Regional integration and 3D modeling of airborne geophysical data: Map of Geophysical Anomalies of Colombia for mineral resources, 2020 version

The Map of Geophysical Anomalies of Colombia for mineral resources, MAGC 2020 version compiles the geophysical information acquired, processed and interpreted by the Servicio Geológico Colombiano (SGC) since 2013. This information was collected via airborne platforms (aircrafts) using magnetometry and gamma spectrometry. This version covers approximately 547 960 km2 of the national territory in the Andean (North and Central), Eastern (Eastern Plains and Amazon) and Caribbean zones (Perijá mountain range). This information consists of 17 blocks of geoscientific interest, covered by flight lines separated by 500 and 1000 m, for a total of more than 907 566 linear km of airborne information, acquired at a nominal altitude of 100 m above the ground, with a sampling resolution that was not previously available at this scale and coverage. This document presents the methodology for compiling, processing and representing the thematic coverage included in MAGC 2020: Map of Total field magnetic anomaly (TFMA), Map of the analytic signal (AS) and radiometric ternary map of the distribution of the relative concentrations of uranium, thorium and potassium. Furthermore, the work identifies 1079 magnetometric anomalies of interest, which were subsequently analyzed and modeled in the Map of magnetic sources modeled from magnetization vector inversion, which contains a total of 1297 magnetic bodies interpreted from these anomalies. Integration of available geological and metallogenic information with each of these bodies allow the suggestion of possible geological sources and possible exploration targets. The objectives of this study were Regional integration and 3D modeling of airborne geophysical data: Map of Geophysical Anomalies of Colombia for mineral resources, 2020 version Integración regional y modelado 3D de datos geofísicos aerotransportados: Mapa de anomalías geofísicas de Colombia para recursos minerales, versión 2020 Citation: Puentes, M., Robayo, A., Moyano, I., Henrique, E., Lara, M., Arias, H., Ospina, D., Rojas, O., Gómez, E., Torrado, S., & Prieto, G. (2021). Regional integration and 3D modeling of airborne geophysical data: Map of Geophysical Anomalies of Colombia for mineral resources, 2020 version. Boletín Geológico, 48(Spl.1). https://doi. org/10.32685/0120-1425/bol.geol.48.Spl.1.2021.586


IntroductIon
Since 2012, the Servicio Geológico Colombiano (SGC) has been advancing the acquisition of geophysical information using magnetometry and gamma spectrometry, with a regional coverage and spatial resolution that were previously not available for Colombia (Moyano et al., 2016). Although prior to this geophysical survey, government entities such as Ecopetrol and the Agencia Nacional de Hidrocarburos (ANH) had conducted airborne geophysical studies, the regional nature and technical parameters of these surveys (altitude, sampling density and resolution) were not detailed enough to detect magnetic anomalies originating from relatively small geological bodies, because as the altitude increases, the magnetic intensity of the latter decreases and is masked by larger regional anomalies. In contrast, the information acquired by the SGC and represented in the Map of Geophysical Anomalies of Colombia for mineral resources -MAGC 2020 (Moyano et al., 2020) reflects acquisition parameters at a much more detailed scale, allowing us to identify and characterize new deposits of minerals and to complement geological mapping.
The survey of airborne geophysical information is based on the need to expand the geoscientific knowledge of the national territory, generate useful information to identify potential mineral resources, provide basic information for better land-use planning, generate geological cartography and advance geoscientific research. Anomalies in the magnetic field are related to variations in the physical properties (magnetic susceptibility) of the materials which compose the earth's crust, which in turn directly reflect the concentration of magnetic minerals in the rocks that constitute them. From a geological-geophysical perspective, magnetic anomalies can be related to a wide variety of geological sources, such as intrusive bodies, dikes, hydrothermal alteration zones and volcanism.
All the geophysical airborne information collected since the first SGC survey in 2012 was processed and integrated, and then presented as four layers of information, including: 1) the total field magnetic anomaly map, 2) the analytical signal map of the total field magnetic anomaly, 3) the ternary radiometric map of the concentration distribution of uranium, thorium and potassium and 4) the map of possible geological sources, based on modeled magnetic sources, 2018 version (Moyano et al., 2018), which includes magnetic sources from the Córdoba and Garzón blocks for the 2020 version. The possible geological sources were identified and interpreted according to their geophysical attributes during modeling and characterized as being in a geological and/or metallogenic environment, information available from the Metallogenic Map of Colombia 2020 (Sepúlveda et al., 2020) together with the Geological Map of Colombia 2015 (Gómez et al., 2015). The four layers generated allowed us to produce the 2020 version of the MAGC (Moyano et al., 2020), featuring magnetometry and gamma ray spectrometry information collected for 17 blocks (Figure 1). This map shows magnetic and gamma ray spectrometric anomalies. One of the blocks (Cesar-Perijá) is located in the Caribbean area, in the departments of Cesar and La Guajira, in the area of influence of the Sierra Nevada de Santa Marta, with heights ranging between 0 and 3000 meters above sea level. Eight blocks (Antioquia-W, Antioquia-E, Bolívar, Urabá, Andes-N, Darién, Córdoba and Buenaventura) are located in the northern part of the Andean zone. One block (Garzón) is located in the central part of the Andean Zone. In general, the Andean region is located in the departments of Antioquia, Bolívar, Boyacá, Caldas, Cesar, Córdoba, Cundinamarca, Chocó, Huila, Risaralda, Santander, Sucre, Tolima and Valle del Cauca, in Central and Western mountain ranges confluence area, with altitudes ranging from 0 to 5200 m. The seven remaining blocks (Amazonas-N, Guainía, Vichada, Vichada-W, Amazonas-S, Vichada-N and Guainía-E) are located in eastern Colombia, in the departments of Amazonas, Arauca, Caquetá, Casanare, Guainía, Guaviare, Meta, Vaupés and Vichada, areas of gentle slopes and heights that range between 50 and 800 meters above sea level.

Survey Specifications
All the blocks were surveyed via airborne platforms fixed wing aircrafts that flew at a nominal altitude of 100 m above the ground, where topographic (particularly in the Andean zone) and operational safety conditions allowed. Each aircraft was equipped with a high-precision positioning system and magnetic and gamma spectrometry sensors configured to record variations in the earth's magnetic field, and count radiogenic particles at rates of 10 Hz and 1 Hz respectively. The magnitude of the total magnetic field was measured every 7m to 9m, and the intensity of the natural gamma radiation from the ground was measured every 70 to 90 m. The distance between flight lines for 14 of the 17 blocks was 500 m. The remaining three blocks, located in the Colombian Amazon, were overflown with a distance between flight lines of 1000 m, and crossed by perpendicular control lines separated by 5000 or 10 000 m. The direction of the flight lines was N20W in the Córdoba, Darién, Urabá, Antioquia W and Antioquia E blocks, and N-S in the other blocks. To level the magnetic data, control lines perpendicular to the flight lines were established at distances of 5000 (blocks with flight lines every 500 m) or 10 000 m (blocks with flight lines every 1000 m).
The magnetometry data were acquired using cesium vapor magnetometers in the aircraft in a tail stinger installation. This type of assembly requires compensating for the aircraft movements during flights. The following are the technical specifications for the magnetometers installed in the aircraft: » Sensitivity 0.01 nT » Absolute accuracy ± 10 nT » Dynamic range 20 000 and 100 000 nT » Sampling interval 0.1 seconds (10 Hz) » Heading effect < 2 nT Diurnal variations in the magnetic field were recorded by a ground base station, with a sampling interval of no more than 3 seconds, synchronized with the magnetometer in the aircraft. The diurnal variations had a maximum tolerance of 3 nT (peak to peak) for a period of one minute for the magnetic base station. Fluctuations above these values and persisting for more than one hour are considered a possible solar storm, meaning flights were cancelled.
Likewise, 2048 cubic inch NaI Tl (sodium iodide crystals treated with thallium) gamma ray spectrometers were used onboard the aircraft to record the full spectrum of gamma rays between 512 and 1024 channels, at a sampling rate of one per second (1Hz). In addition, upward-looking NaI scintillator detectors were used to record the concentration of atmospheric radon. The K, U and Th counts were extracted from the full spectrum during processing according to their energy levels.
In some areas with special topographic or safety characteristics, the aircraft had to be flown at higher altitudes than the nominal flight value. This higher altitude resulted in progressive deterioration and a loss of sensitivity of the gamma spectrometry equipment, which is reflected as noise in the generated data and can lead to erroneous interpretations. For this reason, an analysis of the relative concentrations of the uranium (U), thorium (Th) and potassium (K) channels was performed directly from each block's database. From this analysis, data acquired at an unusual height above the nominal 100 meters showed discordant or even negative values of one or all the radioelements, and can be recognized as along-flight line strips of very high or low counts. These noise measurements were represented independently on the ternary gamma spectrometry map, with a warning regarding the interpretation of these areas.
The airborne magnetometry and gamma ray spectrometry information was compiled, integrated, and processed to generate thematic layers (images) with a spatial resolution of 125 m in the Caribbean and Andean zones and 250 m in the Eastern zone (Amazonas and Orinoquía).

Method
This section describes the methodology and processing to generate the thematic layers for the Caribbean, Andean (North and Central) and Eastern zones. Interpolation methods and micro leveling (MIC) techniques were applied to the 17 selected blocks, which were grouped along the Caribbean, Andean (North and Central) and Eastern zones. From this processing, three thematic images were generated: total-field magnetic anomalies (TFMA), analytic signal (AS) and the Ternary map of the distribution of potassium (K), thorium (Th) and uranium (U) concentrations. Subsequently, the magnetic anomalies were identified using magnetometry data. Therefore, to determine spatial characteristics and magnetic susceptibility, the magnetization Vector Inversion (MVI) algorithm was used to process magnetic field data acquired. From the intensity of the magnetic field data (TFMA) of detected anomalies, the MVI generates a mathematical modeling of a volume of data that represents the variations and the values of magnetic susceptibility in the field. Ideally, the inversion of the magnetic anomaly generated by this magnetic susceptibility model is matched to the geometry by the observed TFMA values, and the analytical signal. Finally, a layer was produced with the selected and modeled anomalies.

Interpolation method for thematic layers
For the interpolation of the magnetometry and gamma ray spectrometry data, a cell size between ¼ and ⅛ was chosen for the spacing of the flight lines, to avoid information loss and re-cover all frequencies. An appropriate algorithm that maintains the values of the original points while generating a continuous and smoothed surface must be chosen. The most commonly used interpolation algorithms are bidirectional, minimum curvature, weighted inverse distance and kriging methods (Watson and Philip, 1985;Briggs, 1974) algorithms.
For the interpolation of the magnetometry data, the bidirectional algorithm was used. This method correlates data taken line by line, by applying cubic splines both along and through the flight lines following a defined grid interval. Additionally, a low-pass filter is applied to remove high frequencies that generate artifacts in the information. This interpolation method is suitable for this type of survey, in which the data are distributed in parallel lines, because it interpolates first in one direction and then in the orthogonal directions, thus fitting an unlimited number of data points. This algorithm cannot be used for data with a random distribution.
The weighted inverse distance method was applied to the gamma spectrometry data. The sampling points values were calculated using their weighted average in a defined search radius. The weight assigned to each value is inversely proportional to its distance. The weighting of each value is assigned by a power parameter that modulates the influence of each data value as a function of distance. The higher the power parameter is, the more influence it will have on the closest points. In addition, the method adds a weighted slope factor that moderates the sharpness of the values' weights, preventing said values from leaving their range in the vicinity of their points.

Micro leveling of the thematic layers
Micro leveling is the elimination of residual errors produced in the regular course of a process such as leveling. These errors are manifested as corrugations along lines that are false artefacts (i.e., non-geological effects), creating distorted images identified as noise, while some data points appear in the same direction as the flight lines. This type of noise was removed from the interpolated grids using directional filters applied to the flight and control lines. The micro-leveling technique developed by Minty (1991) removes the non-geological effects caused by residual and leveling errors without affecting the data frequency spectrum.

Processing of magnetic images
To process and generate the magnetometric layers, it was necessary to apply frequency range filters in the areas, to com- pensate for noise effects in the leveling and in the combination of some images. The Andean zone (north sector) is the result of combining the images of Antioquia-W, Antioquia-E, Bolívar, Urabá, Andes-N, Darién, Buenaventura and Córdoba. Its processing was more exhaustive, as its leveling was more complex due to the steep topography of the area. For processing, directional and frequency range filters were applied to compensate for the combining of the images. For the Eastern zone, which is composed of the Amazonas-N, Guainía, Vichada, Vichada-W, Amazonas-S, Vichada-N and Guainía-E blocks, the images were combined to generate the TFMA image in this area.

Total-field magnetic anomaly map
The TFMA map ( Figure 2) is obtained by removal of temporal variations and geomagnetic field model data (International Geomagnetic Reference Field (IGRF)) (Thébault et al., 2015) from the observed magnetic field. Thus, the TFMA map shows variations in the magnetic field produced by the presence of magnetic sources in the subsurface. However, to generate this image, each zone was processed differently according to the topographic characteristics and the location of the blocks.

TFMA analytic signal
The TFMA analytic signal map (Figure 3) was obtained by applying a mathematical algorithm to the total magnetic field. The variations in the three spatial components in relation to depth were calculated (Roest et al., 1992). This mathematical process was used to highlight the edges of the magnetic sources and to directly locate magnetic anomalies, as this filter is independent of the direction of magnetization.

Preparation of thematic layers for gamma
spectrometry data To generate this coverage, the weighted inverse distance interpolation method was used for each block's gamma spectrometry data. The images were generated with a spacing of 125 m in the Caribbean and Andean zones (North and Central) and 250 m in the Eastern zone. All the images had a power parameter of 2 and a weighting slope of 1. The gamma spectrometry distribution ternary map shows the composition, in RGB colors, of the relative concentration of the three radioelements potassium (red), thorium (green) and uranium (blue). This composite image was generated from the normalization of the counts of each of these three elements in the gamma ray spectrometry data, generating the modulation of the red, green and blue colors in proportion to the concentration of potassium, uranium and thorium. The ternary image reduces the effects of gamma ray attenuation by vegetation or soil moisture (International Atomic Energy Agency, 2003). These colors are quantitatively related in a color triangle that indicates the composition of each radioelement channel to which it belongs (see Figure 4). The processing of the images generated in the Caribbean, Andean (North and Central) and Eastern zones used a multiplication factor to level some images and join them. This factor was applied as the aircraft measurement systems for each  block had a different calibration, yielding differing gamma ray counts. In some areas, aircraft were unable to maintain the nominal altitude due to abrupt topography, which reduced the K U Th K_% U_ppm Th_ppm counts of radioelements potassium (K), thorium (Th), and uranium (U) on the ground. This problem was corrected using deconvolution and height correction techniques to avoid topography effects (Gunn and Almond, 1997). Figure 5 presents the Ternary map of the distribution of K, Th and U concentrations compiled for Colombia.

Selection of magnetic anomalies, 3D modeling and integration of magnetic sources
One of the main objectives of magnetometry information analysis is the identification of areas displaying characteristics associated with the presence of more magnetic bodies than the background. These magnetic anomalies are identified by characteristic attributes in the magnetic information, such as the presence of dipoles in the TFMA map (Figure 6a), areas with magnetic highs in the reduction to pole (RTP) of the TFMA (Baranov, 1957) (Figure 6b) and strong amplitudes of the TFMA analytic signal (Nabighian, 1972) (Figure 6c).
Magnetic anomalies are related to a wide variety of geological sources. Therefore, their geometric and physical parameters must be determined. For this, the magnetization Vector Inversion (MVI) algorithm was used to process acquired magnetic field data affected by remanent magnetization as well as induced magnetization. Remanent magnetization changes the intensity and direction of the total magnetization vector, and thus the magnetic susceptibility. Therefore, remanent magnetization can distort the inversion, if it is assumed that the source is only generated by induced magnetization (Ellis et al., 2012). From the intensity of the magnetic field data (TFMA) for detected anomaly, the MVI generates a mathematical modeling of a volume of data that represents the variations and the values of magnetic susceptibility in the field. Ideally, the inversion of the magnetic anomaly generated by this magnetic susceptibility model is matched to the geometry as observed with TFMA values and the analytical signal.
Finally, the integration of the information on magnetic sources in the blocks of interest was carried out to identify and interpret a possible geological source and exploratory target. For this reason, modeled magnetic sources for each of the 17 blocks were designated following a code consisting of four uppercase alphabetic characters. To identify the geophysical anomalies, a consecutive 3-digit numeric code was used, whilst an alphabetic code (consisting of capital letters, from the letter A to the letter Z) was used for anomalies that required subdividing due to having more than one body. This code is reset for each work block (Table 1). For example: BOLI002, ANTW002A or ANDN002B.

Interpretation of possible geological sources and exploration targets
The resulting compilation of the modeled magnetic sources for each of the 17 blocks is presented in the Map of magnetic sources modelled by magnetization vector inversion (MVI) (Moyano et al., 2020). The map presents the magnetic sources identified, characterized by their geophysical attributes during modeling and their geological and/or metallogenic environment ( Figure  7). Accordingly, the integration of the magnetic sources for the MAGC 2020 was carried out taking into account the geophysical and geological attributes of the deposits, as per the Metallogenic context provided by the Metallogenic Map of Colombia (MMC), version 2020 (Sepúlveda et al., 2020). Within this context, each anomaly may be correlated with either some identifiable surface geological feature, or with a deep geological source devoid of surface expression (intrusive bodies, dikes, hydrothermal alteration zones, volcanism, faults, among others).
In accordance with the above, the interpretation of a possible geological source for each identified magnetic source refers to the available geological cartography, as per the Geological Map of Colombia (Gómez et al., 2015) for the evaluated area, from which the lithological correlation of the evaluated anomaly is established. As an example of the above, Figure 8 shows the 3D inversion model of the ANTW37B magnetic source, a shallow source associated on the surface with porphyritic basalt, and andesites. Similarly, although the magnetic sources do not have a surface geological expression, the existence of common characteristics to a source with geological correlation allows us to infer a possible similar geological origin. As an example of the above, Figure 9 shows the response in the ACT map and the 3D susceptibility model of the ANTW041 deep magnetic source defined as intrusive. Based on the integration of the geological and metallogenic contexts and the response or geophysical signature of each of the magnetic sources modeled, five possible geological sources are proposed for the MAGC 2020 (Moyano et al., 2020): dike (faults, veins and structural pattern), Intrusive, lithological, sedimentary and deposit model (DM). Thus, it is possible to advance the hypothesis that magnetic sources with geophysical characteristics and a similar geological-metallogenic context to those identified as DM may offer an exploratory potential similar to that already established based on the current geological knowledge of the existing deposits.
The differentiation of the possible exploration targets was based mainly on the identified deposit models (DM), and also taking into account parameters of the magnetic sources, such as magnetic susceptibility, depth and vertical extension, geometry and other geophysical attributes observed during the geophysical interpretation, together with the information from the MMC 2020 (Sepulveda et al., 2020) and the 2015 version of the Geological Map of Colombia (Gómez et al., 2015).
Seven possible exploratory targets were defined: intrusive, porphyry, mafic and ultramafic rocks (including laterites), carbonatites, kimberlites and placer deposits. Magnetic sources associated with a surface lithological response, and identified as such in the field as a possible geological source, were not considered.

dIscussIon
The maps were generated by processing the airborne geophysical magnetometry and gamma spectrometry data acquired in 17 blocks between 2018 and 2020. Magnetic anomalies were selected based on the TFMA map dipoles, and 3D inversion models of each were created. This process resulted in a distribution of magnetic susceptibilities related with magnetic bodies in deep and shallow zones with induced and remnant magnetization, defined as magnetic sources, while at the surface, the radioelement ternary gamma spectrometry data enhanced the lithological contrasts.
The maps obtained from the TFMA (Figure 2) of the different zones show the variation in the magnetic field due to the different geological structures. The magnetic anomalies of the Eastern zone are, for the most part, broader than the anomalies of the Andean and Caribbean zones. This finding indicates a clear differentiation of lithologies between the eastern region and the others.
Similarly, the AS shows the location of the bodies causing the magnetic anomalies (Figure 3), in addition to their plan shape. Therefore, the AS shows the extent of the rock formations and the limits of the heterogeneous geological formations and tectonic characteristics (faults). Thus, the AS of the Andean and Caribbean zones shows marked trends in the orientation of their magnetic structures, as is evident in the Urabá-Antioquia West and Bolívar and Cesar-Perijá blocks, in which tectonic activity is present. In addition, the analytic signal shows important geological structures, such as the Antioquia Batholith, which is located in the Antioquia west and East blocks, as in in Figure 10.
The AS of the eastern zone reveals a varied orientation and broad magnetic structures. The high AS values indicate a high gradient in three spatial components and strong contrasts in the magnetic susceptibility of the crystalline basement rocks. Magnetic structures such as extensive dikes are also observed, as shown in Figure 11.
These modeled sources have an estimated susceptibility ranging from 0.0001 to 0.09 SI and can estimate the shape of the source. According to the MVI models, the minimum magnetic susceptibility values were found mainly in the Amazonas S, Amazonas N, Vichada N and Buenaventura blocks (values lower than 0.0005 SI), while the highest susceptibility values were found in the GUAE, GUAINIA, ANTW and URABÁ blocks (values greater than 0.06 SI).
Let us note that the magnetic susceptibility model obtained by inversion represents only one of the several possible solutions that satisfy the predicted mathematical fit. Therefore, the susceptibility on which the geometric parameters of each magnetic source are estimated presents an uncertainty that can be progressively reduced, as long as there are control parameters over the area of interest, such as geological controls and magnetic susceptibility measurements of the rocks present in the area (petrophysics). With regards to the map of Colombian magnetic sources, the geophysical anomalies were interpreted during the early stages of evaluating the mineral resource potential, and as a guide to identifying those areas in which detailed field work may be carried out. Indeed, this type of control information is not always available, therefore the selected magnetic susceptibility has been estimated via parameters such as the anomaly geometry in layers, e.g., the analytic signal layer.
The possible geological source defined for each of the magnetic sources shows that most of them (82.3%) are of an intrusive type distributed throughout all zones, while 8.3% are   Similarly, intrusive exploration targets constitute a higher proportion than other targets (54.6%). The porphyry type and mafic and ultramafic rocks comprise respectively 12% and 25.4% of the targets, and are associated with possible targets for kimberlites, carbonatites and veins, while the remaining 8% comprises the possible exploration targets of the placer, veins and lithologic types.

Possible
In addition, gamma spectrometry information shows different combinations of radiometric element concentrations for surface potassium (K), thorium (Th) and uranium (U). The gamma spectrometry RGB composite ternary map reflects the chemical composition of rocks and soil and may indicate lateral changes in said composition ( Figure 5). These changes can be related to different lithologies (igneous, metamorphic and sedimentary), which reflect the regional tectonic processes and events that occurred in the area under consideration. Similarly, the ternary map may help to identify areas that have experienced alterations owing to the action of fluids during mineral accumulation processes. These alterations may be related to areas of high or low concentrations of radioelements, identified as light or dark areas in the ternary map, respectively. An example of the above is the magnetic source ANTW016 located in Antioquia W block, sector Páramo de Frontino shown in Figure 12. Additionally, areas enri-ched with potassium as a result of hydrothermal alteration are identified by red tones in this same map.
The gamma spectrometry data of the Eastern zone can be correlated with the lithological units already described and mapped, or with unidentified geological units. However, the limits between contrasts may not correspond to contacts between different lithostratigraphic units, but to variations or alterations within the same geological unit. It should be noted that gamma spectrometry data are sometimes of low resolution as a result of flight altitude issues owing to the steep topography of the Andean and Caribbean zones.

conclusIons
Magnetometry and gamma spectrometry data processing techniques were effective in producing geophysical coverage maps that can provide new geological information and detect the potential existence of mineral resources. These techniques are also a valuable tool for land-use planning related to the national territory.
The acquisition and processing of magnetometry data in 17 airborne geophysical blocks enabled the identification and modelling of 1297 magnetic sources associated with bodies with induced magnetization and magnetization perpendicular to the magnetic field. Based on the magnetic sources, potential mineral deposit sites were identified, either covered or outcropping, based on their distribution, depth, size, shape, and modeled magnetic susceptibility.