Natural electromagnetic radiation (EMR) and its application in structural geology and neotectonics

Abstract

Natural electromagnetic radiation (EMR) impulses are emitted from rocks under stress. Electromagnetic emission may start during crystal deformation prior to and during the nucleation phase of nanocracks. The emission direction is either parallel with or normal to the crack surfaces. The EMR magnetic component is measured by the sensor or aerial of an instrument, the Cerescope, at frequencies from 5 to 50 kHz. Measurements at the surface show directions of recent stresses remarkably well. A calibration of EMR intensity in terms of stress magnitude is possible in tunnels, where the overburden pressure can be calculated. Two examples from the Upper Rhine Graben and NW India show EMR line measurements. In both cases, stress concentrations at fault or bedding surfaces can be detected. These surfaces can be regarded as tectonically active. Two further examples of EMR determinations in tunnels give more detailed information on the regional stress field. The example from the Swiss Jura fold-and-thrust belt shows directional results, with different directions beneath and above the regional detachment horizon at the base of the belt. The example from central Scandinavia shows a late Caledonian shear zone as a boundary between two recent stress domains, and gives absolute values of stress.

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

“…The Cerescope is equipped with a ferrite-core antenna, which has its maximum sensitivity for the magnetic component of the electromagnetic field at a frequency of 12.8 kHz (Greiling and Obermeyer, 2010a). Some previous studies, which outline technical data of the Cerescope hardware in more detail, describe the antenna as most sensitive when its long axis is parallel to the propagation direction of the electromagnetic waves (Lichtenberger, 2005(Lichtenberger, , 2006aReuther and Moser, 2009) or perpendicular to the propagation direction of the waves (Greiling and Obermeyer, 2010a).…”

“…Some previous studies, which outline technical data of the Cerescope hardware in more detail, describe the antenna as most sensitive when its long axis is parallel to the propagation direction of the electromagnetic waves (Lichtenberger, 2005(Lichtenberger, , 2006aReuther and Moser, 2009) or perpendicular to the propagation direction of the waves (Greiling and Obermeyer, 2010a).…”

“…The Cerescope is additionally equipped with a filter that "rejects most intransient noise automatically" (Obermeyer, 2005a;see also Lichtenberger, 2006a,b;Greiling and Obermeyer, 2010a). Frequency bands that appear to be less disturbed can be selected, or seemingly disturbed frequency bands can be excluded, respectively, using low-, high-, bandpass or notch filters.…”

“…However, only the following filter settings are available: (1) a band-pass filter only, (2) two notch filters, (3) a low-pass plus one notch filter, and (4) a highpass plus one notch filter (Obermeyer, 2005b). Furthermore, the signal/noise ratio is improved automatically by the pulseaveraging technique which is part of the Cerescope software (Greiling and Obermeyer, 2010a). Based on the fact that natural crack-related EMR signals should occur as transient signals in fast succession (bursts), the measuring device "is also able to detect bursts automatically" (Obermeyer 2005a).…”

“…Reuther et al, 2002;Lauterbach, 2005;Lichtenberger 2005Lichtenberger , 2006aMallik et al, 2008;Reuther and Moser, 2009;Greiling and Obermeyer, 2010a). Even though some of these studies seem to have produced promising results, up to now the EMR method has not been fully accepted and has only been used by a small number of research groups.…”