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.

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Two conjugate sets of active faults oriented NNE-SSW and NNW-SSE have been detected at Landau area in SW Germany. These faults follow the old trends of the rift-related structures predominating in the Upper Rhine Graben (URG), which originated during Late Eocene-Miocene time. Linear and horizontal measurements were performed by using the Cerescope device and interpreted, applying the Electromagnetic Radiation (EMR) Technique. Linear EMRprofiles were helpful for mapping active faults, while the main horizontal stress (σ H , N to NNE) was easily identified with EMR-horizontal measurements. Reactivation of rift-related structures of the Upper Rhine Graben at Landau area produces a new system of active shallow fractures following old trends, and has been detected through the present study by Cerescope applying the EMR-Technique. The present results imply that the Enhanced Geothermal System (EGS) to the south of Landau has a great impact on reactivation of the pre-existing rift-related faults by mechanical hydro-fracturing occurring within the reservoir rocks underneath the area.

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