Application and Significance of Geological, Geochemical, and Geophysical Methods in the Nanpo Gold Field in Laos

As the main part of the Indosinian metallogenic province in the eastern part of the Tethys metallogenic domain, Southeast Asia has experienced multiple stages of tectonic magnetic activities accompanied by the formation of rich mineral resources. However, due to the undeveloped economy, low degree of geological work, dense vegetation cover, and lack of obvious prospecting marks, traditional geological prospecting work in the area is not optimal. Consequently, the combination of high-precision geophysics and geochemistry has become an important method of looking for ore bodies deep underground in this area. The Nanpo gold deposit is a hydrothermal gold deposit that occurs in the Indosinian felsic volcanic rock body, and its mineralization is closely related to felsic magmatism. This study carried out comprehensive geophysical and geochemical exploration methods of soil geochemical survey, induced polarization (IP) survey, and audio-frequency magnetotelluric (AMT) survey. Based on the characteristics of geophysical and geochemical anomalies, geological inference, and interpretation, the integrated geophysical and geochemical prospecting criteria of the ore area have been determined: The large-scale and overlapping Au-Ag-Cu anomaly area in the host felsic magmatic rocks (mainly diorite, monzodiorite and granodiorite) is a favorable metallogenic area. Two anomalies, P1–H1 and P3–H6, with the best metallogenetic conditions and the deepest extensions of the known ore bodies, were further selected as engineering verification targets. After the study of the drill core, gold (mineralized) bodies consistent with the anomalies were found, indicating that the combined method is suitable for the exploration of mineral resources in this area, and the prospecting effect is good. At the same time, the metallogenic prediction shows that the deep part of the mining area still has great metallogenic prospects and prospecting potential. The characteristics of geophysical and geochemical anomalies and prospecting experience in the study area can provide references for the prospecting of hydrothermal gold deposits in the Luang Prabang–Loei structural belt.

Application of 2D Resistivity Modelling to Delineate Lamproites in Parts of North‑Eastern Dharwar Craton

Geophysical methods have extensively been used in exploration of Lamproite bodies. Lamproites are significant source rocks of primary diamond deposits other than Kimberlites. The Eastern Dharwar Craton is unique in the way that it hosts numerous Precambrian Lamproites confined to the crescent-shaped Paleo-Mesoproterozoic Cuddapah Basin and its north-western and north-eastern margins. In the present study electrical method was used as a tool for delineation of Lamproite bodies in contrast with country rocks in Gundrapally, Vattikode and Marepally regions in parts of the North Eastern Dharwar Craton in Telangana State, India. The electrical profiling method was conducted in different direction to identify the variation of the lithology of the area due to the anisotropic nature and smaller size of the 2-D intrusive bodies adopting the Wenner electrode configuration. The electrical properties of the Lamproites tended to change with their composition due to weathering and the presence of Olivine and K-Al rich composition. The electrical response on the Lamproite pipes is shows a decrease in resistivity concerning the country-rock in the area.

An explosive component in a December 2020 Milan earthquake suggests outgassing of deeply recycled carbon

Carbon dragged at sub-arc depths and sequestered in the asthenospheric upper mantle during cold subduction is potentially released after millions of years during the breakup of continental plates. However, it is unclear whether these deep-carbon reservoirs can be locally remobilized on shorter-term timescales. Here we reveal the fate of carbon released during cold subduction by analyzing an anomalously deep earthquake in December 2020 in the lithospheric mantle beneath Milan (Italy), above a deep-carbon reservoir previously imaged in the mantle wedge by geophysical methods. We show that the earthquake source moment tensor includes a major explosive component that we ascribe to carbon-rich melt/fluid migration along upper-mantle shear zones and rapid release of about 17,000 tons of carbon dioxide when ascending melts exit the carbonate stability field. Our results underline the importance of carbon-rich melts at active continental margins for emission budgets and suggest their potential episodic contributions to atmospheric carbon dioxide.

Identification of subsidence hazard zone by integrating engineering geological mapping and electrical resistivity tomography in Gunung Kidul karst area, Indonesia

The presence of natural cavities in karst morphology may cause severe civil engineering and environmental management problems. Karst formations will limit the expansion of urbanization, especially infrastructure development in limestone areas. Geophysical methods, especially electrical resistivity tomography (ERT) techniques, are effective and efficient solutions to detect voids below the surface. This study aimed to develop a subsidence hazard map as basic information for infrastructure development. The identification was made by measuring electrical resistivity tomography on eight profiles in the infrastructure development plan. In addition, it was also supported by geological mapping, particularly the structural geology and types of rocks around the site. The research area consists of massive limestone, bedded limestone, and cavity limestone with generally north-south joints. The analysis of geological mapping data and electrical resistivity tomography measurements showed that the cavity limestone was identified with a north-south elongated pattern in line with the fracture pattern found on the surface at the research area. The surface lithology type, the geological structures density, and the subsurface lithology were used to develop a subsidence hazard map. This information is beneficial in determining the safe location of infrastructure development based on disaster risk mitigation.

Application of geophysical methods in the extrapolation of sedimentological data on unmastered areas of deposits (on the example of Pennsylvanian carbonate reservoir, Akanskoye oilfield, east of Russian Platform)

A feature of the oil-bearing carbonate deposits of the lower Pennsylvanian in the east of the Russian platform is their rapid vertical and horizontal change. It is often difficult to make correlations between sections, especially in the absence of core data when using only geophysical data. In addition, not all facies are reliably identified and traceable from log data and not all have high reservoir properties. Authors made an attempt to trace the promising facies both to adjacent wells and, in general, to the entire field area using core study results and translation of these results using log and seismic data. The data showed pinching of rocks with high reservoir characteristics in the direction of the selected profile (from south to north within the field). Coastal shallow water facies, represented by Grainstones and Packstones, with high reservoir properties in the south of the field, are replaced by lagoon facies and facies of subaerial exposures, represented by Wakestones and Mudstones with low reservoir characteristics, in the north of the field. The authors suggest that this approach can be applicable for rocks both in this region and for areas with a similar structure. Keywords: pinch-out; well data; seismic data; limestone; facies; reservoir rocks.

The role of geophysics in the construction and stability assessment of tunnels

Geophysics play a vital role in the constructions of any major manmade structures in the world. One of those being the tunnels. In depth understanding of geophysical methods and a lot of information are needed in order to design a tunnel construction project. Comprehensive investigation on the ground condition has to be done before the field preparation study that will determine the stand-up time and the groundwater condition that may disrupt the tunnel construction. For tunnel stability assessment, an integration of geophysical methods is a must in order to obtain the most accurate results. Satellite imaging interpretation emphasizes on the structural tracing of negative lineament while field mapping emphasizes on location of underground seepage and major tectonic structures such as faults, joints and shear zones. Geoelectrical resistivity tomography survey is able to identify the differences in resistivity of Earth’s materials based on the water content inside of them. The best course of remediation could only be chosen once the output from all these studies are made available.

Comprehensive analysis and assessment of prospective gold-ore zones using modern geophysical methods

В статье рассматривается комплексный анализ геологической и геофизической информации по проводимым исследованиям с целью выделения перспективных участков для проведения дальнейших геологоразведочных работ. Цель работы. Провести комплексный анализ и оценку перспективных золоторудных зон с применением современных геофизических методов. Методы работы. Вертикальное электрическое зондирование методом вызванной поляризации, метод вызванной поляризации в модификации срединного градиента, геоэлектрические разрезы по профилям, аномалии поляризуемости поля электрических сопротивлений, метод ТЗ электротомографии. Результаты работы. Авторами в 2019 г. в северной части участка на площади 2,87 кв. км были выполнены работы СГ-ВП по сети 50×10м с MN – 10 м. и АВ – 2000 м. Измерения DU и φвппроводились на частоте 1.22 Гц измерителями МЭРИ – 24 и ЭИН – 209 (режим измерения частотный). Использовался генератор ГЭР – 5000 – 25 (мощность 5 кВт). По результатам измерений построены карты изолиний поляризуемости и кажущегося сопротивления. Следует отметить, что при определении сдвига фаз φвп в частотном режиме сигнал, используемый для измерения сдвига фаз при равных DU с импульсным режимом измерений в 30-100 раз выше. Это основное преимущество фазовых измерений над импульсными. Как видно из приведенного сравнения уменьшение длины приемной линии и шага наблюдений позволило увеличить детальность исследований СГ-ВП и выйти на уровень рудных тел, заданных в техническом задании. По результатам ТЗ электротомографии и ВЭЗ-ВП получены близкие результаты, хотя методически и теоретически ТЗ обладает в данных условиях большей детальностью и информативностью. По результатам геофизических работ построены карты изолиний и графиков ρк и φвп, выявлена их связь с рудовмещающими структурами и отдельными рудными телами. Всего выявлено 14 локальных аномалий поляризуемости связанных с золото-кварц-сульфидными и сульфидными рудными телами. Выявлены площадные аномалии повышенной поляризуемости и низких значений электрического сопротивления СГ-ВП вдоль Западной рудно-тектонической зоны. Они связаны с метасоматическими процессами в пределах Западной зоны сбросо-сдвигов. На площади работ отмечены слабоконтрастные аномалии магнитного поля в северо-западной части Северной зоны. Отмечено сильное влияние рельефа на характер магнитного поля, вследствие чего наблюдается корреляция хребтов с минимумами магнитного поля. The article discusses a comprehensive analysis of geological and geophysical information on ongoing research in order to identify promising areas for further exploration. Aim. Conduct a comprehensive analysis and assessment of promising gold ore zones using modern geophysical methods. Methods. Vertical electrical sounding by the induced polarization method, the induced polarization method in the modification of the median gradient, geoelectric sections along the profiles, anomalies of the polarizability of the electrical resistance field, TK method of electrotomography. Results. The authors in 2019 in the northern part of the site on an area of 2.87 sq. km SG-VP works were performed on a network of 50 × 10m with MN -10m. and AB - 2000 m. Measurements of DU and φvp were carried out at a frequency of 1.22 Hz with meters MARI - 24 and EIN - 209 (frequency measurement mode). Used generator GER - 5000 - 25 (power 5 kW). Based on the measurement results, maps of isolines of polarizability and apparent resistivity were constructed. It should be noted that when determining the phase shift φvp in the frequency mode, the signal used to measure the phase shift at equal DU with the pulse measurement mode is 30-100 times higher. This is the main advantage of phase measurements over pulsed ones. As can be seen from the above comparison, a decrease in the length of the receiving line and the step of observations made it possible to increase the detail of SG-VP studies and to reach the level of ore bodies specified in the terms of reference. Based on the results of the technical specification for electrotomography and VES-IP, similar results were obtained, although methodologically and theoretically, the technical specification has greater detail and information content under these conditions. Based on the results of geophysical work, maps of isolines and graphs of ρк and φвп were constructed, their relationship with ore-bearing structures and individual ore bodies was revealed. In total, 14 local anomalies of polarizability associated with gold-quartz-sulfide and sulfide ore bodies have been identified. Areal anomalies of increased polarizability and low values of electrical resistance of SG-VP were revealed along the Western ore-tectonic zone. They are associated with metasomatic processes within the Western fault-strike zone. Low-contrast magnetic field anomalies in the northwestern part of the Northern Zone were noted in the survey area. A strong influence of the relief on the nature of the magnetic field was noted, as a result of which a correlation of the ridges with the minima of the magnetic field is observed