D. Boyer

Characteristics of electromagnetic noise in the Ioannina Region (Greece); A possible origin for so called “seismic electric signal” (SES)

The study of electromagnetic noise in the Ioannina region (NW Greece) shows the existence of long transient electric signals (duration>10 s) without measurable correlated magnetic signals, as is the case for Seismic Electric Signal (SES) observed in the VAN network of short-term earthquake prediction. Our signals have an artificial origin and are emitted by different kinds of transmitters implanted in the Ioannina region. This origin can be clearly ascertained from simultaneous observation, at stations in direct sight of the transmitters, of four electromagnetic components (two electric and two magnetic) in a broad band of frequencies (10−3–10³ Hz). Relying on these observations, it is proposed that the April 18 and 19, 1995 electric signals, interpreted by the VAN group as SES precursors to the May 13 Kozani earthquake, are due to some transmitter located far to the North of the IOA station of the VAN network. The transmitters of this area would be of digital type, as is increasingly common in radio-telecommunication networks. We believe that careful studies of electromagnetic noise in a broad range of frequencies should precede lengthy discussions of the statistical significance of recorded signals.

Identification of a magma chamber in the Ghoubbet-Asal rift (Djibouti) from a magnetotelluric experiment

Abstract A very broad band (10 −3 to 10 4 Hz) magnetotelluric investigation of the axial zone of the Ghoubbet-Asal rift (Djibouti) has revealed a shallow (2–4 km) magma chamber which can be mapped in some detail. The suggested roof of the chamber is shallowest very close to the Ardoukoba volcano which was built during the November 1978 rifting episode.

Sources of anomalous transient electric signals (ATESs) in the ULF band in the Lamia region (central Greece): electrochemical mechanisms for their generation

Abstract Anomalous transient electric signals (ATESs) in the ultra low frequency (ULF) band have been often observed during magnetotelluric (MT) investigations [Nature 319 (1986) 310; Phys. Earth Planet. Int. 114 (1999) 141; Geophys. J. Int. 142 (2000) 948], but their origin was unknown until now. They have the same characteristics as the so-called seismic electric signals (SES) claimed to be earthquake precursors by the VAN group (e.g. [Tectonophysics 110 (1984) 73] and later works by this group). Our analysis suggests that the so-called SES could be of anthropic origin. Following the devastating 7 September 1999 Athens earthquake, the VAN group claimed that a SES had been recorded at LAM station (Lamia, central Greece) some days prior to the main shock and that a second SES, which might correspond to an impending even larger earthquake, had been observed after the main shock. In the 2 years after the Athens main shock, no subsequent large earthquakes have occurred near Athens. We conducted a campaign of measurement in the Lamia region in May and June 2001. The results show that ATESs, which look like SES, have several different sources: pump-stations for ground-water, high power electric lines, and factories located to the SE of Lamia city. The ATESs can be generated by two electrochemical mechanisms of metallic electrode polarization: the “galvanic cell” and the “ac electrolytic cell” which are studied by simulated field experiments and discussed in detail in Appendix A . These two mechanisms can occur over a wide range of length scales in the field. Any isolation failure in buried metallic conductors, such as electrical and telecommunication networks, oil, water and gas pipes, railways, high power electric lines, factories and so on, can produce a galvanic cell or an ac electrolytic cell, or both, which could generate, under some circumstances, an “overvoltage”, the ATES. Finally, the absence of a magnetic signal has been observed during ATES and does not constitute a firm criterion for SES [Acta Geophys. Pol. 44 (1996b) 301]. Thus, great care must be taken when claiming the existence of electric precursors of seismic or volcanic events.

Hydrogeological investigation in the Mekong Delta around Ho-Chi-Minh city (South Vietnam) by electric tomography

In the Mekong Delta (South Vietnam), the agglomeration of Ho-Chi-Minh (HCM) City, with more than 5 million inhabitants, is confronted with a dramatic shortage of fresh water supply because of the pollution of several aquifers at different depths. The electric tomography, obtained by concurrent inversion of two complementary geoelectrical methods, the Vertical Electrical Sounding (VES) and the Magneto-Telluric Sounding (MTS), turned out to be very efficient to provide a complete electrical image of the underground from the surface until about 800 m depth. This methodology constitutes a very cheap guide for the evaluation of the quality of the groundwater resources in the vast alluvial plain of the Mekong Delta. To cite this article: V.N. Pham et al., C. R. Geoscience 334 (2002) 733–740.

Nouveaux arguments sur l’origine de l’anomalie magnétique du Bassin parisien (AMBP) d’après les propriétés électriques de la croûte

The PBMA, by its amplitude and extent, is a major structure of the geology of France, but its origin remains poorly known. Based on the data of two important national programs, the ECORS program and the Deep Geology of France (DGF) program, the last interpretations propose that the anomaly is related to an oblique oceanic suture formed during the Variscan collision between the North Brittany block and the Brabant block. The study of the electrical properties of the crust, by the magneto-telluric (MT) method, along the seismic profile ECORS–‘Nord de la France’, reveals the existence of a strong subvertical electrical discontinuity in the Tourly region, located between the Seine fault and the Bray fault. This discontinuity, marking the frontier between two electrically different crusts, probably corresponds to a subvertical and not oblique Variscan suture zone. As the axis of the PBMA is located about 50 km to the west of this electrical discontinuity, it has no genetic relation with the Variscan suture. Its origin is older, probably Precadomian.

Application of telluric-telluric profiling combined with magnetotelluric and self-potential methods to geothermal exploration in the Fujian Province, China

Abstract In the Fujian Province, southeastern China, most of the hot springs emerge in fluviatile valleys and the geothermal resources are mainly medium and low temperature ones by mixing of hot water with cold superficial groundwater. The occurrence of the thermal waters is controlled by deep tectonic fractures in the bedrock where higher-temperature geothermal reservoirs of economic interest are present. The objective of this study is to detect the deeper active hydrothermal zone under a thick sedimentary cover by geoelectrical methods. In the Gui-An site, the combination of telluric-telluric profiling and magnetotelluric methods turns out very efficient to delineate more accurately the width of the deep conductive fracture zone. Moreover, the self-potential method allows us to localize the most active geothermal zone by electrofiltration processes above a convective cell of hot water which flows up from a deep source. The combined results constitute a possible guide for deep geothermal exploration currently encountered in several geothermal regions over the world.