Evaluation of trenching, ground penetrating radar (GPR) and electrical resistivity tomography (ERT) for sinkhole characterization Domingo Carbonel, 1 Verónica Rodríguez, 1 Francisco Gutiérrez, 1 * James P. McCalpin, 2 Rogelio Linares, 3 Carles Roqué, 4 Mario Zarroca, 3 Jesús Guerrero 1 and Ira Sasowsky 5 1 Departamento de Ciencias de la Tierra, Universidad de Zaragoza, Zaragoza, Spain 2 Geo-Haz Consulting Inc., Crestone, CO, USA 3 Departamento de Geología, Universidad Autónoma de Barcelona, Barcelona, Spain 4 Àrea de Geodinàmica Externa i Geomorfologia, Universitat de Girona, Girona, Spain 5 Department of Geology and Environmental Science, University of Akron, Akron, OH, USA Received 6 March 2013; Revised 6 May 2013; Accepted 16 May 2013 *Correspondence to: Francisco Gutiérrez, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, Edificio Geológicas, C/. Pedro Cerbuna 12, 50009 Zaragoza, Spain. E-mail: fgutier@unizar.es ABSTRACT: This paper explores the suitability and advantages of combining the trenching technique with geophysical surveys [ground penetrating radar (GPR) and electrical resistivity tomography (ERT)] for sinkhole characterization in a mantled karst area. The approach is applied to two active sinkholes concealed by anthropogenic deposits and formed by contrasting subsidence mechanisms; collapse and sagging. The ERT section acquired across the collapse sinkhole images the clayey fill of the depression as an obvious low resistivity area, showing the approximate location of the sinkhole edges. Spatially dense GPR surveys provide information on the position of the boundaries of the concealed subsidence structures and their three-dimensional (3D) internal geom- etry, revealing the dominant subsidence mechanism. We illustrate the impact of several factors on the quality of the GPR data such as sinkhole size, nominal frequency of the antennas, antenna shielding, and the presence of backfilled excavations and above-surface objects. Trenches provided detailed information on the subsurface structure of the sinkhole, subsidence magnitude, partitioning of the strain, and the position of the sinkhole edges, especially when they are deep enough and excavated across the central sector and perpendicular to the boundaries. The stratigraphic and structural relationships observed in the trench were then used to infer the spatial evolution of the sinkholes (e.g. enlargement), their kinematic behavior (episodic versus progressive), and to differentiate discrete subsidence events and their associated magnitude. Numerical dates were used to estimate average subsidence rates and the recurrence of subsidence events. Such integrated data sets may be used as an objective basis to forecast the future behavior of potentially damaging sinkholes and to assess the associated hazard and risk. Copyright © 2013 John Wiley & Sons, Ltd. KEYWORDS: sinkhole hazard; subsidence rate; retrodeformation analysis; GPR; ERT Introduction Sinkholes or dolines are closed depressions with internal drainage characteristic of karst landscapes. Two main genetic groups of sinkholes may be differentiated: (1) solution sinkholes resulting from differential dissolutional lowering of the surface, where karst rocks are exposed at the surface or merely soil mantled; and (2) a range of depressions created by internal erosion and gravitational deformation processes caused by subsurface karstification, which may be collectively termed subsidence sinkholes. This latter group is the most important from the applied perspective, since its development involves the subsidence of the ground surface on timescales of human concern. Traditionally, two main subsidence mechanisms have been proposed in most sinkhole classifications (Williams, 2004; Beck, 2005; Waltham et al., 2005): (1) collapse that involves the brittle deformation of soil or rock through the development of discrete failure planes or brecciation; and (2) suffosion by downward migration of cover deposits through underlying conduits. Recently, sagging was introduced as an additional subsidence mechanism, which is the ductile flexure (passive bending) of sediments caused by the lack of basal support, resulting in the development of structural basins with centripetal dips (Gutiérrez et al., 2008b; Gutiérrez, 2010). The sagging mechanism is particularly frequent in evaporite karst areas, where the bedrock typically has a more ductile rheology and dissolves much faster than in carbonate terrains (Gutiérrez and Cooper, 2013). In practice, several subsidence mecha- nisms may operate simultaneously or consecutively in the development of each sinkhole. Sinkholes are the most important hazard in karst areas (e.g. Waltham et al., 2005; Gutiérrez, 2010; Zhou and Beck, 2011). Ground deformation related to the development of sub- sidence sinkholes may cause severe damage to any human EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms 39, 214–227 (2014) Copyright © 2013 John Wiley & Sons, Ltd. Published online 20 June 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.3440