1. Introduction
Geophysical prospecting is a widely used methodology in geological mining research. There are numerous publications in which different geophysical techniques (e.g., gravimetric, magnetic, electrical, electromagnetic or seismic) are applied in the exploration of mineral deposits. The choice of one or the other method will depend on different factors, including the depth of research to be achieved, the required resolution or the type of lithologies existing in the subsoil. In this sense, in order to obtain satisfactory results with a given method, there must be a good contrast of the measured property between the host rock and the mineralization investigated [1,2,3].
In addition, it should be noted that the interpretation of the data obtained by these geophysical techniques is based on the resolution of the inverse method [4,5,6], with which it is intended to obtain a geological model that is capable of responding to the values measured in the field. To this end, the inverse iteration software currently used [4,5,6], through a series of iterative calculations, adjusts the starting model until the difference between the data calculated by the model and the data measured in the field is minimal, depending on the error criterion assigned to the iterative algorithm. Despite this, the process is always associated with a certain degree of uncertainty. In many cases, several methods are used together [7,8,9,10,11] to reduce the degree of uncertainty in the interpretation of the results obtained by using only one of these indirect techniques.
The aim of this paper is to analyze the effectiveness of the combined use of different geophysical techniques for the study of geological structures with mining implications. Thus, a frequent problem in the exploitation of a mining deposit is the disruption due to the action of subsequent fractures, which displace the mineralization or even make it disappear. Therefore, identifying the associated fracture systems, as well as their movements, will be essential when planning the exploitation [12,13,14]. In this work, a detailed geological study was first carried out on the outcrops, analyzing direct field data (geological cartography and petrological study). This information will be of special interest for the subsequent interpretation of the geophysical data obtained.
Two very versatile and widely used methods were chosen as indirect techniques for the characterization of fractured geological models: electrical resistivity tomography (ERT) and the electromagnetic method (time domain electromagnetic, TDEM, method) [7,9,10,12,13]. In addition, magnetic techniques were also used in a blended manner, as they have recently demonstrated a great capacity to detect and map hidden structures on the surface [11,15].
For this purpose, the old mining district of Linares-La Carolina (southeast Spain, Figure 1) was selected. This mining district is characterized by the presence of Philonian deposits, including galena (PbS), which are hosted by Paleozoic granitoids [15,16,17]. The sector was previously the subject of intense mining and was abandoned in the 1980s. The closure was conditioned by the decrease in the price of this mineral and by the depletion of the known reserves. Despite this, in the final stages prior to closure, intense direct research campaigns (mechanical soundings) and indirect research campaigns (electrical prospecting in the SEV modality and resistivity profiling) were carried out to look for new reserves. This effort was in vain, as the results were negative [18].
Therefore, reinstating this mining district would require the discovery of new mineral sites. Currently, the rise in the price of the mineral makes the exploitation of these deposits economically profitable. This encourages the search for new reserves, and therefore, new research campaigns in this region.
A normal fault in the N40° E direction, known as the Linares fault (Figure 1), marks the southern limit of this district. In the hanging wall, the Paleozoic basement and associated seams are fossilized by a thick sedimentary cover of Triassic and Miocene materials. The veins present in the uplifted block disappeared when the fracture was reached, so they were not exploited in the hanging wall block. It is assumed that the trace of the deposit, although displaced, must continue in the sunken block under a powerful sedimentary cover. This makes the southern boundary of the district of special interest as a research area.
First of all, the possible mining investigations in this sector would require advanced knowledge of the functioning of this complex regional fracture. In this geological context, different combined geophysical techniques were used to analyze this fracture system geometry in detail in a sector near Linares (Figure 2). In addition, this multidisciplinary study allowed us to analyze the advantages and disadvantages of the use of each of these techniques.
2. Local Geological Survey
In the studied region, two geological units can be distinguished: the Paleozoic basement and the sedimentary cover (Figure 1 and Figure 2, respectively). The first unit is basically composed of intensely folded phyllites intruded by a granite stock in the last stages of the Hercynian Orogeny [15,16,17]. The seams affect the basement and are fossilized by a subhorizontal sedimentary cover. This cover includes Triassic, Miocene, and Quaternary materials (Figure 1). At the bottom, the Triassic strata are composed of conglomerates or breccias on which there are alternating levels of sandstones and lutites of approximately 10 m in thickness (T1 in Figure 2 and Figure 3). The sandstones are very immature (generally arkose), and the rudites contain pebbles and blocks of granite, from which it can be deduced that the granite stock outcropped during the Triassic constituted the bedrock for these detrital rocks. In the upper part of the Triassic, red lutites predominate, with thicknesses that vary in the sector studied between 50 and 100 m (T2 in Figure 2). Materials of Miocene age are deposited on this set and separated by a stratigraphic discontinuity. At the bottom, the calcarenitic breccia changes laterally to marl and vertically to limestone marl with fine calcarenitic intercalations. The Quaternary is characterized by the presence of silt, sand and gravel and is associated with the filling of river channels [15].
In the mining district, which constitutes the uplifted block of a horst structure, the sedimentary cover is practically dismantled by erosion, and only the base of the Triassic or the granite stock directly emerges [15]. The mineral deposits have hydrothermal origins: the fluid phase and the metals were injected through old fractures in the N35°–N60° E and north–south (NS)–N20° E directions in the phyllites and granite body, respectively. The ores of these hydrothermal veins are mainly composed of galena, sphalerite, chalcopyrite, and pyrite. Moreover, gangue, an association consisting of quartz, ankerite and calcite, was described [15,16,17].
At the southern boundary of the mining district, the presence of a normal fault (Linares fault) is notable, with an approximate direction of N40° E, which is responsible for the subsidence of the Paleozoic basement in the southern sector (Figure 1). This fault affects the Miocene materials; thus, it is younger than them. However, the faults that bound the horst of the mining district of Linares have functioned in different previous stages [10], and there are other associated fracture systems. Figure 3 shows two photographs of the contact between the granite and the subhorizontal Triassic materials in the raised block of the Linares fault (one with a general view and the other with a detailed view). The sedimentary cover fossilizes this granite with a very irregular surface. There are fractures that affect both sets (basement and cover), but others are limited to the granite, from which it is deduced that some of them formed prior to the Triassic deposits. The aim of this work is to characterize these structures through the combined use of the aforementioned geophysical techniques.
The realization of detailed cartography in the studied sector allows us to deduce two fracture systems: the first in the N10° E direction (F0 in Figure 2) and the second, associated with the previously mentioned normal fault, in the N40° E direction (F1 in Figure 2). From the field information, a vertical displacement is deduced for the F0 fault. On both sides of the fault, the base of the Triassic outcrops at elevations of 448 m and 428 m above sea level, which allows us to deduce a throw of approximately 20 m. However, there are no field arguments that allow us to calculate the throw of the F1 fault.
3. Materials and Geophysical Methods
3.1. Electrical Resistivity Tomography
This geophysical prospecting technique consists of determining the distribution of the apparent resistivity in the subsoil from a very large number of measurements performed from the ground surface [1,19]. The variability in the geoelectric behavior of the materials allows us to obtain 2D profiles, making it a highly effective non-destructive tool for investigating and characterizing the subsoil [19,20]. This technique has proven useful, with satisfactory results in stratigraphic studies, especially for detecting fractures in sedimentary basin analysis [14,21,22,23,24], hydrogeological [7,9,24] and environmental [25,26,27,28] studies. This technique has also been used as an efficient way of detecting subsurface voids and cavities [29,30].
The method involves the arrangement of numerous electrodes along rectilinear profiles, with a specific spacing between the electrodes that is determined by the desired resolution and depth. Thus, the lower the separation between the electrodes, the greater the resolution achieved, while the greater the separation, the greater the depth [20]. In this study, a Wenner–Schlumberger electrode configuration is adopted because of its good performance and stability against both vertical and horizontal changes in resistivity [31,32]. This configuration is especially useful for the investigation of horizontal or slightly inclined layers that may present lateral changes in facies and/or verticalized structures, as is the case in this paper.
The electrodes are connected to a measuring device, and through a sequential program, the sets of electrodes that must operate in each measurement and their arrangement are chosen. The voltage and current intensity are measured for each electrode arrangement, from which the apparent resistivity of the ground is calculated. This resistivity is assigned to a specific geometric point in the subsoil.
The electrical tomography equipment used is the RESECS model from Deutsche Montan Technologie (DMT) [33]. The interpretations of the electrical tomography profiles were performed from the apparent resistivities obtained in the field work, as treated through the specific software RES2DINV version 4.10 [4]. This calculation program is based on the method of least squares with forced smoothing, modified with the quasi-Newton optimization technique. The inversion method allows us to build a model of the subsoil using rectangular prisms and to determine the resistivity for each of them, minimizing the differences between the observed and calculated apparent resistivity values [4,5]. The field data have subsequently been processed with the Surfer software version 13, which offers the possibility of representing the variation of resistivities in the profiles in a continuous and progressive way.
In this work, three electrical tomography profiles are obtained, each with an electrode spacing of 5 m. The injection time is 256 ms, with a delay of 128 ms. The first profile is in the southeast (SE)–northwest (NW) direction, with a length of 315 m and 64 electrodes (ERT 1 in Figure 2). This profile is designed to comprehensively detect the two sets of faults observed on the surface (F0 and F1). The second profile (ERT 2 in Figure 2), which is 315 m in length and has 64 electrodes (northeast (NE)–southwest (SW) direction), is subparallel to the F1 fault. The contact between the Paleozoic basement and the overlying sedimentary cover in the sunken block is analyzed. A third profile is designed parallel to the ERT 2 profile, with a length of 475 m and 96 electrodes (ERT 3 in Figure 2). With this greater length, greater depths of investigation are expected in order to effectively identify the irregular geometry of the contact between granite and Triassic materials where they are found at greater depths.
3.2. Time Domain Electromagnetic Method
The TDEM operates by circulating electrical current through a transmitting coil for short intervals of time. By abruptly interrupting the flow of current, a variation is generated in the associated primary magnetic field, causing a variable electrical current. This current creates a transient secondary magnetic field. These currents follow closed paths and migrate in depth, decreasing in intensity over time. Variations in the secondary magnetic field over time induce a transient voltage in the receiving coil. The mechanism by which this voltage decays provides information concerning the distribution of the conductivity of the subsurface, which can be used for its characterization [1,34,35].
There are many studies in which this geophysical prospecting technique is used. Thus, although it was originally used in the prospecting of deep mineral deposits [35], its ability to detect changes in conductivity associated with variations in the salinity of aquifers was later confirmed. This led to its widespread use in the investigation of marine intrusion in coastal areas [36,37,38], in the study of aquifers [11] or in the detection of pollutant plumes in environmental studies [39]. It has also been used in the analysis of sedimentary basins, especially to detect the depth of the fractured basement under sedimentary covers [9,12,38,40].
The TDEM equipment used in this work was the AIE-2 model of the ELGEO Research and Production Company. This device has a maximum output power of 200 W and a current intensity of up to 10 A. The TDEM receiver is based on a 16-bit analogue–digital converter with a signal processor that provides an analogue–digital conversion immune to input voltage noise and real-time signal preprocessing [41].
In the field campaign, a total of 17 TDEM measurement stations were established. The points located in Figure 2 refer to the center of the reading stations for square loops with a side of 50 m. At each of these points, measurements were taken with central-loop and single-loop devices, varying the measurement parameters (intensity, measurement time, and voltage), making it possible to compare the effectiveness of the different configurations. A 6 mm2 section cable was used, which reduces its resistance and increases the achievable effective current intensity. In cases in which a central-loop device was used, the receiving coil had dimensions of 0.8 × 0.8 m, using a loop of 63 turns. TEMBIN software version 1.1was used to display and edit the different curves. The modeling and inversion processes were carried out with the ZondTEM1D program version 1.1.
3.3. Magnetic Method
Magnetic prospecting is a passive geophysical technique that is based on the identification of local variations (anomalies) in the Earth’s magnetic field. These anomalies, measured in nanoteslas (nT), originate from the presence of materials with high magnetic susceptibility. Specifically, these anomalies are capable of being magnetically polarized under the influence of the Earth’s magnetic field or an inductive field. Before interpreting these measurements, it is essential to correct for diurnal variations in the magnetic field [42]. This correction is carried out through a fixed auxiliary station, which provides information on the variations in the magnetic field at that point throughout the campaign. The diurnal variation identified is subtracted from the values obtained at the mobile workstation. A detailed description of this methodology can be found in the specialized literature [1,2,3,42].
Magnetic prospecting is a technique widely used in the mining industry, particularly in the detection of ferromagnetic elements [43,44,45]. Magnetic prospecting is applicable in various fields of Earth science, demonstrating its usefulness in the geological mapping of extensively covered or inaccessible areas [46,47,48,49,50]. In addition, this technique is useful for the prospecting of hydrocarbon deposits [51] and land-use planning [52,53].
In this study, field work was carried out with the portable magnetometry equipment model GSMP-35G v 8.0 potassium from Geophysical Electromagnetic (GEM) Systems. This potassium magnetometer offers high-quality data collection due to its high sensitivity (0.0003 nT), minimum reading error, resolution (0.0001 nT) and high precision ±0.05 nT). This device has the capacity to take 20 readings per second, yielding a remarkable sampling density while walking [54].
For the analyzed sector, continuous profiles were produced in walking mode, with a separation between them of 10 m. Given the stability of the equipment used, measurements were taken every 50 ms. Georeferencing was carried out with a NovAtel Global Positioning System (GPS) device mounted on the top of a mobile device. A total of 19 profiles were produced in the NW–SE direction (perpendicular to the Linares fault, Figure 2), with lengths between 100 and 150 m. In addition, a base station was used with a GSMP-35 fixed magnetometer, which allowed the diurnal variations in the field to be calculated. This information was later used to correct the data. Surfer version 13 and Geosoft Oasis Montaj software version 9 were used to process the magnetic data.
4. Results and Discussion
4.1. Electrical Resistivity Tomography
From the electrical tomography study, three units can be differentiated in the subsoil (Figure 4). In general, at great depths, a resistive unit appears with values that exceed 70–80 Ωm, which corresponds to granite (G in Figure 4). There is a second unit with an intermediate resistivity (values between 20 and 80 Ωm). Due to the position occupied in the studied outcrops, this unit is related to the base of the Triassic materials (rudites and sandstones) and to the altered granite (T1 and Ga, respectively, in Figure 4). From the electrical information, it is not possible to differentiate these two lithologies; thus, they are considered a single set in the interpretation of the profiles. Finally, clays with very low resistivities (less than 20 Ωm) appear, with few intercalated sandstones from the upper part of the Triassic (T2 in Figure 4).
The Triassic clays (T2) outcrop in the origin (southeast) of the profile 1 (ERT 1, Figure 4). The resistivity values associated with these facies are very low, in the order of 10–20 Ωm. At approximately 100 m from the origin of the profile, a contact surface appears with the resistive facies of the granite, which makes it possible to locate the position of the F1 fault. The position of this contact coincides with the fault trace observed in the outcrop, where the Triassic shales and the altered granite come into contact at the surface. This correlation is observed by comparing the trace of profile 1 in the geological mapping in Figure 2 and the results of the electrical tomography profile in Figure 4. However, in this profile, it is not possible to deduce the vertical displacement of the fault since it does not reach a sufficient depth to detect the resistive facies related to the granite (the presence of a perimeter fence associated with a railway line prevents further extension of the profile toward the SE, which would allow great vertical depths to be reached). In the sunken block of profile 1, in contact with the F1 fault, more than 30 m thick highly conductive facies (T1 shales) appear, so a vertical displacement of this fracture of more than 30 m can be assigned. If the elevation of this point in the profile (390 m a.s.l.) and the base of T1 in the uplifted block (448 m a.s.l.) is compared, it can be estimated that the fault has a vertical displacement of more than 58 m at this place (profile 1 in Figure 4).
Approximately 160 m from the origin of profile 1, there is a zone of decreasing resistivity in the granite. This electrical behavior is related to an alteration process, which is probably favored by the presence of a fracture. This hypothetical fracture has not been observed in the outcrops. Approximately 240 m from the origin, fault F0 outcrops, which is difficult to recognize in this profile. The position of F0 and that of the Triassic base shown in Figure 4 are supported by field data, which allow estimation of a vertical displacement of 20 m.
In the hanging wall of the F1 fault, two subparallel profiles were determined (ERT 2 and ERT 3). The first profile, with a short AB distance, features the sedimentary cover and the granite contact. The ERT 3 profile, with a great AB distance, allows greater depths to be reached. In both profiles, the presence of the three units mentioned above is deduced based on their electrical response: granite (resistive unit), altered granite and Triassic base (intermediate facies) and Upper Triassic clays (conductive unit). The depth at which the granite appears is variable (between 15 m and 40 m) and is conditioned by the irregular morphology of the granite prior to the deposition of the Triassic materials and by the presence of faults.
In profile 1, according to the field data, the contact between the granite and the base of the Triassic (T1) is located at 448 m a.s.l. in the raised block. Profile 2 in Figure 4 shows the intersection between ERT 1 and ERT 2. At the vertical of this point, the contact between the T2 unit and the T1 + Ga unit is estimated to be at 380 m a.s.l., and the contact between the T1 + Ga unit and the unaltered granite is at an elevation of 370 m a.s.l. The difference in the elevation of the contact between the basement and the sedimentary cover in the raised and sunken blocks allows us to estimate the vertical displacement for the F1 fault: between 68 and 78 m at this point. The exact throw cannot be calculated as the top of the altered granite has been included in the T1 + Ga unit. In many cases, the resistivities of T1 and Ga are very similar, so it is difficult to identify the contact between the basement and the cover by electrical tomography.
In profile 3 of Figure 4 (closest to fault F1), the intersection with ERT 1 is also represented. At the vertical of this point, the contact between T2 and T1 + Ga is estimated to be at about 390 m of elevation, while the contact between T1 + Ga and unaltered granite is about 375 m a.s.l. That is, the contact between the sedimentary cover and the granite basement would be between 58 and 73 m a.s.l at this point (the difference between the elevation of the geological contact in the raised block and the corresponding values of these two surfaces in the sunken block). In this case, although an accurate value for the throw cannot be calculated either, it turns out to be somewhat lower than at the previous point (about 5 m), which could suggest the existence of several subparallel faults that sink the SE sector.
In the lower part of Figure 4, the ERT 3 (BIS) profile is represented, taken directly from the results of the RES2DINV inversion, without any type of resistivity smoothing through the Surfer software. According to this representation, it is better noticed as some of the faults that affect the granite are fossilized by the Triassic materials. Therefore, there was a fracturing stage prior to the deposition of the sedimentary cover. Between 100 and 200 m from the origin of the profile, the presence of bodies of variable thickness with prograding morphologies is deduced, which tend to fossilize these irregularities of more than 40 m of vertical displacement.
4.2. Time Domain Electromagnetic Method
Figure 5 shows the TDEM curves processed at fifteen of the measurement stations. For each of these stations, the induced voltage curves measured in the field (blue line), the apparent resistivity curves obtained as a function of time (red line) and the resistivity model generated by the inversion software (stepped red line) are represented. In general, there is a good fit of the curves, with a root mean square (RMS) error of between 0.5% and 5%. However, in some cases, the fit is not as perfect, and the RMS reaches 7.6%. After inverting the data, three sections from the surface are usually observed in the models obtained. The first section shows a decrease in resistivity, which is associated with the Triassic (clayey facies). The second section shows a progressive increase in resistivity, which is related to the Triassic base (rudites and sandstones) and the altered granite. Finally, in deep areas, the highest resistivities correspond to granite. These three units coincide with those deduced from the electrical method, as discussed in the previous section.
The stations represented in Figure 5 are used to construct three profiles, the locations of which are shown in Figure 2 (X-X’, Y-Y’ and Z-Z’). In Figure 6, these three geological sections are represented based on surface and TDEM data. For each of the measuring stations, the figure indicates the topographic elevation of the station (above sea level), the depth to reach the transitional facies (T1 − Ga) and the roof of the granite (G). This information makes it possible to deduce the elevations of this contact, and therefore, the vertical displacement of the faults. In section X-X`, the vertical displacement of fault F0 is determined from the surface data (approximately 20 m). The F1 displacement varies between 47 m and 70 m (stations D6, D3 and D7), suggesting progressive subsidence due to the movement of several associated faults (profile X-X’ in Figure 6).
The correlation of the B2, B1, A3, A2, and A1 stations (profile Y-Y’ in Figure 6) also allows us to deduce the movement of several fractures that tend to progressively sink the SE block, with a total throw ranging from 58 m in the vicinity of the F1 fault (B2 station) to 91 m (A3 station). If the values obtained for the vertical displacements using the electrical and electromagnetic techniques are compared, very similar results are observed.
Figure 6 also shows the Z-Z’ profile as a correlation of the E3, E2, E1, A3, D4, D3 and D2 stations. This profile allows us to deduce that the morphology of the granite roof is very irregular and is likely controlled by a complex system of fractures. The greatest thicknesses of the sedimentary cover appear in the SE sector, where the granite may exceed 90 m in depth (Figure 6). The Z-Z’ profile is parallel to the ERT 1 and ERT 2 electrical tomography profiles, occupying an intermediate position between them. As can be seen, there is a great parallelism between the resulting models from both techniques, with equivalent vertical displacements of the granite roof (between 40 and 50 m in the distance of the profile).
4.3. Magnetic Methods
Quality control of the magnetic field data is carried out with the GEM Link program version 5.4. First, the diurnal correction is carried out using the base station data. Subsequently, the despiking method is used to eliminate environmental noise and extreme values. Histograms and variograms are used to evaluate the data’s spatial distribution. The minimum curvature and kriging are the most successful interpolation methods tested. On the other hand, the data collection and representation of the results of the total magnetic field values are carried out using the Surfer program version 13. The result is represented in Figure 7A. To discriminate shallow geological structures, a map of residual anomalies is generated. For this, a first-degree polynomial fit is used that better represents planar surfaces [55]. The polynomial is subtracted from the total magnetic field and a map of residual anomaly values is obtained (Figure 7B).
In general, the lowest magnetic values (less than 44,030 nT) appear in the SE sector, precisely where the electrical information (ERT profiles) and electromagnetic information (TDEM stations) point to the Paleozoic basement (granite) at greater depths. At the points where the granite outcrops, values greater than 44,035 nT are reached. Thus, as described in other studies [46,48,50], fractures affecting sedimentary basins bring into contact basement and sedimentary cover materials with different magnetic susceptibility values, which would justify the linear anomalies.
In Figure 7, the positions of the F0 and F1 fractures observed in the outcrops are indicated (black line). Magnetic anomalies in the order of 20 nT are noticed, and they are associated with such fractures and aligned in the same directions (10° E and N40° E). From these findings, the effectiveness of this technique for fault detection can be deduced.
In addition, the presence of another N120° E alignment (represented by red lines in Figure 7) might correspond to a younger fracturing stage (F2), cutting the previous ones. The fractures that affect the granite in the ERT 2 profile (Figure 4) may be related to this third fracturing stage. Both the electrical study (ERT 1 and ERT 2 profiles in Figure 4) and the electromagnetic study (Z-Z’ profile and Figure 6) deduced the presence of these fractures, to which throws of between 40 and 50 m are assigned. In regional maps, these fracture orientations have been described in other sectors of the mining district. For example, these alignments are observed in the geological cartography of Figure 1 to the N of the study area. The map representation of the magnetic field values facilitates the detection of the different fracturing directions.
5. Conclusions
In this study, the direct information obtained from outcrops and the indirect information obtained by different geophysical methods are used in an integrated manner to analyze the effectiveness of each method in modeling fracture zones. This work is focused on the old mining district of Linares-La Carolina, where the continuity of the exploitation is influenced by the presence of a normal fault in the N40° E direction (F1 in this work). The geological study of the outcrops allows us to deduce the presence of another previous fracture system with a direction of 10° E (F0 in this work). The indirect study methods used (ERT, TDEM and magnetic methods) make it possible to verify the presence of these structures in the subsoil.
From the lateral and vertical variations in the resistivity values in the ERT profiles, the position of the contact between the granite and the sedimentary cover is identified, which allows the reconstruction of the irregular geometry of the paleo-relief in the region prior to the deposition of Triassic materials. Moreover, different fractures are identified. This technique even makes it possible to differentiate the ages of some of these fractures since certain fractures that affect the Paleozoic are detected and fossilized at the base of the Triassic, which limits their age. Other fractures affect the Triassic materials, and these ones are considered relatively recent. When the granite roof is altered, it features an electrical behavior similar to the sands and rudites of the base of the Triassic (transition facies), which makes it difficult to precisely mark the contact between the basement and the sedimentary cover. In these cases, it will not be possible to calculate the vertical displacement of the faults accurately. Thus, for the F1 fault, values between 58 and 73 m are estimated. In cases where the transition facies have a small thickness, the throw of this fault is approximately 58 m. A system of associated fractures causes the depth of the sunken block to increase toward the SE, up to more than 90 m in the sector studied.
The TDEM is very versatile and can be used with different configurations and ranges of amplitude, voltage, and time and with various devices, depending on the objective. In general, a good fit between electrical (ERT) and electromagnetic (TDEM) data is found in this study. The TDEM has made it possible to detect the depth of the Paleozoic basement under the sedimentary cover. The correlation of the data from the different stations allows us to verify the lateral changes in the thickness of the cover associated with the presence of a paleo-relief and with the previously mentioned fractures. This methodology has even made it possible to quantify the vertical displacement of some of these faults. The results of this electromagnetic technique (TDEM) are compared with those obtained with electrical techniques (ERT), and it can be stated that very similar body morphologies and throw magnitudes are obtained for faults.
The magnetic method was chosen because of its efficacy in discerning among rock bodies with different magnetic susceptibilities, which provides a way of delineating subsurface geologic features as faults over broad areas. Magnetic prospecting reveals three anomaly alignments associated with three stages of fracturing. Two of these fractures had already been observed by surface studies (F0 and F1), and a third one was observed in the N120°E direction (F2 in this work). The map representation of the magnetic values allows us to detect these three stages and to deduce their chronology.
Therefore, the combined use of electrical, electromagnetic and magnetic techniques is of great interest in fracture analysis. In this case study, it provides a comprehensive understanding of the complex fracture zone at the southern boundary of the mining district. Thus, it constitutes a useful tool to help in a subsequent research campaign concerning the seams in the hanging wall of the Linares fault. Future research should explore the border of the former mining district, where the exploitations stopped, integrating aeromagnetic techniques that can cover large areas.
Conceptualization, J.R.; investigation, J.R., R.M., J.V. and M.C.H.; software, R.M., I.F., S.B. and J.R.; writing, J.R. and M.C.H.; review, M.C.H. All authors have read and agreed to the published version of the manuscript.
The data that support the findings of this study are available upon request from the corresponding author, M.C.H.
Author Isla Fernández has received research grants from Company Geognosia S.L., author Sara Berman has received research grants from Company Gaia Exploración. The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 1. Geographical location and regional geological map of where the studied sector is located. 1. Granitoids. 2. Sandstones and rudites (Triassic, T1). 3. Lutites (Triassic, T2). 4. Calcarenitic breccia (bottom) and alternation of marls and limestone marl (Miocene). 5. Alluvial (Quaternary).
Enlarge this image.Figure 2. Detailed geological mapping of the studied sector where two types of fractures (F0 and F1) in the outcrops are indicated. Three types of materials are differentiated: G (granite), T1 (rudites and sandstones of the base of the Triassic) and T2 (lutites of the upper part of the Triassic). The limits of the magnetic study (solid yellow line), the positions of the TDEM stations (A1, A2, A3, B1, B2, B3, C1, C2, C3, D2, D3, D4, D6, D7, E1, E2 and E3), and the traces of the three ERT profiles (ERT 1, ERT 2 and ERT 3) are marked. The positions of the geological sections in Figures 4–6 (X-X’, Y-Y’ and Z-Z’) are indicated.
Enlarge this image.Figure 3. Photographs taken adjacent to the study area where the contact surface between the Paleozoic basement (granite) and the sedimentary cover (Triassic) is observed. An irregular morphology is observed prior to the deposition of the Triassic materials (paleo-relief). The paleo-relief and some fractures are fossilized by subhorizontal Triassic materials. In contrast, other fractures affect the granite and the sedimentary cover. The first photograph (A) corresponds to a general view of the outcrop, and the second photograph (B) corresponds to a detailed view.
Enlarge this image.Figure 4. Representation of the three electrical tomography profiles produced in this study (ERT 1, ERT 2 and ERT 3). The vertical axis is represented in m above sea level. The three profiles obtained with the RES2DINV inversion software version 4.10 were treated and represented using Surfer software to offer a gradual change in the variation of the resistivities. In addition, for profile 3, the data directly obtained from the inversion are represented (ERT 3BIS). Through the final modality, the base of the Triassic materials infilling the irregularities of the basement is better observed. Granite (G), altered granite (Ga), sandstones and conglomerates from the base of the Triassic (T1), and clays and some sandstone from the upper part of the Triassic (T2) are detected. In addition, in each profile, the crossing position with the others (ERT 1, ERT 2 and ERT 3) is indicated.
Enlarge this image.Figure 5. Induced voltage curves (blue line) and apparent resistivity (red line) as a function of time at different stations: B2, B1, A3, A2, A1 (profile Y-Y’), E3, E2, E1, A3, D4_bis, D3_bis, D2_bis (profile Z-Z’), and D6, D3, D7 (profile X-X’). The interpretation after the inversion of the data is included (Zondtem1D software); red straight lines represent the resistivity as a function of the depth. Granite (G); altered granite (Ga); Triassic conglomerates/sandstones (T1); lutites (T2).
Enlarge this image.Figure 6. Three geological sections (X-X’, Y-Y’, and Z-Z’) are represented based on the surface information and the correlations of the different stratigraphic columns deduced from the TDEM stations. The position of each station is indicated (the corresponding topographic elevation appears in parentheses). For each station, two other numerical values appear, including the depths of the lithological changes (T2/T1 and T1 + Ga/G). The fractures described in this work (F0, F1 and F2) are indicated in the different geological cross-sections. The vertical axis represents m a.s.l.
Enlarge this image.Figure 7. Magnetic values of the studied sector. Three different anomalies are detected that correlate with three fracture systems (F0, F1 and F2). F0 and F1 (black lines) are faults observed in the outcrops. F2 (red lines) are faults deduced from magnetic study. Map of total magnetic field values (A) and map of magnetic residual anomaly values (B) are represented.
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; Mendoza, Rosendo 2
; Vilchez, José 1
; Hidalgo, M Carmen 1
; Fernández, Isla 3 ; Berman, Sara 4 1 Department of Geology and CEACTEMA, Higher Polytechnic School of Linares, Technological-Scientific Campus, University of Jaen, 23700 Linares, Spain;
2 Department of Mechanical and Mining Engineering and CEACTEMA, Higher Polytechnic School of Linares, Technological-Scientific Campus, University of Jaen, 23700 Linares, Spain;
3 Geognosia S.L., 21600 Valverde del Camino, Spain;
4 Gaia Exploración, 21600 Valverde del Camino, Spain;