Visualization of tectonic structures in shallow-depth high- resolution ground-penetrating radar (GPR) profiles Martin Meschede 1 *, Ulrich Asprion 1 and Klaus Reicherter 2 1 Institute of Geology, University of Tuebingen, Sigwartstr. 10, D-72076 Tuebingen, Germany, 2 Geological Institute, University of Hamburg, Bundesstr. 55, D-20146 Hamburg, Germany GPR GPR profiling has been used success- fully in many places for hydrogeologi- cal and sedimentological studies to in- vestigate electrically low conductive areas (e.g. Davis and Annan, 1989; Huggen- berger, 1993; Basson et al., 1994; Beres et al., 1995; Meyers et al., 1996). It is a relatively inexpensive and highly por- table non destructive geophysical tool for obtaining subsurface information. We present an example for the applic- ability of GPR on tectonic structures. Reflections of radar waves are caused by changes in physical proper- ties. These can be related to grain-size, density and porosity variations, and changes in mineralogy. Clay minerals and water have mostly a negative effect on signal quality (e.g. attenuation, dis- tortion) and penetration depth. Good reflectors, however, are provided by discrete horizons with a detectable con- trast within the physical parameters. This study was performed in a quarry (Fig. 1) where ideal conditions for a correlation of reflection lines and ex- posed tectonic structures are realized. The surfaces of the terraces are soil-free and horizontal thus avoiding effects of attenuation or distortion of the signal by high contents of water or clay miner- als of the soil. The measurements were taken after a dry climatic period to avoid water effects. We used a GPR system with 300 Mhz centre frequency antennae (Geophysi- cal Survey Systems, GSSI SIR-10 A). Spacing between the transmitter and reciever antennae was maintained at 1.8 m and recordings along the profile lines were made approximately every 9± 10 cm. The relative dielectric permitivity (e r ) for limestone ranges between 7 and #1997 Blackwell Science Ltd 167 ABSTRACT Ground-penetrating radar (GPR) is applied to detect subsurface tectonic structures and to map the geometry of faulted blocks. Tectonic interpretations from a profile crossing the graben fault and a grid in a second-order graben structure providing a 3D data set are correlated to the structural inventory of the outcrop. Folded layers of the roll-over anticline are identified by continuous curved reflectors and an increasing dip towards the main graben fault. Faults are indicated by arrays of reflector terminations. Variations in the water and clay content caused by karstification and brecciation on fault planes are displayed by changing amplitudes of the detected signal. The 3D visualization of the second-order graben structure with a grid of GPR profiles illustrates the local stress pattern which coincides with structural observations in the outcrop and photo lineations. Terra Nova, 9, 167±170, 1997 Ahed Bhed Ched Dhed Ref marker Fig marker Table marker Ref end Ref start *Correspondence: E-mail: mesch@uni- tuebingen.de Fig. 1 (a) Lithostratigraphic section of the Kaltenbach quarry (after Eissele et al., 1966). The position of the GPR profiles is indicated in the right column. 1, lumachelle beds; 2, limestone; 3, marlstone; 4, nodular limestone (`Wellenkalk'); 5, dolomitic limestone. (b) Simplified geological map of the Freudenstadt Graben (FG), p, pre-Mesozoic basement; s, Buntsandstein; m, Muschelkalk (modified after Metz, 1971). (c) Map of the Kaltenbach quarry and location of the GPR profiles (indicated by numbers 1±19). The composite radar line is indicated by heavy lines. mo 1 , Trochitenkalk; so, Upper Buntsandstein. Note that the main fault is not exposed in the quarry. Paper 132 Disc