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Abstract: Complex geological shallow subsurface environments represent an important handicap in urban and building projects. The geological features of the Central Ebro Basin, with sharp lateral changes in Quaternary deposits, alluvial karst phenomena and anthropic activity can preclude the characterization of future urban areas only from isolated geomechanical tests or from non-correctly dimensioned geophysical techniques. This complexity is here analyzed in two different test fields, (i) one of them linked to flat-bottomed valleys with irregular distribution of Quaternary deposits related to sharp lateral facies changes and irregular preconsolidated substratum position and (ii) a second one with similar complexities in the alluvial deposits and karst activity linked to solution of the underlying evaporite substratum. The results show that different geophysical techniques allow for similar geological models to be obtained in the first case (flat-bottomed valleys), whereas only the application of several geophysical techniques can permit to correctly evaluate the geological model complexities in the second case (alluvial karst). In this second case, the geological and superficial information permit to refine the sensitivity of the applied geophysical techniques to different indicators of karst activity. In both cases 3D models are needed to correctly distinguish alluvial lateral sedimentary changes from superimposed karstic activity. [Copyright &y& Elsevier]

Abstract: Alluvial karst represents a complex system where cavities, collapses, subsidence and suffusion processes are the main result of dissolution of soluble rocks in the substratum underlying the detrital cover. Different states of filling of subsiding areas or paleocollapses and the anthropogenic filling of these areas provide a wide range of variability of physical properties, resulting in ambiguities in the interpretation of geophysical surveys. This complex system can be analyzed from a historical approach by means of aerial photographs, old maps and farmers interviews, permitting a regional planning. However, local analysis for building purposes needs a change on the working scale, especially when karst activity is not self-evident from superficial indicators, as it is the case of sudden collapses related to near surface cavities below carbonate crusts. In this case geophysical methods are especially useful. The application of geophysical techniques in urban and building planning must deal with equilibrium between resolution, ambiguity, discrimination capacity and budget. In this work we propose a geophysical routine to be applied in alluvial karst areas to obtain a final model based on the integration of different techniques. For each technique, the geophysical properties involved, and its resolutions and ambiguities were analyzed. The proposed sequence of applied techniques represents a progressive increase of time consumption in surveys, decrease of ambiguities and higher resolution. The first step is related to fast techniques (magnetometry and electromagnetic, EM, multifrequency radiation) that permit to examine the whole survey area to determine anomalous subsoil behaviours. The objective of the second step is to get an indirect view of subsoil structure by means of ground penetrating radar (GPR), electric tomography, or seismic methods. The last and more time-consuming step concerns the development of microgravimetric surveys on anomalies identified by other techniques. [Copyright &y& Elsevier]