applied sciences Article Geophysical Characterization of Aquifers in Southeast Spain Using ERT, TDEM, and Vertical Seismic Reflection Javier Rey 1,* , Julián Martínez 2 , Rosendo Mendoza 2 , Senén Sandoval 3, Vladimir Tarasov 4, Alex Kaminsky 5, M. Carmen Hidalgo 1 and Kevin Morales 1 1 Departamento de Geología, EPS de Linares y CEACTEMA, Universidad de Jaén, Campus Científico Tecnológico, 23700 Linares, Jaén, Spain; [email protected] (M.C.H.); [email protected] (K.M.) 2 Departamento de Ingeniería Mecánica y Minera, EPS de Linares y CEACTEMA, Universidad de Jaén, Campus Científico Tecnológico, 23700 Linares, Jaén, Spain; [email protected] (J.M.); [email protected] (R.M.) 3 Everest Geophysics SL, 28270 Colmenarejo, Madrid, Spain; [email protected] 4 Geophysicist in ElGeo Ltd., Petrovskaya Kosa, 1, 190000 St. Petersburg, Russia; [email protected] 5 Zond Software Ltd., 8010 Paphos, Cyprus; [email protected] * Correspondence: [email protected] Received: 29 July 2020; Accepted: 17 October 2020; Published: 21 October 2020 Abstract: We assess the effectiveness of complementary geophysical techniques to characterize a Jurassic dolomite confined aquifer at Loma de Ubeda, Spain. This aquifer, which is penetrated by wells in the 100–600-m depth range, is confined by Triassic clays (bottom) and Miocene marls (top). The Jurassic dolomite is characterized by prominent seismic reflectors of high amplitude. Thus, it is readily differentiated from the low-amplitude reflectors of the confining clay-rich Triassic and Miocene materials. Electrical resistivity tomography (ERT) allowed us to detail the characteristics of the aquifer up to a maximum depth of 220 m. Lateral changes in facies and small faults have been identified using ERT. Time-domain electromagnetic (TDEM) is an excellent complement to the two above-mentioned techniques in order to widen the analyzed depth range. We acquire TDEM data with different configurations at multiple study sites while simultaneously varying measurement parameters. In doing so and by comparing the effectiveness of these different configurations, we expand the use of TDEM for aquifer characterization. Keywords: Jurassic dolomite aquifer; seismic reflection; electrical resistivity tomography; time-domain electromagnetic; loma de Úbeda; Spain 1. Introduction Geophysical prospecting techniques for groundwater exploration have evolved considerably in recent years. The automatic data acquisition systems, the availability of powerful computers, and the development of two- and three-dimensional modeling software have greatly improved the resolution of complex geological models [1–4]. A great diversity of geophysical methods (gravimetric, magnetic, electrical, electromagnetic, and seismic) can contribute to the groundwater exploration. Electrical, electromagnetic, and seismic methods are the most efficient techniques in this kind of setting and they are more cost-effective. Electrical methods are particularly suitable for groundwater research because hydrogeological parameters, such as porosity and permeability, can be correlated to electrical resistivity values. In addition, they are cheap and easy to implement techniques [5]. Electromagnetic sounding techniques, on the other hand, have not been used as extensively as electrical methods for groundwater research Appl. Sci. 2020, 10, 7365; doi:10.3390/app10207365 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7365 2 of 16 Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 15 because the the equipment equipment is is more more expensive expensive and and the the interpretation interpretation methods methods are more are more complicated. complicated. The Theuse has use focused has focused on detecting on detecting the interface the interface between between fresh groundwater fresh groundwater and saline and seawater saline intrusion seawater intointrusion coastal into aquifers coastal [6]. aquifers These [two6]. Thesetechniques two techniques have been haveused beenrecently used in recentlyan unconfined in an unconfined sandstone aquifersandstone in aquifersemiarid in southwestern semiarid southwestern Niger [7]. Niger This [study7]. This is studya good is example a good example of the efficiency of the effi ciencyof using of complementaryusing complementary geophysical geophysical techniques techniques.. Herein, we analyze the effectiveness effectiveness of electrical (electrical resistivity resistivity tomography, tomography, ERT), ERT), electromagnetic (time-domain(time-domain electromagnetic, electromagnetic, TDEM), TDEM) and reflection, and reflection seismic techniquesseismic techniqu to characterizees to characterizedolomite aquifers. dolomite Our aquifers study. area Our (Figure study 1area) not (Figure only has 1) detailednot only surfacehas detailed geological surface information, geological information,but also numerous but also pumping numerous boreholes pumping and seismic boreholes lines, and which seismic were lines, employed which to verifywere employed and correlate to verifythe new and geophysical correlate the data new collected geophysical in this data research. collected in this research. Figure 1. RegionalRegional hydrogeological hydrogeological map. map. The The location location of ofthe the study study area area is isshown. shown. Legend: Legend: (1) (1)Paleozoic Paleozoic basement basement (Phyllites). (Phyllites). (2) (2) Paleozoic Paleozoic basement basement (Granite). (Granite). (3) (3) Triassic Triassic clays clays from from Chiclana Chiclana d dee Segura Formation (Impervious base). (4) Jura Jurassicssic tabular dolomites (Aquifer). (5) Prebetic Jurassic dolomites (Aquifer). (6) Late Late Miocene Miocene marls marls with with detrital detrital levels. levels. (7) (7) Pliocene Pliocene–Quaternary–Quaternary clays, clays, marls, marls, and conglomerates. (8) (8) Undifferentiated Undifferentiated stratigraphic contact. contact. (9) (9) Undifferentiated Undifferentiated tectonic tectonic con contact.tact. (10) Groundwater flowflow paths.paths. The economy of Loma de ÚbedaÚbeda area in Southern Spain is based almost exclusively on the cultivation ofof oliveolive groves.groves. Groundwater Groundwater exploitation exploitation has has increased increased olive olive production production and and ensured ensured the thesurvival survival of crops of crops during during the severethe seve droughtsre drought thats that affect affect this this region. region. Until the the late late 1980s, 1980s, only only a aMiocene Miocene shallow shallow aquifer aquifer was was exploited, exploited, with with pumping pumping flows flows on the on orderthe order of 1 L/s. of 1 In L/ thes. Inearly the 1990s, early 1990s,the Loma the de Loma Úbeda de JurassicÚbeda Jurassicdolomite dolomite aquifer began aquifer to be began exploited to be atexploited depths atbetween depths between100 m and 100 m600 and m, 600 with m, withpumping pumping rates rates exceeding exceeding 40 40L/s L /ins insome some cases cases [ [88].]. Numerous deepdeep boreholesboreholes were were drilled, drilled, many many of themof them without without the mandatorythe mandatory permits permits from thefrom Water the Authorities.Water Authorities Estimated. Estimated irrigation irrigation in 2005 in exceeded2005 exceeded 35 hm 353 /hmyear3/year over over an irrigation an irrigation area area of 24,050of 24,050 ha haof oliveof olive groves groves [9]. [9]. The The aquifer aquifer has has an an average average annual annual deficit deficit close close toto 13.313.3 hmhm33 andand it is clearly overexploited [ 10]. W Wee applied different different geophysical techniques to make comparisons with existing boreholes and surface surface geol geologicogic data data.. This This allowed allowed us us to to verify verify the the effectiveness effectiveness of ofeach each technique technique to investigateto investigate this this aquifer aquifer.. We We emphasized emphasized TDEM TDEM and and we we used used different different configurations configurations to to find find the optimal parameters. The The different different techniques used here can be applied to investigat investigatee o otherther carbonate aquifers showingshowing similar behavioral.behavioral. Appl. Sci. 2020, 10, 7365 3 of 16 2. DescriptionAppl. Sci. 2020 of, 10,the x FOR Study PEER REVIEW Region 3 of 15 2.1. Geological2. Description Setting of the Study Region Two2.1. Geological geological Setting units can be differentiated in the study area: the Paleozoic basement and the post-Hercynian sedimentary cover. The first unit is dominated by intensely folded phyllites intruded Two geological units can be differentiated in the study area: the Paleozoic basement and the post- by a granitic batholith (Figure1: 1,2). Subhorizontal to gently dipping, the post-Hercynian sedimentary Hercynian sedimentary cover. The first unit is dominated by intensely folded phyllites intruded by covera unconformablygranitic batholith ( overliesFigure 1: 1 the,2). Subhorizontal basement. It to is gently composed dipping of, the Triassic, post-Hercynian Jurassic, sedimentary and Neogene formationscover un (Figureconformably1). Sparse overlie Quaternarys the basement alluvial. It sedimentsis composed are of also Triassic, present. Jurassic, and Neogene Theformat Triassicions (Figure strata 1). belongs Sparse to Quaternary the Chiclana alluvial de Segura sediments Formation are also [ 11present,12],. which extensively outcrops at the northThe of Triassic the Guadalimar strata belongs River to (Figure the Chiclana1: 3). This de Segura formation Form variesation in[11,12 thickness], which (50–400