ORIGINAL ARTICLE Collapse hazard assessment in evaporitic materials from ground penetrating radar: a case study Toma ´s Martı´n-Crespo Æ David Go ´ mez-Ortiz Received: 26 June 2006 / Accepted: 12 December 2006 / Published online: 10 January 2007 Ó Springer-Verlag 2007 Abstract Evaporitic materials have been studied by means of ground penetrating radar (GPR) in order to evaluate the collapse hazard. The obtained 200 MHz GPR profiles show a low signal-noise ratio over the first 3 m depth, where well-defined and continuous reflectors can be observed. Between 3 and 4.5 m depth, the signal to noise ratio decreases due to attenuation of the electromagnetic (EM) waves. As a result, reflectors located deeper than 3 m become more discontinuous and poorly defined. GPR profiles show trails of con- tinuous and subhorizontal reflectors, with a slightly undulated and irregular geometry. Although some of these reflectors laterally vanish or seem to disappear, sudden interruptions or hyperbolic reflections that could be originated by the occurrence of cavities have not been detected. These reflectors have been inter- preted as corresponding to several evaporitic layers (gypsum) that constitute the main lithology in the area. Clear interruptions of reflectors have only been ob- served in some GPR profiles, and they could be attributed to small (1–2 m long) subvertical faults, with only a few centimetres offset. These faults may be generated by the accommodation of the evaporitic layers to local collapses affecting deeper materials. Keywords Risk assessment Á Geohazards Á Ground penetrating radar Á Spain Introduction Dissolution processes are common in soluble sedimen- tary materials such as limestone and gypsum, where the presence and circulation of liquid water can generate caves and cavities by dissolution. If this process is intensive enough, collapses are generated by the sinking of large cavities in the ground. If dissolution occurs on a small scale, only little cavities are generated, and the upper materials deform and adapt to the new mor- phology, filling up the generated cavity. Deformed and folded materials, on occasion affected by small faults and fractures, are usually defining these structures just above the dissolved zone. Such features are not always indicative of a cavity beneath, although dissolution processes producing adaptation of the surface materials can be expected in depth. Dissolution cavities are often a major hazard occurring in soluble sedimentary mate- rials. Location and size determination of these cavities and associated collapse structures are essential in order to avoid engineering hazards like road subsidence or building collapse (e.g. Batayneh et al. 2002; Zhou et al. 2002; Gutie ´rrez-Santolalla et al. 2005; Soupios et al., in press). Benito et al. (1995) studied the environmental and engineering problems associated with natural and human-induced sinkholes in gypsum-rich terrains. In that study, gravimetric and ground penetrating radar methods were used in order to determine both the location and depth of anomalies and cavities along the Zaragoza–Barcelona railway, NE Spain, in evaporitic facies similar to those studied in the present work. Ground penetrating radar (GPR) is a geophysical technique that is used to characterize the physical properties, thickness, spatial distribution, internal structures and discontinuities of subsurface materials. A T. Martı ´n-Crespo (&) Á D. Go ´ mez-Ortiz A ´ rea de Geologı´a, Dpto. de CC. de la Naturaleza y Fı´sica Aplicada, ESCET, Universidad Rey Juan Carlos, C/Tulipa ´n s/n, 28933 Mo ´ stoles (Madrid), Spain e-mail: tomas.martin@urjc.es 123 Environ Geol (2007) 53:57–66 DOI 10.1007/s00254-006-0618-1