time course of pain-relieving properties of 7 TCA in heat-induced paw withdrawal after subcutaneous administration. Mixed inhibitors of norepinephrine and serotonin uptake (amitriptyline, nortriptyline, imipramine, desipramine, doxepin) and selective inhibitors of serotonin uptake (fluoxetine and fluvoxamine) were assayed. The TCA with the longest analgesic activity were selected to test its antiallodynic effect in the neuropathic pain model of chronic sciatic nerve constriction injury. Bupivacaine was used as a reference drug in both experiments. A dose versus time of maximal analgesic effect curve was constructed for each drug. The longest analgesic effect was obtained for doxepin and imipramine. Although low doses of amitriptyline showed the same activity than doxepin, higher doses failed to reach the same effect. Selective inhibitors of serotonin showed no action at all doses tested. In the chronic sciatic nerve constriction injury model, doxepin and, to a smaller degree, amitriptyline and imipramine protected from allodynia; bupivacaine was ineffective. The antiallodynic effect always lasted less long than the analgesic effect. These observations provide support for the potential use of TCA as durable analgesics. Doxepin overall showed the most outstanding results in pain relief.