## 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; 2; 3; 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 because the equipment is more expensive and the interpretation methods are more complicated. The use has focused on detecting the interface between fresh groundwater and saline seawater intrusion into coastal aquifers [6]. These two techniques have been used recently in an unconfined sandstone aquifer in semiarid southwestern Niger [7]. This study is a good example of the efficiency of using complementary geophysical techniques.  
Herein, we analyze the effectiveness of electrical (electrical resistivity tomography, ERT), electromagnetic (time-domain electromagnetic, TDEM), and reflection seismic techniques to characterize dolomite aquifers. Our study area (Figure 1) not only has detailed surface geological information, but also numerous pumping boreholes and seismic lines, which were employed to verify and correlate the new geophysical data collected in this research.  
The economy of Loma de Ubeda area in Southern Spain is based almost exclusively on the cultivation of olive groves. Groundwater exploitation has increased olive production and ensured the survival of crops during the severe droughts that affect this region.  
Until the late 1980s, only a Miocene shallow aquifer was exploited, with pumping flows on the order of 1 L/s. In the early 1990s, the Loma de Ubeda Jurassic dolomite aquifer began to be exploited at depths between 100 m and 600 m, with pumping rates exceeding 40 L/s in some cases [8]. Numerous deep boreholes were drilled, many of them without the mandatory permits from the Water Authorities. Estimated irrigation in 2005 exceeded 35 hm\({}^{3}\)/year over an irrigation area of 24,050 ha of olive groves [9]. The aquifer has an average annual deficit close to 13.3 hm\({}^{3}\) and it is clearly overexploited [10]. We applied different geophysical techniques to make comparisons with existing boreholes and surface geologic data. This allowed us to verify the effectiveness of each technique to investigate this aquifer. We emphasized TDEM and we used different configurations to find the optimal parameters. The different techniques used here can be applied to investigate other carbonate aquifers showing similar behavioral.  
Figure 1: Regional hydrogeological map. The location of the study area is shown. Legend: (1) Paleozoic basement (Phyllites). (2) Paleozoic basement (Granite). (3) Triassic clays from Chiclana de Segura Formation (Impervious base). (4) Jurassic tabular dolomites (Aquiref). (5) Prebetic Jurassic dolomites (Aquiref). (6) Late Miocene marks with detrital levels. (7) Pliocene–Quaternary clays, marks, and conglomerates. (8) Undifferentiated stratigraphic contact. (9) Undifferentiated tectonic contact. (10) Groundwater flow paths.