Francisco Martínez receives "Best Geophysical PhD Thesis" award, attributed by the J. García-Siñeriz Foundation

Francisco Martínez, IDL's researcher, received the award that distinguishes the best Geophysical Pure or Applied Doctoral Thesis made in Spanish or Ibero-American Universities. The topic of Francisco's PhD thesis is "Detection and characterization of karstic caves: integration of geological and geophysical techniques".More information about the J. García-Siñeriz Foundation and the award here.

Extended Abstract of the PhD ThesisThe study of karstic cavities has undergone substantial growth due to its high socio-economic and scientific value. In addition to attracting tourists and having economic potential, caves imply large reservoirs of water and may even serve for housing, wine cellars or food preservation. The study of caves that are accessible from the outside can be conducted in a direct way, using classical geodetic and geological techniques. However, there are many undiscovered cavities lacking direct access, and their study must be conducted indirectly.Geophysical methods are presented as a suitable tool for new cavity detection. Still, each cave has different boundary conditions to be taken into account. Geological features such as lithology, bedding, foliation, dip, faults and joints, as well as the hydrogeological features, determine cave morphologies and therefore condition the interpretation of results when applying geophysical prospecting techniques.In this Ph.D. Thesis, a combination of geophysical techniques was applied over cavities having different morphologies and geological settings. Included are caves hosted in marbles (Gruta de las Maravillas cave, Aracena, Southwest Spain), limestone (Algaidilla Cave, Estepa, Southern Spain), and gypsum (El Bosque cave, Sorbas, Southeast Spain). Diverse physical properties between the cavity and surrounding rock (density, resistivity, seismic wave velocity or dielectric contrast) are analysed to detect the presence of cavities. The response of the applied techniques varies due to the different boundary conditions: caves partially submerged in water (Gruta de las Maravillas and Algaidilla), metallic mineralized walls (Gruta de las Maravillas), or completely dry caves (El Bosque). Another essential aspect is the depth and magnitude of the void, from small and shallow caves (Gruta de las Maravillas and El Bosque), to deep and large ones (Gruta de las Maravillas and Algaidilla).By integrating various geophysical methods, more reliable results are ensured. Microgravity, magnetic, electrical methods (resistivity and induced polarization), seismic methods (refraction and reflection) and electromagnetic (Ground Penetrating Radar) were combined in this research. All the geophysical procedures were supported by high-precision geodetic techniques for the positioning and adjustment of field measurements.This combination of geophysical methods was furthermore complemented by a detailed geological survey of the areas surrounding the cavity. Such data aids the interpretation of findings, so that they better fit reality.The obtained results show that regional microgravity studies, performed at regular grid measurement spacing according to the expected size and depths of the studied cavity, may be used as a first approximation to locate and estimate sectors with a high probability of containing cavities. A comparison of the different methods for separating the Bouguer anomaly into regional and residual anomalies indicated that low-order polynomial fitting was most adequate for this purpose. Because gravity methods offer a non-unique solution for morphology, size and depth estimations, they must be complemented with other geophysical methods. Electrical resistivity tomography and velocity profiles in seismic refraction provide high resolution with accurate cave morphologies, which can be tested using direct gravity 2.5D models. Unlike electric, magnetic and GPR methods, microgravity and ray tracing coverage in seismic refraction methods are not restricted to the boundary conditions exclusively for cave detection. Time-lapse electrical tomography measurements served to reveal that variable soil humidity conditions increase the resistivity contrast between the unsaturated cavity and host rock.By combining geophysical methods applied in different settings, the morphologies of unexplored cavities could be identified (Algaidilla cave and continuity of the Gruta de las Maravillas). The main features of the anomalies related to the cavities, according to each technique applied, may help detect unknown cavities under similar conditions. In sum, microgravity and ray tracing coverage in seismic refraction are the most reliable methods overall for cave detection.