TSK 11 Göttingen 2006 Greiling et al. Electromagnetic radiation (EMR) and its interpretation in terms of stresses in the lithosphere Vortrag Reinhard O. Greiling1 Marco Lichten- berger1 Hennes Obermeyer2 Electromagnetic radiation (EMR) as measured at the surface of the litho- sphere or underground shows preferred orientations, which can be related to mi- crocracks and other brittle structures at micro and nano scales (see Bahat et al. 2005 and references therein). During the last years, numerous studies showed the applicability of EMR measurements for the determination of active fractures and stress orientations. EMR is deter- mined with a ‘Cerescope’, which picks up EMR signals at frequencies from 5– 50kHz (Obermeyer, 2005) with a ferrite aerial and processes them electronically so that the results can be displayed on a screen or copied to a computer. With the help of oriented EMR mea- surements, intensity variations are de- termined, which can be related to pre- ferred crack fracture orientations. From this information, orientations of the principal stresses can be calculated. In addition, the intensity of the EMR is related to stress magnitudes. Sev- eral EMR measuring methods with the Cerescope can be applied to study re- gional and local stress fields. For hori- zontal measurements the aerial is moved in a horizontal circle. Every 5 degrees a measurement is taken. The entire hori- zontal measurement consists of 72 read- ings, which are illustrated in a polar di- 1 Geologisch-Paläologisches Institut, Hei- delberg University, Im Neuenheimer Feld 234, 69120 Heidelberg, FR Germany 2 Gesellschaft für Erkundung und Or- tung, Yorckstraße 36, 76185 Karlsruhe, FR Germany agram. Calculating the directions of the cracks from EMR intensities, the prin- cipal directions of the horizontal princi- pal stresses can be determined. For Lin- ear measurements the aerial is moved along a straight or curved line in this method. Every metre (can also be any other distance) a measurement is taken. Using this method it is possi- ble to find loci of high stresses, such as active faults, where stress accumu- lates. It is also appropriate to find loci of possible rock burst in underground fa- cilities. If several linear measurements are arranged in grids it is possible to map the orientation of structures such as faults, where stress is accumulated. Cross section measurements can only be applied in long underground exca- vations such as tunnels. The measure- ments are undertaken along the cross section of the tunnel and are oriented normal to the tunnel’s long axis. Be- ginning with a vertical orientation of the aerial every 5 degrees a measure- ment is taken until a full circle of 360 degrees consisting of 72 measurements is completed. Fractures around tun- nels originate from a secondary stress field which is induced by the regional stress field and the empty space of the tunnel. This secondary stress field is described by radial, tangential and shear stresses. Shear stresses are most likely produce fractures, since shear strength is much smaller than compres- sive strength. Therefore, the EMR mea- sured in cross section measurements is proportional to shear stress of the sec- ondary stress field. By determining the shear stress distribution along the long axis of the tunnel it is possible to calculate directions and magnitudes of the regional stresses, which induce the secondary stress field. The tun- 1 Greiling et al. TSK 11 Göttingen 2006 nels in which this technique can be ap- plied have to be curved and need to have a maximum overburden of at least 75m and a minimum overburden of less than 20m. These parameters are neces- sary to produce shear stress distribution along the tunnels’ long axis from which regional stresses can be determined re- liably and with small standard devia- tions. In order to build up experience and a comprehensive database, EMR is deter- mined in different regions and different tectonic environments. Two examples will be presented, one from the shoulder of the Upper Rhine rift (Odenwald) and one from northern Sweden and adjacent Norway. In the example from the Odenwald (tunnel at Wald-Michelbach), EMR re- sults are generally compatible with pub- lished data on the regional stress field (azimuth of major horizontal principal stress 103°). In addition, a minor N-S tensional component and the influence of local faults can be discerned. In the investigated area of Scandinavia the regional stress field as determined from EMR is uniform, the major hori- zontal principal stress has a ENE–WSW orientation. Published results from dif- ferent stress determination methods ap- plied on the eastern coast of Sweden show a major horizontal principal stress with NE–SW orientation. At the Steinfjellet road tunnel residual stresses with a maximum of 3 times the regional stress magnitude are found at a thrust contact, which represents the Caledonian suture between Baltica and Laurentia. There, the major horizontal principal stress has a NW–SE orienta- tion. References Bahat D, Rabinovitch A & Frid V (2005) Tensile Fracturing in Rocks — Tectonofrac- tographic and Electromagnetic Radiation Methods. Springer Verlag, Berlin Heidel- berg, pp 569 Obermeyer H (2005) Measurement of Natural Pulsed Electromagnetic Radiation (EMR) with the Cerescope, Ceres GmbH, Staffort, pp 8 2