Common Geophysical Methods Research Articles

Natural source-field induced polarisation exploration of an iron-oxide copper-gold (IOCG) deposit under thick cover

Induced polarisation (IP) is a common geophysical method of exploration for disseminated sulphides. However, in areas with deep (>200 m) and conductive (10 Ωm or less) cover, the technique is less successful as it requires a significant transmitter source and large-offset dipoles. An alternative approach is to use natural-variations in Earth’s external magnetic field as the polarising source of the signal, known as natural-field IP. This paper presents a study of extracting IP information from electrical dipole observations during a broadband magnetotelluric (MT) program with a 9 km by 9 km grid of 100 sites above the Vulcan IOCG deposit beneath 750 m of cover, ∼40 km north of the Olympic Dam IOCG mine in South Australia. Inter-site transfer functions between 95 sites and five reference sites at the southeast and southwest corners of the grid were computed to determine a phase shift between horizontal electric fields in the bandwidth of 1–100 s period. Phase shifts of up to – 5 degrees were centred on a region of brecciated hematite where drill holes intersected pyrite, and an inferred fault-zone from passive seismics that marks the boundary between upper crust that is resistive (>100 Ω.m) with high magnetic susceptibility to the south of the fault, and a region of conductive crust (<10 Ω.m) which is low magnetic susceptibility. Our study suggests that the natural-field IP method can identify regions of polarizable minerals beneath deep cover where artificial power sources cannot be feasibly deployed. Such surveys are cheaper, safer, and easier to deploy as only receivers are required, and 3D coverage can be obtained as the source-field is of much larger dimension than the survey array. In addition, the natural-field IP signals are observed as part of the MT program so that both electrical resistivity and polarisation parameters can be determined.

Identification of sliding surface using electrical-resistivity tomography for landslide mitigation: A case study of the Cibitung Landslide

Landslide mitigation efforts require a knowledge of the geometry and depth of the sliding surface present in a landslide body. Electrical resistivity tomography is the most common geophysical method used in landslide investigation. This paper presents the results of a series of electrical resistivity tomography surveys performed using a dipole-dipole array configuration to identify the sliding surfaces within a landslide body located in the Cibitung landslide area. The ERT images parallel the landslide body suggest that the sliding surface located below the crown is characterized by a bedrock layer with very low resistivity values. This bedrock layer continues down-slope at a maximum depth of 9 m below the ground surface. In contrast, the landslide body is characterized by a higher resistivity value. Based on the interpretation of the ERT images, the landslide involved a non-circular deep sliding surface. The results of this study have been used, in combination with the geotechnical drilling data, to construct the landslide cross-section necessary to analyze landslide stability and subsequently to recommend a landslide stabilization measure.

Exploration of Erdaohe Silver Polymetallic Deposit in Inner Mongolia based on Induced Polarization Method

The exploration of Erdaohe Cu-Ag-Pb-Zn deposit in Zhalantun City, Inner Mongolia has entered the stage of blind exploration. It is a difficult problem at this stage whether common geophysical methods can find anomalies caused by 300∼1000 m or deeper Pb-Zn deposits in this area. Whether the problem is solved or not has a strong practical significance for the current and future prospecting work in this area. According to the theory of induced polarization sounding in time domain, it is considered in this paper that when the parameters such as high power, suitable device, and time delay are well coupled with the deep blind ore, it is possible to find anomalies caused by deep lead-zinc ore larger than 300 m. In order to prove the feasibility of the deep application of this method, the high-power induced polarization sounding experiment with different devices and different power supply cycles was carried out in Erdaohe copper-silver lead-zinc mine. The test results show that by adopting the measurement method of the encrypted equipotential device and selecting the appropriate polar distance, when the power supply period is 32 s, the time delay is 200 s and the sampling width is 200 s, a good deep lead-zinc mine anomaly can be found. The equipotential symmetrical quadrupole sounding MN/AB = 1/10 and the depth correction coefficient is 0.41 for inversion can accurately determine the anomaly location, which is worthy of popularization and application in this area. The ore bodies in this area are controlled by structures and distributed in lenticular layers along the fault footwall interlayer fracture zone. The ore bodies may change with the occurrence of the fracture surface. The characteristics of low resistivity, low slow, and high polarization of IP and single peak anomaly of IP sounding are important signs of lead-zinc prospecting in this area.

STUDI POTENSI AIR TANAH MENGGUNAKAN METODE GEOLISTRIK KONFIGURASI SCHLUMBERGER DAN WENNER DI KAMPUNG NAMBON KABUPATEN JAYAPURA

Kampung Nambon is a village in Jayapura regency which has no access to clean water. Groundwater is an alternative water source for villages which are prone to surface water sources. Groundwater aquifers are porous and permeable subsurface layers which can hold and transmit groundwater. Geoelectric method is a common geophysical method for groundwater exploration due to direct relationship between resistivity and water content. The objective of this study is to characterize subsurface structure of Kampung Nambon and to estimate the depth of potential groundwater aquifer using Vertical Electrical Sounding (VES) of Schlumberger array and Electrical Resistivity Tomography (ERT) of Wenner array. An ERT line of 100 metres whose midpoint is the sounding point of VES with maximum electrode spacing (AB/2) of 250 metres were performed. The results show that subsurface at Kampung Nambon is heterogeneous with significant lateral variation where karst formation was estimated at depth range of 5 – 15 meters. This karst formation might contain groundwater flow as indicated by relatively low resistivity values compared to high resistivity background of Limestone from Jayapura formation (Qpj).

Review on the Geophysical and UAV-Based Methods Applied to Landslides

Landslides (LS) represent geomorphological processes that can induce changes over time in the physical, hydrogeological, and mechanical properties of the involved materials. For geohazard assessment, the variations of these properties might be detected by a wide range of non-intrusive techniques, which can sometimes be confusing due to their significant variation in accuracy, suitability, coverage area, logistics, timescale, cost, and integration potential; this paper reviews common geophysical methods (GM) categorized as Emitted Seismic and Ambient Noise based and proposes an integrated approach between them for improving landslide studies; this level of integration (among themselves) is an important step ahead of integrating geophysical data with remote sensing data. The aforementioned GMs help to construct a framework based on physical properties that may be linked with site characterization (e.g., a landslide and its subsurface channel geometry, recharge pathways, rock fragments, mass flow rate, etc.) and dynamics (e.g., quantification of the rheology, saturation, fracture process, toe erosion, mass flow rate, deformation marks and spatiotemporally dependent geogenic pore-water pressure feedback through a joint analysis of geophysical time series, displacement and hydrometeorological measurements from the ground, air and space). A review of the use of unmanned aerial vehicles (UAV) based photogrammetry for the investigation of landslides was also conducted to highlight the latest advancement and discuss the synergy between UAV and geophysical in four possible broader areas: (i) survey planning, (ii) LS investigation, (iii) LS dynamics and (iv) presentation of results in GIS environment. Additionally, endogenous source mechanisms lead to the appearance of deformation marks on the surface and provide ground for the integrated use of UAV and geophysical monitoring for landslide early warning systems. Further development in this area requires UAVs to adopt more multispectral and other advanced sensors where their data are integrated with the geophysical one as well as the climatic data to enable Artificial Intelligent based prediction of LS.

Ground Penetrating Radar Measurements in Shallow Water Environments—A Case Study

In this study, we investigate GPR measurements in freshwater of less than 5 m at four different locations to derive rules of thumb in terms of depth penetration, resolution, and material contrasts of the method for 200 and 400 MHz antennas under field conditions. The objective is to improve the attractiveness of the method for archaeological issues in water, as there are hardly any studies on this subject so far. The depth penetration of 2–4 m is negligibly influenced by the choice of the 200 or 400 MHz antenna. Organic material in the water column also does not affect the water depth but offers new fields of applications for mapping and volume estimation of biomass in lakes with GPR. The horizontal resolution in the cm range in the direction of the profile and in the dm range across the profile could not be improved by the narrow antenna radiation pattern of &lt;30° at the 3 dB level. In the crossline direction, the use of an antenna array would be necessary here. Still, the narrow antenna pattern reduces side reflections. Most common archaeological material contrasts can be resolved with the method. The method shows reflection coefficients &gt;0.1 for materials of &lt;80% porosity to the water column and for materials of &lt;25% porosity and of &gt;45% porosity to water-saturated sand. Large reflection coefficients also show, for example, granite to sand and gyttja to wood. The water column has a considerable effect on the data quality of the 400 MHz antenna from a depth of 2 m due to the antenna ringing. Furthermore, multiples must be expected in a water column &lt;0.5 m. The method can especially complement the common geophysical methods of seismics and geoelectrics to exclude material ambiguities. The major advantage is the simple setting of the land equipment in the water.

The forward modeling for surface-borehole observation responses of polarized sphere in uniform electric field

Surface-borehole induced polarization method is a common geophysical method in metal exploration. The forward calculation results of the borehole observation responses can provide reference and support for practical work. In particular, the forward results of spherical polarized target can be equivalent to massive ore body or karst caves and other low-resistivity anomalies. In this paper, the forward simulation of borehole induced polarization responses is carried out by using the electric dipole model equivalent spherical polarized body. The results show that the polarization degree of the excitation source at different positions is different, and the closer the proximity is, the stronger the secondary field anomaly is. Borehole observation has advantages for the identification of deep ore bodies. Under the same offset condition, the closer the distance from the borehole is, the stronger the responses will be. The results of this paper can provide reference and basis for relevant research work.

Investigating of soil features and landslide risk in Western-Atakent (İstanbul) using resistivity, MASW, Microtremor and boreholes methods

AbstractIn Western-Atakent (İstanbul), population density is increasing day by day and settlement areas are expanding. Soil properties and landslide conditions of these expanding regions must be absolutely examined. In the geophysics, there are many methods used to investigate landslide risks and geotechnical structure. The most common geophysical methods used for this purpose are the Electrical resistivity tomography (ERT), Multi-channel Analysis of Surface Waves (MASW) and Microtremor Survey Method (MSM) methods. These methods are very successful techniques for defining underground layers as geological structures, stratigraphic elements, soil layer thickness and landslide. Because of that reason in this study, soil properties and possibility of landslides of the Western-Atakent (İstanbul) region were investigated by using ERT, MASW, MSM and drilling methods. In this study the first stage, electrical resistivity data have been measured using dipole-dipole method on two profiles for ERT. In the second stage, MASW measurements have been made at 25 points on 5 seismic profiles in the field. In the third stage, MSM measurements have been made to determine the fundamental period in the 5-measure station in the study area. In the fourth and final stage, 10-pieces boreholes with a depth of 20 m were drilled to reveal the lithological structure of the study area. As a result of the evaluation of all data, parts of the region that could form landslides were revealed.

Interpretasi Bawah Permukaan Daerah Penambangan Batuan Andesit Awang Bangkal Kabupaten Banjar Kalimantan Selatan Menggunakan Metode Magnetik

This research has been done in the mining area of andesite rock of Awang Bangkal village by using magnetic method. Magnetic method is one of the most common geophysical methods used for preliminary surveys on petroleum, earth, and mineral rock exploration. It is intended to determine the subsurface distribution in the area by performing qualitative and quantitative interpretations based on the magnetism value of the earth. The result of data processing using geosoft oasis montaj software shows that the total magnetic field anomaly is -807,3 to -361,7 nT. Qualitative interpretation is done by looking at the contour map of the anomaly, the result of upward continuation and the reduction to the poles. While the interpretation is done by making a modeling of the path that has been determined using mag2dc software. The model shows that the rocks that dominate the area are andesite rocks with their susceptibility value of 0.0992 to 0.2305 in SI units.

Geoelectrical Investigation Along Miri Coast, East Malaysia: Evaluate the Vulnerability of Coastal Aquifer

The coastal zones are subject to rapid development with growing and conflicting demands on the natural resources, and often they are subject to irreversible degradation. Many coastal aquifers around the world are experiencing some level of salinity encroachment. The most common geophysical method used for the groundwater investigation is Geoelectrical method. In this study, a total of 15 vertical electrical sounding (VES) were carried out using Schlumberger electrode arrangement along Miri coast. The field data was interpreted by using curve matching techniques using IPI2Win software. 53% of the area is dominated by ‘Q’ type curve indicates decreasing resistivity with depth. The resistivity spatial maps and pseudo cross-sections reveal that the low-resistivity values were observed in the northern side of the study area due to the natural saline water intrusion in the deeper aquifer and anthropogenic influence in the shallow aquifer.

CONTRIBUTION OF DEEP ELECTRICAL RESISTIVITY TOMOGRAPHY TECHNIQUE TO HYDROGEOLOGICAL STUDIES: CASES FROM AREAS IN KAVALA (NORTH GREECE)

The most common geophysical method widely used in hydrogeological surveys concerning deep investigations (150-300m of depth) is the resistivity method and particularly the Vertical Electric Sounding (VES) using the Schlumberger array. VES interpretations assume 1D geoelectrical structure yet it is obvious that such an interpretation assumption is not valid in many cases where 2D and 3D geological features exist. In such cases the application of geoelectrical techniques which can provide both vertical and lateral information concerning the resistivity variations is required. Techniques such as the electrical resistivity tomography, mostly used for the 2D and 3D geoelectrical mapping of near surface applications can be adapted to be used for larger investigation depths provided that modified equipment (viz. cables) is used. In the present paper, the application of deep electrical resistivity tomography (ERT) techniques is applied. ERT array of 21 electrodes, at a distance of 50 meters between them (total length 1000 meters) has been used in several studied areas located in the prefecture of Kavala (North Greece). In several cases near surface structure has been compared with VLF data. The aim of the survey was to study in detail the geological-hydrogeological structure the area of interest in order to suggest the best location for the construction of hydrowells with the most promising results. The 2D images of the geological structure down to the depth of at least 200 meters allowed the better understanding of the behaviour of layered geological formations, since in several cases resistivity values have been calibrated with data from pre-existing boreholes.

Effects of submarine groundwater discharge on the present‐day extent of relict submarine permafrost and gas hydrate stability on the Beaufort Sea continental shelf

AbstractWe investigate the role of submarine groundwater discharge on the offshore temperature and salinity field and its effect on the present‐day extent of submarine permafrost and gas hydrate stability on the North American Beaufort Shelf with a two‐dimensional numerical model based on the finite volume method. This study finds that submarine groundwater discharge can play a large role in submarine permafrost evolution and gas hydrate stability, suggesting that local hydrology may control the evolution of submarine permafrost as strongly as does sea level or paleoclimatic conditions. Submarine permafrost evolution shows transient behavior over potentially long time scales (e.g., several glacial cycles) before a balance of density‐ and pressure‐driven flows is established with the permeability variations imposed by the overlying permafrost layer. The “detectable” offshore permafrost extent is related to the quasi‐stationary location of the saltwater‐freshwater transition. Larger values of submarine groundwater discharge allow permafrost to extend farther offshore because fresh pore water preserves relict ice. Therefore, differences in the permafrost extent at locations that share similar paleoclimatic history may be explained in part by differences in the local hydrology. Gas hydrate stability on the North American Beaufort Shelf may be more widespread than currently thought because low‐ice saturation, highly degraded submarine permafrost likely exists beyond the boundary detectable by common geophysical methods.

Denoising magnetic data using steering kernel regression

ABSTRACTGround‐based magnetic surveying is a common geophysical method to explore near‐surface environments in a non‐destructive manner. In many typical applications (such as archaeological prospection), the resulting anomaly maps are often characterized by low signal‐to‐noise ratios and, thus, the suppression of noise is a key step in data processing. Here, we propose the steering kernel regression (SKR) method to denoise magnetic data sets. SKR has been recently developed to suppress random noise in images and video sequences. The core of the method is the steering kernel function which represents a robust estimate of local image structure. Using such a kernel within an iterative regression based denoising framework, helps to minimize image blurring and to preserve the underlying structures such as edges and corners. Because such filter characteristics are desirable for random noise attenuation in potential field data sets, we apply the SKR method for processing high‐resolution ground‐based magnetic data as they are typically collected in archaeological applications. We test and evaluate the SKR method using synthetic and field data examples and also compare it to more commonly employed denoising strategies relying, for example, on fixed filter masks (e.g., Gaussian filters). Our results show that the SKR method is successful in removing random and acquisition related noise present in our data. Concurrently, it preserves the local image structure including the amplitudes of anomalies. As demonstrated by derivative based transformations, the mentioned filter characteristics significantly impact subsequent processing steps and, therefore, result in an improved analysis and interpretation of magnetic data. Thus, the method can be considered as a promising and novel approach for denoising ground‐based magnetic data.