Geophysical Response Research Articles

Reinterpreting a “Roman villa” site: First results of nondestructive archaeological research in Dračevica (Donji Radišići, Bosnia and Herzegovina)

AbstractThe article presents the results of archaeological prospection of site Dračevica (Donji Radišići, Ljubuški municipality, West Herzegovina Canton, Bosnia and Herzegovina) which was partly excavated in the late 19th century and so far has been interpreted as a Roman villa, possibly with a special function (road station) and a name attested in ancient sources (Bigeste). Through the use of various nondestructive prospecting methods (aerial photogrammetry, fieldwalking survey, magnetic susceptibility, magnetometry, earth resistance), precise location with RTK GNSS and integration of the data in GIS, the site was restudied and reinterpreted. It was established that the structures are not isolated, as was assumed in the earlier research but rather were a part of a much larger Roman settlement. The research proves that the internal spatial division of the site is more complicated than previously thought, consisting of several building complexes located along a road, as well as land division systems. Such settlement pattern suggests similar, average‐sized land plots and farms. The importance of the research lies also in the methodological approach and the outcome of the geophysical research. Our research was the first application of nondestructive methods of archaeological prospection on Roman sites in the karst landscapes of West Herzegovina, proving the usefulness of the applied methodology in these specific and difficult conditions. We have found dependencies in between the geological background (thin layer of low resistive, colluvial soil on shallow limestone bedrock and the occurrence of highly magnetic terra rossa, formed on weathered limestone) and the geophysical responses registered by various instruments, that is, moderate to good magnetic and electric contrasts between the features and their surroundings, with relatively strong anomalies from geological features. The most important was finding relationships between particular magnetic and electrical anomalies and the presence of very shallow bedrock or faint archaeological remains (e.g. limestone walls). Our methodology, as well as some of our interpretations, will be helpful as a reference for further surveys in the region.

Geoscience Poster G13: A regional strike-slip fault under the Canning Basin

Poster G13 Gravity data in the Canning Basin exhibit a northeast-trending linear feature that offsets a gravity ridge in the vicinity of the Jurgurra–Mowla Terraces with a displacement of about 30 km. The sharp displacement is aligned with similar disruptions on the Lennard Shelf to the northeast and Broome Platform to the southwest. The northeast part of the lineament also shows an anomalous offset on aeromagnetic images. However, the lineament does not correspond to fault displacement within the sedimentary succession of the Canning Basin based on seismic profiles. The geophysical responses to the linear feature vary in different areas, possibly caused by the different thickness of overlying sedimentary rocks across the basin. The lineament is interpreted as a left-lateral strike-slip fault that pre-dates the Canning Basin to account for the lack of seismic responses in the sedimentary section. This northeast-trending fault lies in the basement and might have originated during one of the younger Proterozoic to earliest Paleozoic orogenies pre-dating the Canning Basin. The northeast-oriented fault in the basement was orthogonal to the strike of the basin, thus was less likely to have significant impact on the basin deposition compared to the northwest-oriented faults, such as the Pinnacle and Fenton Fault system. However, these perpendicular features probably created a deeply seated crustal weakness near the intersection where the cluster of lamproite pipes of the Ellendale field intruded during the Miocene. To access the poster click the link on the right. To read the full paper click here

A regional strike-slip fault under the Canning Basin

Gravity data in the Canning Basin exhibit a northeast-trending linear feature that offsets a gravity ridge in the vicinity of the Jurgurra–Mowla Terraces with a displacement of about 30 km. The sharp displacement is aligned with similar disruptions on the Lennard Shelf to the northeast and Broome Platform to the southwest. The northeast part of the lineament also shows an anomalous offset on aeromagnetic images. However, the lineament does not correspond to fault displacement within the sedimentary succession of the Canning Basin based on seismic profiles. The geophysical responses to the linear feature vary in different areas, possibly caused by the different thickness of overlying sedimentary rocks across the basin. The lineament is interpreted as a left-lateral strike-slip fault that pre-dates the Canning Basin to account for the lack of seismic responses in the sedimentary section. This northeast-trending fault lies in the basement and might have originated during one of the younger Proterozoic to earliest Paleozoic orogenies pre-dating the Canning Basin. The northeast-oriented fault in the basement was orthogonal to the strike of the basin, thus was less likely to have significant impact on the basin deposition compared to the northwest-oriented faults, such as the Pinnacle and Fenton Fault system. However, these perpendicular features probably created a deeply seated crustal weakness near the intersection where the cluster of lamproite pipes of the Ellendale field intruded during the Miocene.

Contrasting geophysical signatures of a relict and an intact Andean rock glacier

Abstract. In semi-arid Chile, rock glaciers cover more surface area than glaciers and are potentially important water reserves. To understand their current and future hydrological role, it is necessary to characterize their internal structure (e.g. internal boundaries and ice, air, water and rock content). In this study, we present the results and interpretations of profiles of electrical resistivity and refraction seismic tomography collected on two contrasting rock glaciers in the Chilean Andes located at the headwaters of the Elqui River within the Estero Derecho nature reserve. These geophysical measurements are interpreted both independently and jointly through a scheme of petrophysical four-phase inversion. These first in situ measurements in Estero Derecho confirm that El Ternero (intact rock glacier) contains a significant volume of ground ice, while El Jote contains little to no ice (relict rock glacier). Within our study, we highlight the strong differences in the geophysical responses between intact and relict rock glaciers and propose a diagnostic model that differentiates between them.

Characterizing groundwater salinity patterns\xa0in a coastal sand aquifer\xa0at Magilligan, Northern Ireland,\xa0using geophysical and geotechnical methods

Tidal forcing influences groundwater flow and salt distribution in shallow coastal aquifers, with the interaction between sea level variations and geology proving fundamental for assessing the risk of seawater intrusion (SI). Constraining the relative importance of each is often confounded by the influences of groundwater abstraction and geological heterogeneity, with understanding of the latter often restricted by sampling point availability and poor spatial resolution. This paper describes the application of geophysical and geotechnical methods to better characterize groundwater salinity patterns in a tidally dominated ~ 20 m thick sequence of beach sand, unaffected by groundwater abstraction. Electrical resistivity tomography (ERT) revealed the deposit to consist of an upper wedge of low resistivity (< 3 Ωm), reaching over 8 m thick in the vicinity of the low water mark, overlying a higher resistivity unit. Cone penetrometer testing (CPT), and associated high-resolution hydraulic profiling tool system (HPT), coupled with water quality sampling, revealed the wedge to reflect an intertidal recirculation cell (IRC), which restricts freshwater discharge from a relatively homogeneous sand unit to a zone of seepage within the IRC. The application of CPT and HPT techniques underscored the value of geotechnical methods in distinguishing between geological and water quality contributions to geophysical responses. Survey results have permitted a clear characterization of the groundwater flow regime in a coastal aquifer with an IRC, highlighting the benefit of combining geophysical and geotechnical methods to better characterize shallow SI mechanisms and groundwater flow in coastal hydrogeological environments.

Potential field modelling and U–Pb geochronology reveal the pluton emplacement dynamics of the Lower Devonian Tarnagulla Granodiorite, southeast Australia

The Tarnagulla Granodiorite in the western Lachlan Fold Belt is poorly exposed; however, magnetic data reveal its distinct U shape (in plan view) and patterns characterised by multiple nested cusp-shaped anomalies of contrasting magnetic intensity. We interpret this magnetic response to reflect incremental assembly of the pluton above a persistent magma point source that migrated laterally. Here, we use potential field data, geophysical modelling and high-precision zircon U–Pb dating to constrain the shape of the intrusion at depth and deduce the migration path of the magmatism epicentre. Potential field profile modelling across three traverses, constrained by petrophysics, resolves the subsurface geometry of the Tarnagulla Granodiorite. In cross-section, the intrusion is funnel-shaped, with a lower boundary that varies from ∼2 km depth in the north to ∼6 km depth in the southwest, reaching a maximum depth of ∼8 km, which may coincide with a magma feeder zone. New precise U–Pb geochronology provides ages that confirm the interpreted magnetic phases were emplaced sequentially along a clockwise intrusion path that progressed stepwise from the north to the southwest between 406.62 ± 0.27 Ma and 399.3 ± 1.1 Ma. We attribute migration of the epicentre of magmatism within the Tarnagulla Granodiorite to syn-intrusion (Early Devonian) reactivations of the underlying Tarnagulla Fault, with relative displacements between the fault hanging wall (which hosts the pluton) and footwall (which we interpret to have hosted a persistent point-source magma feeder zone during pluton emplacement). Our model explains the intrusion’s U-shaped magnetic response and reveals critical information about the paleo-stress and paleo-strain conditions during emplacement, characterised by three main displacement vectors and fault slip magnitudes. The geophysical characteristics, path of emplacement and mineralisation distribution all suggest that progressive fractional crystallisation contributed to the formation of an enriched final pulse of magma that resulted in Mo, Cu and W mineralisation hosted in the Moliagul Granodiorite. KEY POINTS High-precision U–Pb zircon geochronology reveal that the Tarnagulla Granodiorite was emplaced along a clockwise pathway from the northeast to the southwest between 406.62 ± 0.27 Ma and 399.3 ± 1.1 Ma. Active faulting during pluton emplacement is the suggested explanation for the Tarnagulla Granodiorites’ U-shaped magnetic response. Fractional crystallisation throughout pluton emplacement produced the varying internal geophysical response of the pluton as well as the enrichment of Mo and W at Mt Moliagul.

ERT and GPR Prospecting Applied to Unsaturated and Subwater Analogue Archaeological Site in a Full Scale Laboratory

Geophysical techniques are widely applied in the archaeological field to highlight variations of the physical behaviour of the subsoil due to the presence of ancient and buried remains., Considerable efforts are required to understand the complexity of the relationship between archaeological features and their geophysical response where saturated conditions occur. In the case of lacustrine and wetland scenarios, geophysical contrasts or electromagnetic signal attenuation effects drastically reduce the capabilities of the geophysical methodologies for the detection of structures in such conditions. To identify the capability of the electrical and electromagnetic methods in different water-saturated scenarios, an experimental activity was performed at the Hydrogeosite CNR laboratory. The test allowed us to analyze the limits and potentialities of an innovative approach based on the combined use of the ground-penetrating radar and 2D and 3D electrical resistivity tomographies. Results showed the effectiveness of the ground-penetrating radar for detecting archaeological remains also in quasi-saturated and underwater scenarios despite the em signal attenuation phenomena; whilst the results obtained involving the resistivity tomographies offered a new perspective for the archaeological purposes due to the use of the loop–loop shaped array. Moreover, the radar signal attenuation, resolution and depth of investigation do not allow to fully characterize the archaeological site as in the case of the scenarios with a limited geophysical contrast (i.e., water-saturated and arid scenarios). The experimental tests show that these limits can be only partially mitigated through the integration of the geophysical methodologies and further efforts are necessary for improving the results obtainable with an integrated use of the adopted geophysical methodologies.

COMPARATIVE ANALYSIS OF THE K-NEAREST-NEIGHBOUR METHOD AND K-MEANS CLUSTER ANALYSIS FOR LITHOLOGICAL INTERPRETATION OF WELL LOGS OF THE SHUSHUFINDI OILFIELD, ECUADOR

The lithological interpretation of well logs is a fundamental task in Earth science that can be accomplished with the application of various machine learning algorithms. The current investigation attempts to evaluate the performance of the K-nearest-neighbour Density Estimate (KNN) and K-means cluster analysis methods for predicting lithology in a dataset of logs measured in the siliciclastic reservoir of the Shushufindi Oilfield of Ecuador. The comparison of lithological interpretation is assembled using classical methods, such as qualitative interpretation and density-neutron cross plot. The lithological interpretation results showed that the supervised method KNN has a higher fitting level with the comparison interpretation data (87.3%, 1145 m predicted of 1311.1 m interpreted) than the results of the K-means method (71.6%, 939.7 m predicted of 1311.1 m interpreted). The geological nature of the reservoir creates a level of a discrepancy because of the near geophysical responses between limestone and intermedia grain size rocks. The possibility of controlling this in the KNN algorithm makes it preferable for usage in these types of reservoir lithological interpretation.

Imaging indigenous sacred sites: A tale of clay, silt, and sand

When studying indigenous sites, especially sacred sites such as burials, the needs and wishes of the indigenous people are paramount; the site integrity must be respected, and the site must be left intact and undisturbed. Noninvasive, nondestructive geophysical imaging is well suited to such investigations, but suspicion within indigenous communities because of past transgressions is a barrier to widespread use; skepticism is not uncommon, but we have always managed to convince skeptics. We have developed an overview of results from 10 Māori (the indigenous peoples of New Zealand) sites in the South Island of New Zealand: one historic burial site, six modern burial sites that also had historical use, one historic battle site, one prehistoric site that may have been a burial site, and one prehistoric site used for food storage that was tapu (sacred). Our approach was to let them ask us to delineate areas of importance, for example, old burial sites. There are, naturally, processes and protocols, cultural and technical, that we followed. The sites comprised a range of lithologies and soil types: one site where clay soil overlays limestone, five sites where loess (airborne silt) overlays basalt, one set of inland silty soil sites, one site on peat soils overlying sandy gravels, and two sites in coastal sands. The geophysical responses of the sites cluster into three groups: Horizontal loop electromagnetic (HLEM) and magnetic field methods worked well for the clay soil site, and once the effects of the conductive clay response were removed by filtering, ground-penetrating radar (GPR) worked well. HLEM and magnetic results were good to equivocal on the silty and peaty sites, whereas GPR excelled at delineating anomalous features, particularly burials, which yielded clear characteristic diffraction responses. Finally, results for coastal sand sites were disappointing. Such sites appear to be too dynamic to yield useful results.

Growth pattern of dispersed methane hydrates in brine-saturated unconsolidated sediments via joint velocity and resistivity analysis

Although the different morphologies of methane hydrates have been identified in pore spaces, their growth mechanism has not yet been fully resolved. In this study, we performed two sets of experiments using dissolved methane in brine-saturated unconsolidated sediments composed of 95.5 wt% quartz sand and 4.5 wt% kaolin to simulate the formation of dispersed marine gas hydrates. The in situ ultrasonic velocities (VP and VS), electrical resistivity, and methane hydrate saturation (Sh) were simultaneously measured and were jointly analyzed and compared to elastic and electrical rock physics models of various hydrate morphologies in pore spaces to quantify the invisible dynamic evolution of the hydrates’ morphologies. The results indicate that the hydrate growth pattern is characterized by distinct stages as Sh increases. During the first set of experiments, the frame-strengthening morphology was dominant in all of the stages, the contact-cementing morphology occurred infrequently when Sh > 8%, and the pore-filling morphology gradually became identifiable when Sh > 25%. It is proposed that the frame-strengthening hydrates primarily grow on grain surfaces when Sh < 25%, then they extend toward the center of the pore, and finally they begin to plug the pores when Sh > 35%. In the second set of experiment, the geophysical responses were significantly affected by the presence of free gas, and the observed hydrate growth behavior was differed from that observed in the first set of experiments.

Geophysical characterization, design and construction of the Teoloyucan Geophysical Test Site for archaeological and engineering applications, Central Mexico

The Teoloyucan Geophysical Test Site (TGTS) is designed to replicate and simulate scenarios encountered in engineering and archaeological contexts. It is meant for calibrating instruments, testing near-surface geophysical techniques, training students and developing new ideas for shallow surveys and data interpretation. TGTS has been built within the Teolyucan Geomagnetic Observatory of the Universidad Nacional Autónoma de México (UNAM) in the town of Teoloyucan, N of Mexico City. The test site covers an area of 864.00 m2, consisting of 17 structures buried less than 2.00 m deep with regular and irregular shapes, and vertical and horizontal layers simulating stratigraphic sequences. We selected materials similar to those commonly used since the Pre-Hispanic period (adobe, wood, basalt, tezontle), and current materials (concrete, reinforced concrete, plastic containers and PVC). Prior to the construction, the field area was characterized by a combination of non-invasive near-surface geophysical techniques including magnetic gradiometry, electromagnetic induction, ground penetrating radar, seismic refraction tomography and electrical resistivity tomography. The structures' physical properties (density, elastic modules, electrical conductivity and magnetic susceptibility) and their geometrical characteristics are well known, therefore the near-surface geophysical response of the TGTS is controlled. The geophysical results indicate three flat interfaces at approximate depths of 0.45 m, 0.91 m and 1.82 m, respectively. The lithological analysis indicate that the layers are well graded coarse soil layers, including two volcanic hard-horizons (tepetates). Geophysical results coincide with Teoloyucan's lithology, proving that the field area selected is suitable for a test site.

Not all faults are conductive: the effects of seasonal weather changes on the near‐surface expression of faults

ABSTRACTThe near‐surface electrical properties of faults can change with seasonal climate, depending on the nature of the materials filling the faults, e.g. fault gouge vs fractured rock, and be subject to seasonal weather changes. A period of dry weather, especially in a hot climate, can dry out the near‐surface sections of faults that are filled with porous materials, thus increasing the electrical resistivity, yet may not affect a fault that is filled with fine‐grained material that retains moisture well. Conversely, a sustained period of wet weather can fill porous faults with water, thus decreasing the resistivity. To illustrate the impact of seasonal weather changes on the near‐surface properties of faults, we conducted electrical imaging and ground‐penetrating radar surveys during the late spring, summer, and early autumn on the Glen Lyon and Benmore segments of the Ostler Fault, respectively, in the South Canterbury and Central Otago provinces of the South Island of New Zealand. The changes in the response are clear: seasonal weather changes can affect the near‐surface properties of faults, and the geophysical responses from the near‐surface faults can vary depending on the nature of the material filling the fault.

Evidence of magmatism and rifting in the southern superior craton from the Temagami geophysical anomaly

The Temagami Geophysical Anomaly (TGA) is situated in the Paleoproterozoic Huronian Supergroup near the southern margin of Archean Superior craton, along strike from the Temagami Greenstone Belt, 50 km northeast of world-class metal endowment in the Sudbury Igneous Complex. It has been attributed to coincidental geophysical responses from Neoarchean iron formation and a Paleoproterozoic mafic–ultramafic intrusion. The TGA is investigated using integrated magnetotelluric, magnetic, gravity and seismic reflection datasets to further define the geometry of its sources; to define structures in overlying Huronian Supergroup rocks; and to examine the geometrical relationship of Archean basement structure, the Proterozoic mafic–ultramafic intrusion, and Proterozoic rift structures.Geophysical results reveal a strong conductor, interpreted to be iron formation, extending downwards from the Archean basement surface and enveloping the upper northern margin of a large magnetic, dense body at 5 km depth. The geometry of the features indicates mafic–ultramafic intrusion adjacent to greenstone rocks. Alignment of the long-axis of the intrusion with the greenstone belt suggests control on the intrusion process by the Neoarchean structures. The 60x10x10 km TGA intrusion lies within the mantle-plume, rift-related Huron-Nipissing magmatic belt and interpreted to be a member of the 2.491–2.475 Ga East Bull Lake intrusive suite. Based on this genetic relationship, the intrusion probably contributes significantly to Ni-Cu-PGE endowment of the Sudbury region. The spatial correlation of the intrusion with an overlying 20 km wide, 4.5 km deep, fault-bounded rift basin is attributed to crustal subsidence triggered by the intrusion. Seismic reflection and magnetotelluric results show younger Huronian Supergroup rocks record the transition to sedimentation on a laterally-extensive passive margin. An electrically-anisotropic, moderately-conductive layer in the Gowganda Formation is related to alteration during intrusion of the 2.200 Nipissing Diabase.

Drilling formation perception by supervised learning: Model evaluation and parameter analysis

Formation and lithology identification based on well logging curves reflecting geophysical response characteristics is fundamental for drilling planning and reservoir recovery. For the purpose of providing efficient, accurate, and comprehensive insights for drilling operation decisions, the present research evaluates three typical supervised learning algorithms based on machine learning, e.g. Adaboost, decision tree and support vector machine (SVM). By comparing the prediction results from three typical classification algorithms based on performance metrics such as accuracy, precision, recall, F1 score, Adaboost and decision tree are found to present more accurate prediction with relatively higher accuracy, precision, recall and F1 score. The prediction accuracy is positively related to the training data set proportion for all three approaches. Decision tree approach spends less computation time while still provides favorable prediction scores. The accuracy of prediction gradually increases as the number of logging features increases. Accuracy for most parameter combinations beyond four logging parameters can be up to 90%. The neutron porosity and the spontaneous potential are considered as the most influential parameters affecting the prediction accuracy.

Machine learning - A novel approach of well logs similarity based on synchronization measures to predict shear sonic logs

This study proposes a novel approach to predict missing shear sonic log responses more precisely and accurately using similarity patterns of various wells with similar geophysical properties, which is important in decision making and planning of hydrocarbon exploration. Deep Neural Network (DNN) along with the similarity metrics such as Jaccard and Overlap similarities are employed to examine the relationship between the wells. Further, dimensionality reduction techniques including Multi-Dimensional Scaling (MDS) and well-ranking process are applied to extract common geophysical responses of the wells. A higher response indicates the existence of a strong similarity. This can also be verified by the superimposed of well log data. The potential benefits of our novel method are following; (a) it does not follow the zone-by-zone prediction of the missing logs such as rock physics methods, (b) it outputs the uncertainties facilitated that is by the least-squares method. Having the potential of demonstrating shear sonic log prediction in hydrocarbon-bearing zones, which cannot be precisely predicted by the Greenberg-Castagna method that only works in brine-saturated rocks, this approach will provide improved accuracy, where shear sonic logs are missing and need to be predicted for geomechanics, rock physics, and other applications.

The geophysical response of the Goldrush-Fourmile orebody and implications for camp-scale Carlin-type deposit exploration, Cortez District, Nevada

Since discovery of the Goldrush and Fourmile deposits, numerous geophysical surveys have been acquired over the footprint of mineralization and surrounding areas to focus exploration. The Goldrush-Fourmile system extends more than 7 km in strike length and averages approximately 300 m wide. Most of the orebody lies more than 300 m below ground surface and continues to depths of more than 900 m. Direct detection of the ore system using geophysical tools is impeded by several factors. The system is relatively flat lying and sits in the hinge of a doubly plunging anticline. This gives the shallowest zones a very small lateral footprint. The ore consists of thin strata-bound zones of silica-sulfide mineralization with a tight alteration selvage. Structural controls along faults are limited, with only small displacements. Strong petrophysical contrasts exist between unaltered rock units. Finally, the terrain is rugged. Resistivity inversions of airborne electromagnetic data show that mineralization sits within a complex zone of resistivity responses. Close to intrusions, the host stratigraphy is resistive. Away from intrusions, the same stratigraphic units show highly variable but commonly very low resistivities. This suggests the possibility of redistribution of carbon around intrusions during premineralization metasomatism. Within the orebody, sulfide content increases conductivity within individual formations related to the distribution of fine-grained sooty pyrite. The geometry of the Red Hill Anticline is imaged as a positive density contrast in the observed gravity and geologically constrained gravity inversions. The positive density contrast represents the gross geophysical signature of denser carbonates within the Paleozoic stratigraphy. Magnetic surveys are crucial for mapping the distribution of igneous rocks and potentially hornfelsed sedimentary rocks. The challenges associated with exploring for deeply buried Carlin mineralization notwithstanding, the application of innovative geophysical tools tuned to assess specific geologic questions, combined with best-practice geologic and geochemical modeling, is helping drive exploration for additional Carlin-type mineralization across the Cortez District.

Application of seismic sedimentology to the western slope of northern Songliao basin

During the sedimentary period of Saertu reservoir on the western slope of the northern Songliao basin, delta front and shore-shallow lake subfacies are mainly developed, which have the characteristics of few sandstone layers and thin single layer thickness. The lithology of thin layer or thin interlayer can not be distinguished clearly by seismic response on conventional seismic section, and it is difficult to identify them. Geophysical response characteristics of channel sand bodies are defined by well-seismic combination. Under the guidance of seismic sedimentology, the qualitative and quantitative prediction of channel sand bodies is carried out by using 90°-phase conversion, stratal slicing and waveform indication inversion techniques. The results show that the seismic reflection axis is symmetrical with respect to the top and bottom surface of sandstone, and the channel sand body has obvious characteristics and completely corresponds to wave peak reflection. The channel bodies of S1 and S23 reservoir formation are separated respectively into two stages by using the amplitude attributes of stratal slices, and the coincidence rate of reservoir prediction to wells is 78%~84%, with an average of 79.7%. The waveform indicator inversion technique is used to predict the channel sand body thickness of the four stages, and the error of sand body thickness to well is 0~1.6m, with an average of 0.32m.

Magnetic and radiometric signatures of alkaline rocks and gabbros from the Ponta Grossa Arch, southeastern Paraná Basin, Brazil

We present airborne magnetic and radiometric characteristics of some igneous suites from the Ponta Grossa Arch Alkaline Province (southern Brazil), namely the Bairro da Cruz, Banhadão, Barra do Itapirapuã, Mato Preto and Tunas complexes, Barra do Teixeira Phonolite, Itapirapuã Nepheline syenite, and José Fernandes Gabbro. We applied magnetic susceptibility 3D inversions using the total magnetic anomaly data together with its analytic signal of the vertical integral and the vertical integral of the analytic signal for the Bairro da Cruz, José Fernandes, and Tunas intrusions. Gamma-ray spectrometric methods involved analyses of basic, ternary, and ratio maps, thorium-normalized parameters, and radioelement concentrations at sampled data points. Stacked profiles from geophysical data were generated for each rock. The Bairro da Cruz, José Fernandes, and Tunas intrusions are associated with dipolar anomalies with normal polarity whereas their inversions show low magnetic susceptibility values. Gamma-ray spectrometric results allowed us to verify that all complexes are enriched mainly in eTh. Carbonatite bodies showed the highest contents of eTh and eU. Our findings are consistent with geophysical responses of alkaline bodies and impact the understanding of geophysical signatures, especially radiometric ones, of alkaline provinces in Brazil.

Is there hidden potential in Carajás? Insights through the geophysical signature of Cristalino deposit

Geophysical methods are fundamental to exploration of IOCG deposits because they are associated with magnetite, hematite, and sulfides, which can produce different and significant geophysical responses. Because of the frequent occurrence of ferromagnetic minerals, magnetic data have been used extensively in exploration for IOCG deposits. The geophysical data at the Cristalino IOCG deposit in Carajás, Brazil, provide two important insights: 1) magnetic rocks are not fully mapped by total gradient data when these rock units have the same strike direction as the declination of the Earth’s magnetic field in low latitude areas, and 2) deposits that are mainly composed of brecciated to massive sulfide minerals are only weakly magnetic or even non-magnetic. These characteristics are important for exploration programs because qualitative analysis of magnetic data , especially by the commonly used total gradient calculated from total field anomaly, is the main tool for selecting prospects in low latitude regions, and important targets could be undetectable by this method. Gravity data have been extensively applied recently to exploration programs in Carajás, but similarly to magnetic data, the core of the brecciated to massive chalcopyrite mineralization can have lower densities than the surrounding alteration with hematite and magnetite and, therefore, be associated with low gravity anomalies that are not usually targeted in mineral exploration. The electromagnetic response will increase with the increasing connection between sulfides, varying from moderate in vein systems, to moderate to high in stringer and stockwork systems, to high anomalies in massive systems. To better understand the impact of these features on exploration programs, we show in this study that the magnetic, gravity, and electromagnetic responses vary with mineralization end-members. From a vein system formed at greater depths that produce high magnetic and gravity anomalies (associated with iron oxides from alteration) and moderate electromagnetic anomaly (poor connection between sulfides), to stringer, stockwork, and massive systems formed at shallower depths that exhibit lower magnetic and gravity anomalies but a high electromagnetic anomaly. Our analyses indicate that it is possible that the deposits mostly associated with massive sulfide could have been missed, and there could be significant hidden potential in the Carajás region. We illustrate that the detection of such potential depends on the quantitative integration of magnetic data with gravity and electromagnetic data through the inversion and multiphysics approach.

Integrated Approach to Investigating Historic Cemeteries

Ground-penetrating radar (GPR) and terrestrial laser scanning (TLS) surveys were conducted at a historic cemetery at Cape Canaveral Air Force Station, Florida, U.S., in order to confirm the presence of burials corresponding to grave markers and detect potential unmarked burials. Noise in the GPR data from surface features and subtle terrain differences must be addressed to determine the extent of anomalies of interest. We use singular value decomposition (SVD) to isolate and remove energy from GPR data. SVD allows one to remove unwanted signals that traditional processing techniques cannot. With SVD filtering, we resolve an anomaly adjacent to confirmed burials otherwise overprinted by unwanted signal. The migration of SVD-filtered data produces more distinct, spatially constrained point reflectors. Ground elevation is derived from georeferenced TLS data and compared to that from airborne laser scanning (ALS) to highlight subtle terrain that can assist data interpretation. TLS elevations show a subtle modern mound over the burial plot where ALS elevations show a depression. The targets of interest are approximately 20–30 cm higher in elevation if a topographic correction is performed using TLS versus ALS. In archaeological applications, a notable change is often recorded at the sub-meter scale. The combined approach presented here better resolves geophysical response of buried features and their positions in the ground relative to each other.