To better understand and interpret seismoelectric measurements acquired over vadose environments, both the existing theory and the wave propagation modelling programmes, available for saturated materials, should be extended to partial saturation conditions. We propose here an extension of Pride's equations aiming to take into account partially saturated materials, in the case of a water-air mixture. This new set of equations was incorporated into an existing seismoelectric wave propagation modelling code, originally designed for stratified saturated media. This extension concerns both the mechanical part, using a generalization of the Biot-Gassmann theory, and the electromagnetic part, for which dielectric permittivity and electrical conductivity were expressed against water saturation. The dynamic seismoelectric coupling was written as a function of the streaming potential coefficient, which depends on saturation, using four different relations derived from recent laboratory or theoretical studies. In a second part, this extended programme was used to synthesize the seismoelectric response for a layered medium consisting of a partially saturated sand overburden on top of a saturated sandstone half-space. Subsequent analysis of the modelled amplitudes suggests that the typically very weak interface response (IR) may be best recovered when the shallow layer exhibits low saturation. We also use our programme to compute the seismoelectric response of a capillary fringe between a vadose sand overburden and a saturated sand half-space. Our first modelling results suggest that the study of the seismoelectric IR may help to detect a sharp saturation contrast better than a smooth saturation transition. In our example, a saturation contrast of 50 per cent between a fully saturated sand half-space and a partially saturated shallow sand layer yields a stronger IR than a stepwise decrease in saturation.

The behaviour of CO 2 deposition sites-and their surroundings-during and after carbon dioxide injection has been matter of study for several years, and several geophysical prospection techniques like surface and crosshole seismics, geoelectrics, controlled source electromagnetics among others, have been applied to characterize the behaviour of the gas in the reservoirs. Until now, Seismolectromagnetic wave conversions occuring in poroelastic media via electrokinetic coupling have not been tested for this purpose. In this work, by means of numerical experiments using Pride's equations-extended to deal with partial saturations-we show that the seismoelectric and seismomagnetic interface responses (IR) generated at boundaries of a layer containing carbon dioxide are sensitive to its CO 2 content. Further, modeling shear wave sources in surface to borehole seismoelectric layouts and employing two different models for the saturation dependence of the electrokinetic coefficient, we observe that the IR are sensitive to CO 2 saturations ranging between 10% and 90%, and that the CO 2 saturation at which the IR maxima are reached depends on the aforementioned models. Moreover, the IR are still sensitive to different CO 2 saturations for a sealed CO 2 reservoir covered by a clay layer. These results, which should be complemented by the analysis of the IR absolute amplitude, could lead, once confirmed on the field, to a new monitoring tool complementing existing ones.

International audienceSeismoelectromagnetic conversions are induced through electrokinetic phenomena occurring when passing seismic waves induce relative fluid to solid displacements. Information on porous material properties provided by the type I coseismic electric field accompanying surface and body waves is limited to the vicinity of the electrical receivers, as opposed to the type II electromagnetic interface response (IR) which can help detect porous contrasts and/or fluid changes within the subsurface 'at depth'. For general field geometries, the problem is that type II disturbances exhibit lower amplitudes than type I fields and are difficult to access directly. Hence, separating both types of waves is a critical step when processing seismoelectric data. Synthetic seismograms and electrograms, generated with a full-waveform seismoelectric forward modelling code written by Garambois & Dietrich, enabled us to study the behaviour of the IR's characteristic dipolar amplitude pattern recovered through filtering. The definitive version is available at www.blackwell-synergy.com To better preserve the IR amplitudes, we have developed a new filtering strategy based on the Fast Discrete Curvelet Transform. Seismic or seismoelectric wave fronts can be optimall described using this multiscale decomposition over multidirectional anisotropic needle-shape structures. We have built a mask in the curvelet domain zeroing out seismoelectric samples corresponding to non-zero samples from the accelerogram, thus taking advantage of the relationship between seismic and seismoelectric waves for type I fields. This mask consists of a threshold function combined with a Gaussian distribution promoting horizontal (i.e. zero- slowness) directions. When applied to synthetic data, this filter enabled to successfully extract the IR although less altering its dipolar radiation pattern than the conventional dip-based techniques. This strategy was successfully applied to a seismoelectric data set acquired in sedimentary deposits, and permitted to isolate an IR generated at the water table

Assessing the statistical significance of electromagnetic anomalies in the ultralow frequency (ULF) range observed prior to earthquakes is a necessary step toward determining whether these perturbations constitute actual earthquake precursors. A statistical epoch analysis (SEA) was recently performed by Han et al. (2014, https://doi.org/10.1002/2014JA019789) to analyze earthquakes happening between 2001 and 2010 near the geomagnetic observatory of Kakioka, Japan; the authors found a significant number of anomalies 6 to 15 days prior to the earthquake day within 100 km from Kakioka, while no significant pre-earthquake activity was observed for the farther region 100 to 216 km from the observatory. In this current paper, we describe the application of our independent software implementation of their method. Despite using a different outlier rejection scheme, we manage to approximate their results. Upon validation of our program, we conduct multiple sensitivity studies. First, we explore how different outlier rejection schemes impact the results. We then restrict the analysis to only mantle earthquakes, highlighting a marginally significant number of anomalies prior to the earthquake day. Next, we test a higher band-pass filter than the one initially used but find no anomalous pre-earthquake activity in this higher-frequency band. We then use a different catalog to establish the list of qualifying "earthquake days" which also leads the anomalous pre-earthquake episode to vanish, thus raising concerns about the robustness of the results. Finally, we apply the SEA to another time window, ranging from 2013 to 2018: No significant pre-earthquake episode can be observed for this interval. We conclude our study by providing guidelines for upcoming work.

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