Ginette Saracco

An estimate of average lower mantle conductivity by wavelet analysis of geomagnetic jerks

It has recently been proposed that geomagnetic jerks observed at the Earths surface could be viewed as singularities in the time behavior of the geomagnetic field with a regularity of about 1.5 when wavelet analyzed. Such a signal should have suffered some distortion when diffusing from the core-mantle boundary (CMB) through the conducting mantle. Assuming that the upper mantle is an insulator and given the electromagnetic time constant of the mantle, we compute the distortion that a pure singularity introduced at the CMB suffers as it traverses the mantle. We compute this distortion through its effects on the so-called ridge functions extracted from the wavelet transform of the signal. This distortion is very similar to the small but significant one that we observe in real data. We therefore speculate that jerks must have been pure singularities at the base of the mantle and infer an average estimate for the mantle electromagnetic time constant from the way the signal is distorted by fitting the synthetic ridge functions to the experimental ones. Assuming, for example, a thickness of 2000 km for a uniform lower conducting mantle, we find an electrical conductivity smaller than 10 S m(-1). This value is in reasonable agreement with values derived from high-pressure experiments for a silicate mantle.

MWTmat-application of multiscale wavelet tomography on potential fields

Wavelet analysis is a well-known technique in the sciences to extract essential information from measured signals. Based on the theory developed by previous studies on the Poisson kernel family, this study presents an open source code, which allows for the determination of the depth of the source responsible for the measured potential field. MWTmat, based on the Matlab platform, does not require the wavelet tool box, is easy to use, and allows the user to select the analyzing wavelets and parameters. The program offers a panel of 10 different wavelets based on the Poisson kernel family and the choice between a fully manual and a semiautomatic mode for selection of lines of extrema. The general equations for both horizontal and vertical derivative wavelets are presented in this study, allowing the user to add new wavelets. Continuous wavelet analyses can be used to efficiently analyze electrical, magnetic, and gravity signals; examples are presented here. The MWTmat code and the multiscale wavelet tomography approach are an efficient method for investigating spatial and temporal changes of sources generating potential field signals.

New insights into the Kawah Ijen hydrothermal system from geophysical data

Abstract The magmatic–hydrothermal system of Kawah Ijen volcano is one of the most exotic on Earth, featuring the largest acidic lake on the planet, a hyper-acidic river and a passively degassing silicic dome. While previous studies have mostly described this unique system from a geochemical perspective, to date there has been no comprehensive geophysical investigation of the system. In our study, we surveyed the lake using a thermocouple, a thermal camera, an echo sounder and CO2 sensors. Furthermore, we gained insights into the hydrogeological structures by combining self-potential surveys with ground and water temperatures. Our results show that the hydrothermal system is self-sealed within the upper edifice and releases pressurized gas only through the active crater. We also show that the extensive hydrological system is formed by not one but three aquifers: a south aquifer that seems to be completely isolated, a west aquifer that sustains the acidic upper springs, and an east aquifer that is the main source of fresh water for the lake. In contrast with previous research, we emphasize the heterogeneity of the acidic lake, illustrated by intense subaqueous degassing. These findings provide new insights into this unique, hazardous hydrothermal system, which may eventually improve the existing monitoring system.

Wavelet transform: A tool for the interpretation of upper mantle converted phases at high frequency

P to S converted receiver functions recorded at VBB FDSN California stations are studied in the frequency range of 0.1 to i Hz. Microseismic noise is maximum in this frequency range, but signal processing by the wavelet transform enables us: (1) to enhance seismic phases associated with seismic velocity gradients and discontinuities at the base of the upper mantle, (2) to extract accurate arrival times, and (3) to obtain an accurate insight into the frequency content. In the case of California, one seismic discontinuity and two zones of high gradients in the depth range of 625 to 720km are recurrently observed for two different data sets.

Characterization of elastic shells by the use of the wavelet transform

The characterization of elastic targets immersed in a fluid and subjected to an acoustic impulse is discussed. The wavelet transform has been chosen for its particular properties, such as linearity and local analysis at Delta f/f=c/sup ste/. An algorithm based on the behavior of the phase of the transform has been developed. It allows the extraction of modulation laws (related to the dispersion law of the phase velocity), even for close echoes. In the case of spherical elastic shells, the method was used on both experimental and computer-generated signals, and the good relation between theoretical and experimental results is presented.<<ETX>>

Ultrasonic seismic imaging of lava samples by viscoacoustic asymptotic waveform inversion: calibration and developments

The object of this study is to estimate the viscoelastic parameters (compressional velocity and the intrinsic attenuation factor Q) of lava samples in acoustic tomography experiments by the use of an asymptotic viscoacoustic diffraction tomography method developed by Ribodetti et al. (2000). 2.5-D common-offset ultrasonic experiments are performed in a water tank laboratory.

Deconvolution from instrumental devices and source effect in acoustic experiments

Two methods to deconvolve experimental data from the distortions introduced by instrumental devices or source effects are presented. Considering a total acquisition system (emission-reception line, amplifier, pre-amplifier) as a global experimental filter, we can define its properties (module and phase) experimentally from the generation of a family of source signals dilated in time. The estimation of this filter allows the deconvolution of the recorded output signal. The first approach is based on the simple reconstruction formula of the continuous wavelet transform (CWT). The second method is based on the construction of a normalized family of a finite number of specific filters, independent of the frequency range used. In both cases, experimental results in an acoustic tank are presented. We show that after deconvolution, the source signal is correctly reconstructed from the recorded output signal and the global instrumental filter.

3D Azimuthal seismic endoscopy: application to acoustic borehole experiment

Summary A method to perform a three dimensional imaging around a borehole is presented. A specific tool was developed in our laboratory to allow us a directive and azimuthal acquisition. The main characteristics of this prototype and the basic algorithms of the method called seismic endoscopy are reported. The problem to obtain a correct 3D image around the borehole is the strong amplitude of borehole waves that mask the reflections coming from the far field that we want to analyze. A method based on the continuous wavelet transform is used to detect and filter in time and frequency domain these waves. A dynamic azimuthal correction called azimuthal move out (AMO) is then applied to refocus the energy that is spread out in the azimuthal-time domain. Some acoustic experiment conducted in a water tank will be presented to prove the efficiency of this