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A critical discussion of the electromagnetic radiation (EMR) method to determine stress orientations within the crust

Solid Earth (2012) 3(2) 401-414
DOI: 10.5194/se-3-401-2012
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Abstract

In recent years, the electromagnetic radiation (EMR) method has been used to detect faults and to determine main horizontal stress directions from variations in intensities and directional properties of electromagnetic emissions, which are assumed to be generated during microcracking. Based on a large data set taken from an area of about 250 000 km2 in Northern Germany, Denmark, and southern Sweden with repeated measurements at one location during a time span of about 1.5 yr, the method was systematically tested. Reproducible observations of temporary changes in the signal patterns, as well as a strongly concentric spatial pattern of the main directions of the magnetic component of the EMR point to very low frequency (VLF) transmitters as the main source and hence raise serious concerns about the applicability of the method to determine recent crustal stresses. We conclude that the EMR method, at its current stage of development, does not allow determination of the main horizontal stress directions. © Author(s) 2012. CC Attribution 3.0 License.

Figures

  • Fig. 1. Principal model of the polarisation of a micro-crack associated with electromagnetic emissions. During the formation and/or growth of a micro-crack, oscillating dipoles are generated that are orientated perpendicular to the micro-crack walls (Rabinovitch et al., 2007). Therefore, the measured magnetic component (B, blue) of the resulting electromagnetic field propagates parallel to the orientation of the micro-crack.
  • Fig. 2. EMR measuring setup. (A) The main orientation of the magnetic component of EMR is expected to be parallel to the associated micro-cracks (Obermeyer, 2005b). (B) The directional EMR properties are determined by measuring its magnetic component with the ferrite-core antenna. (C, D) Sketch of determined EMR intensities that results from measurements of its magnetic component as a function of direction. In this example the maximum values are measured in E–W direction.
  • Fig. 3. Distribution of the main directions of the magnetic component of EMR (BEMR) of this study in comparison with σH as published in the WSM (Heidbach et al., 2008). Red trajectories display the measured BEMR main directions with typical intensities. Blue trajectories indicate remarkably high intensities and green trajectories remarkably low intensities. Grey and black trajectories show σH directions as displayed in the WSM. Orange trajectories show representative results of Lauterbach (2005) and Lichtenberger (2006a).
  • Fig. 4. Fluctuation of main BEMR direction over time. (A) Measurement results and (B) device settings of the Cerescope of horizontal measurements carried out near Göttingen between November 2008 and July 2009. During a two-week interval at the turn of the year, the measured main BEMR direction (area shaded in grey) differed from the commonly observed main BEMR direction (C). During this time, lower intensities of the single pulses and the defining energy were measured.
  • Fig. 5. (A) Maximum signal strength of horizontal measurements (cf. Fig. 3) as a function of distance to DHO38. The strength of the defining energy (red, blue, and green) is a function of the distance to DHO38, while the maximum number of single pulses in each measurement (black) is mostly constant and often coincides with the transmitting frequency of DHO38. (B) Device settings of the Cerescope for the measurements displayed in (A).
  • Fig. 6. Signal-strength variation of DHO38, modified after SID Monitoring Station: http://sidstation.loudet.org/home-en.xhtml. (A) Daily broadcasting pattern as received near Toulouse on 15 July 2009. Between sunrise and sunset the received signal strength is nearly constant. During night, sunrise, and sunset the received signal strength varies over a wide range. Between 07:00 and 08:00 UTC a regular daily intermission in the broadcasting of DHO38 is abided. This daily intermission occurs throughout the year. (B) Annual broadcasting pattern. A seasonal influence is related to the solar altitude. An intermission in the broadcasting during several days is scheduled during the turn of every year.
  • Fig. 7. Results of Cerescope measurements on 15 July 2009 near Göttingen (Germany) showing the effects of the daily intermission in the regular broadcasting of DHO38. (A) Horizontal measurement results before the intermission. (B) Time-triggered measurement during the turn off of DHO38 with the antenna oriented horizontally pointing towards 45◦. (C) Results of a horizontal measurement during the intermission. (D) Time-triggered measurement during the turn on of DHO38 with the antenna oriented horizontally pointing towards 45◦.
  • Fig. 9. Comparison of three horizontal measurements using different frequency ranges but otherwise the same device settings: amplification 107 dB and discrimination level of 24. (A) Intensities of single pulses and (B) defining energy vs. the azimuth (long axis of antenna with regard to north = 0◦). The small anomaly coincides with direction measured during the broadcasting intermission of DHO38 on 15 July 2009 and is probably related to RDL Krasnodar in Russia.

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CITATION STYLE

APA

Krumbholz, M., Bock, M., Burchardt, S., Kelka, U., & Vollbrecht, A. (2012). A critical discussion of the electromagnetic radiation (EMR) method to determine stress orientations within the crust. Solid Earth, 3(2), 401–414. https://doi.org/10.5194/se-3-401-2012

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