Source Audio Magnetotellurics Research Articles

Exploring Medium and Low-Temperature Convective Geothermal Resources Through Controlled Source Audio Magnetotelluric Mothod

Medium and low-temperature convective geothermal resources are a crucial component of China's geothermal wealth, primarily concentrated in the eastern coastal regions. To maximize the utilization of these resources, geophysical surveys are essential, with controlled source audio magnetotelluric(CSAMT) being a prominent and effective method employed in geothermal resource assessments. This study focuses on applying CSAMT to survey specific geothermal sites, including the Xiu Yan Gou Tang Hot Spring, Fengcheng Gou Tang Hot Spring, and Hai Cheng Xi Huang di Geothermal Field. The results obtained through this method provide detailed insights into the distribution of both deep and shallow thermal storage spaces within these fields, shedding light on the intricate thermal circulation systems associated with them. These findings serve as a reliable foundation for planning and executing further geothermal resource development projects. CSAMT emerges as a comprehensive tool for understanding the nuances of medium and low-temperature convective geothermal resources.

Tectonic Setting of the Kenya Rift in the Nakuru Area, Based on Geophysical Prospecting

In this paper, we present results of tectonic and geophysical investigations in the Kenya Rift valley, in the Nakuru area. We compiled a detailed geological map of the area based on published earlier works, well data and satellite imagery. The map was then integrated with original fieldwork and cross sections were constructed. In key areas, we then performed geophysical survey using Electrical Resistivity Tomography (ERT), Hybrid Source Audio MagnetoTelluric (HSAMT), and single station passive seismic measurements (HVSR). In the study area, a volcano-sedimentary succession of the Neogene-Quaternary age characterized by basalts, trachytes, pyroclastic rocks, and tephra with intercalated lacustrine and fluvial deposits crops out. Faulting linked with rift development is evident and occurs throughout the area crosscutting all rock units. We show a rotation of the extension in this portion of the Kenya rift with the NE–SW extension direction of a Neogene-Middle Pleistocene age, followed by the E–W extension direction of an Upper Pleistocene-Present age. Geophysical investigations allowed to outline main lithostratigraphic units and tectonic features at depth and were also useful to infer main cataclasites and fractured rock bodies, the primary paths for water flow in rocks. These investigations are integrated in a larger EU H2020 Programme aimed to produce a geological and hydrogeological model of the area to develop a sustainable water management system.

Boat-towed radio-magnetotelluric and controlled source audio-magnetotelluric study to resolve fracture zones at Äspö Hard Rock Laboratory site, Sweden

SUMMARY Boat-towed radio-magnetotelluric (RMT) measurements using signals between 14 and 250 kHz have attracted increasing attention in the near-surface applications for shallow water and archipelago areas. A few large-scale underground infrastructure projects, such as the Stockholm bypass in Sweden, are planned to pass underneath such water zones. However, in cases with high water salinity, RMT signals have a penetration depth of a few metres and do not reach the geological structures of interest in the underlying sediments and bedrock. To overcome this problem, controlled source signals at lower frequencies of 1.25 to 12.5 kHz can be utilized to improve the penetration depth and to enhance the resolution for modelling deeper underwater structures. Joint utilization of boat-towed RMT and controlled source audio-magnetotellurics (CSAMT) was tested for the first time at the Äspö Hard Rock Laboratory (HRL) site in south-eastern Sweden to demonstrate acquisition efficiency and improved resolution to model fracture zones along a 600-m long profile. Pronounced galvanic distortion effects observed in 1-D inversion models of the CSAMT data as well as the predominantly 2-D geological structures at this site motivated usage of 2-D inversion. Two standard academic inversion codes, EMILIA and MARE2DEM, were used to invert the RMT and CSAMT data. EMILIA, an object-oriented Gauss–Newton inversion code with modules for 2-D finite difference and 1-D semi-analytical solutions, was used to invert the RMT and CSAMT data separately and jointly under the plane-wave approximation for 2-D models. MARE2DEM, a Gauss–Newton inversion code for controlled source electromagnetic 2.5-D finite element solution, was modified to allow for inversions of RMT and CSAMT data accounting for source effects. Results of EMILIA and MARE2DEM reveal the previously known fracture zones in the models. The 2-D joint inversions of RMT and CSAMT data carried out with EMILIA and MARE2DEM show clear improvement compared with 2-D single inversions, especially in imaging uncertain fracture zones analysed in a previous study. Our results show that boat-towed RMT and CSAMT data acquisition systems can be utilized for detailed 2-D or 3-D surveys to characterize near-surface structures underneath shallow water areas. Potential future applications may include geo-engineering, geohazard investigations and mineral exploration.

APPLICATION OF CONTROLLED SOURCE AUDIO MAGNETOTELLURIC (CSAMT) AT GEOTHERMAL

<p class="abstrak">CSAMT or Controlled Source Audio-Magnetotelluric is one of the Geophysics methods to determine the resistivity of rock under earth surface. CSAMT method utilizes artificial stream and injected into the ground, the frequency of artificial sources ranging from 0.1 Hz to 10 kHz, CSAMT data source effect correction is inverted. From the inversion results showed that there is a layer having resistivity values ranged between 2.5 Ω.m – 15 Ω.m, which is interpreted that the layer is clay.</p><p class="abstrak"><span lang="EN-US"><br /></span></p>

Controlled Source Audio Magneto Telluric (CSAMT) Studies for Uranium Exploration in Durgi Area, Palnad Sub-basin, Cuddapah Basin, India

ABSTRACT Cuddapah basin is known for hosting unconformity proximal uranium deposits viz., Lambapur, Peddagattu, Chitirial and Koppunuru along the northern margin of the basin. It is well known that these deposits are mostly associated with basement granitoids in Srisailam Sub-basin, and with cover sediments in Palnad sub-basin where basement topography and fault/fracture system influence the fluid flow causing basement alteration and ore deposition. Geological setup, surface manifestation of uranium anomalies and association of the hydro-uranium anomalies near Durgi area in southern part of the Palnad sub-basin, have prompted detail investigation by geophysical methods to probe greater depths. Controlled Source Audio Magneto Telluric (CSAMT) survey conducted over five decades of frequency (0.1-9600 Hz) delineated the various lithounits of Kurnool and Nallamalai Groups along with their thicknesses as there exist an appreciable resistivity contrast. Interpretation of CSAMT sounding data are constrained by resistivity logs and litholog data obtained from the boreholes drilled within the basin indicated three to four layered structure. Sub-surface 2-D and 3-D geo-electrical models are simulated by stitching 1-D layered inverted resistivity earth models. Stitched 1-D inverted resistivity sections revealed the unconformity between the Kurnool Group and Nallamalai Group along with basement undulations. The faults/fractures delineated from the CSAMT data corroborated well with the results of gravity data acquired over the same area. Simulated 3-D voxel resistivity model helped in visualising the faults/fractures, their depth extent, thickness of the Banganapalle quartzite and basement configuration. Integrated interpretation of CSAMT, gravity and borehole data facilitated in delineating the unconformity and the structural features favourable for uranium mineralisation in deeper parts of the Palnad sub-basin.

Multi-geophysical approaches to detect karst channels underground — A case study in Mengzi of Yunnan Province, China

Mengzi locates in the south 20km away from the outlet of Nandong subsurface river, and has been suffering from water deficiency in recent years. It is necessary to find out the water resources underground according to the geological characteristics such as the positions and buried depths of the underground river to improve the civil and industrial environments. Due to the adverse factors such as topographic relief, bare rocks in karst terrains, the geophysical approaches, such as Controlled Source Audio Magnetotellurics and Seismic Refraction Tomography, were used to roughly identify faults and fracture zones by the geophysical features of low resistivity and low velocity, and then used the mise-a-la-masse method to judge which faults and fracture zones should be the potential channels of the subsurface river. Five anomalies were recognized along the profile of 2.4km long and showed that the northeast river system has several branches. Drilling data have proved that the first borehole indicated a water bearing channel by a characteristics of rock core of river sands and gravels deposition, the second one encountered water-filled fracture zone with abundant water, and the third one exposed mud-filled fracture zone without sustainable water. The results from this case study show that the combination of Controlled Source Audio Magnetotellurics, Seismic Refraction Tomography and mise-a-la-Masse is one of the effective methods to detect water-filled channels or fracture zones in karst terrains.

Unravelling aquifer-wetland interaction using CSAMT and gravity methods: the Mollina-Camorra aquifer and the Fuente de Piedra playa-lake, southern Spain

The hydrological regime of Fuente de Piedra playa-lake (Málaga, southern Spain) has been significantly affected by the intensive exploitation of groundwater in the area. The playa-lake is situated above clays, marls, and gypsum, and under unaltered conditions received surface-subsurface runoff within the watershed as well as groundwater discharge from two carbonate aquifers. We have analyzed the structure of the main one, the Mollina-Camorra carbonate aquifer, by combining controlled source audio magnetotellurics (CSAMT), gravity prospecting, and time-domain electromagnetic (TDEM) soundings. This geophysical information, together with new structural and hydrogeological data, was gathered to develop a new conceptual hydrogeological model. This model allows the hydrological linkage of the carbonate aquifer with the playa-lake system to be established. Moreover, the intensive exploitation in the carbonate aquifer, even outside the watershed of the playa-lake, has affected the hydrological regime of the system. This multidisciplinary work demonstrates the potential of geophysical methods for understanding wetland-aquifer interaction, having important groundwater management implications.

CSAMT investigations of the Caferbeyli (Manisa/Turkey) geothermal area

Western Turkey is one of the most remarkable regions of very active continental extension in the world. The most significant structures of this region are Alaşehir, Gediz, and Büyük Menderes Grabens. Geothermal activity around city of Manisa in Gediz Graben has been investigated by many researchers and many geothermal boreholes were drilled in order to produce electricity and for heating purposes. The Caferbeyli geothermal area is on the southern side of the Gediz Graben just west of Salihli, Manisa, Turkey. According to rising demand on thermal water around Salihli, geophysical studies were performed using the Controlled Source Audio Magnetotellurics (CSAMT) measurements near the area of Caferbeyli, and they were interpreted by the two-dimensional modelling. Vertical and horizontal resistivity sections were mapped, and it was determined that a low-resistivity layer exists in the SW part of the survey area. As a result of the studies in the area, the boundaries of the low-resistivity layer were mapped and a test drilling was recommended.

Study on Well Location Methods in Deep Buried Area of Karst Aquifer-Giving an Example in Boai County, Henan

Based on the groundwater exploration project in serious water shortage region (Jiaozuo, Henan) of Huaihe valley, the paper explores a comprehensive borehole location method from water detection experience in northern carbonate rock hilly area of Boai county, Henan. The results show that in the deep buried area of Karst aquifer, the combination of hydrogeologic survey-structural controlled water theory, high density electrical method and V8-controlled source audio magnetotelluric is effectively in borehole location ,which can greatly reduce the rate of "dry well" and improve the proportion of completion.

Geophysical techniques for low enthalpy geothermal exploration in New Zealand

Shallow warm water resources associated with low enthalpy geothermal systems are often difficult to explore using geophysical techniques, mainly because the warm water creates an insufficient physical change from the host rocks to be easily detectable. In addition, often the system also has a limited or narrow size. However, appropriate use of geophysical techniques can still help the exploration and further investigation of low enthalpy geothermal resources. We present case studies on the use of geophysical techniques for shallow warm water explorations over a variety of settings in New Zealand (mostly in the North Island) with variable degrees of success.A simple and direct method for the exploration of warm water systems is shallow temperature measurements. In some New Zealand examples, measurements of near surface temperatures helped to trace the extent of deeper thermal water.The gravity method was utilised as a structural technique for the exploration of some warm water systems in New Zealand. Our case studies show the technique can be useful in identifying basement depths and tracing fault systems associated with the occurrence of hot springs.Direct current (DC) ground resistivity measurements using a variety of electrode arrays have been the most common method for the exploration of low enthalpy geothermal resources in New Zealand. The technique can be used to detect the extent of shallow warm waters that are more electrically conductive than the surrounding cold groundwater. Ground resistivity investigations using the electromagnetic (EM) techniques of audio magnetotellurics (AMT or shallow MT), controlled source audio magnetotellurics (CSAMT) and transient electromagnetic (TEM) methods have also been used. Highly conductive clays of thermal or sedimentary origin often limit the penetration depth of the resistivity techniques and can create some interpretation difficulties. Interpretation of resistivity anomalies needs to be treated in a site specific manner.

Metodologia exploratória para demilitação de condutos vulcânicos kimberlíticos com o uso do método CSAMT, MG, Brasil

No presente trabalho apresentamos um estudo de caso onde o método CSAMT (audiomagnetotelúrico com fonte controlada) foi aplicado para determinar a assinatura geoelétrica da estrutura geológica do kimberlito Régis, localizado e conhecido no Estado de Minas Gerais, Brasil. Afloramentos desses kimberlitos foram mapeados na década de 1970. Três perfis CSAMT, dois com direções norte-sul e um leste-oeste foram adquiridos ao longo do corpo. As estações CSAMT foram adquiridas com eletrodos e boninas dispostos na configuração clássica em cruz. O intervalo típico entre estações variou de 50 a 100 m ao longo de cada perfil. A faixa de frequência utilizada foi de 10 Hz a 100 kHz, adequadas para investigações rasas (profundidades menores que 500 m). O presente trabalho deu subsídio para a locação de dois furos de sondagens dentro do pipe estudado e permitiu a proposição de um novo modelo geológico para o kimberlito.

Enhancing VLF data for qualitative interpretation: An example of massive chalcopyrite exploration

Economic ores usually consist of some combination of semiconductor sulfide minerals that respond to applied electric and electromagnetic fields. Consequently, electric and electromagnetic methods are the main exploration tools to locate the sulfide minerals. Models obtained from two- or three-dimensional inversion of induced polarization (IP) or controlled source audio-magnetotellurics (CSAMT) data can provide a good description of orebodies (Basokur et al., 1997). However, these methods are cumbersome in field procedures, survey costs are relatively high, and acquisition systems are expensive. For these reasons, they are not suitable for reconnaissance surveys and for exploration of small targets. The very low frequency (VLF) method, on the other hand, is appropriate under these conditions. It is fast, can often be implemented by 1–2 people, and the receiver unit is less expensive than those used with other ground EM methods. Moreover, the method uses existing VLF radio transmitters as electromagnetic so...

Recent base metal discovery at the Hill 800 Prospect

Hill 800 Prospect is located northwest of Licola, near the Jamieson River in the eastern highlands of Victoria. It was discovered in 1994 and is currently held under EL 3592 by Mount Wellington Gold N.L. (Mount Wellington Gold). The exploration model is for volcanic-hosted massive sulphides in the Cambrian Jamieson Volcanics. Mount Wellington Gold's interest in the area was sparked by new regional magnetic and radiometric data that became available in 1993 under the Victorian Initiative for Minerals and Petroleum. Investigation of potassium anomalies resulted in the discovery of gossans at Hill 800. Soil geochemical data show high gold values over Hill 800 and high copper values surrounding this, to the south and west. The best assay values in drillholes are 37 m @ 3.16 g/t gold from 0.5 m, and 6 m @ 1.1% copper from 253 m in HED1. The high gold values occur in an oxidised and altered cap near the top of Hill 800. The high copper values occur separately from the gold values, in altered volcanics. The geophysical surveys carried out by Mount Wellington Gold include: airborne radar (AIRSAR), airborne frequency-domain electromagnetics (DIGHEM), ground magnetics, time-domain electromagnetics (SIROTEM), induced polarization, downhole electromagnetics, downhole magnetometric resistivity (DHMMR), spontaneous potential and controlled source audio magnetotellurics (CSAMT). The gold and copper mineralisation currently detected at Hill 800 does not have a significant geophysical response. Responses recorded to date are generally from geological rather than mineralised sources. The latest induced polarization results accurately predicted pyrite enrichment (+- gold). A drilling target was defined from a 3-D model of the inverted 100 m line-spaced data. The induced polarization chargeability anomaly was associated with high resistivity, and when drilled, was found to be caused by disseminated sulphides (pyrite). The DHMMR survey on diamond drillhole HED4A indicates a conductive source, possibly massive sulphides, above the hole at about 460 m depth. The SIROTEM data also show subtle high conductivity at late times in this area. These data have many anomalies, which have not yet been effectively tested by the drilling program. Magnetic susceptibility measurements on rock chips from the drilling program indicate that the weathered and altered volcanics have lower susceptibility than the fresh volcanics - perhaps due to magnetite destruction associated with the alteration/weathering. The regional magnetics show a highly magnetic source beneath Hill 800, probably an intrusion. Modelling shows that this intrusion may come to within a hundred metres of the surface, so it is a likely source of the alteration fluids.

NICKEL: Geophysical response of the Rocky's Reward nickel sulphide deposit, Leinster, Western Australia

The Rocky's Reward nickel sulphide deposit is located in the Agnew-Wiluna greenstone belt, about 2 km north of the Perseverance (Agnew) nickel mine. The belt lies within the northern portion of the Eastern Goldfields Province of the Archaean Yilgarn Craton, Western Australia. Ore-grade mineralisation was discovered at Rocky's Reward in 1984 as a result of drill testing a geochemically anomalous gossan. Geophysical surveys (airborne and ground magnetics, induced polarisation/resistivity) had been carried out over or in the vicinity of the deposit well before the discovery of mineralisation. However, even though a magnetic anomaly was clearly delineated over the Rocky's Reward deposit, the target was not selected for follow up at that stage as the surface geological expression did not fit the existing conceptual geological model. A large amount and variety of geophysical work, including airborne and surface time-domain electromagnetics, induced polarisation/resistivity, controlled source audiomagnetotellurics, gravity and downhole surveys was subsequently completed following the discovery of mineralisation at Rocky's Reward. The object of these surveys was to map and characterise the deposit geophysically, in order to assist in the delineation of the extent and geometry of the mineralisation, and to evaluate geophysical techniques applicable to further exploration in the area. The deposit represents an excellent target for several geophysical techniques because of its shallow depth, geometry, and physical property contrasts of the ore and its host with surrounding rocks. A combination of ground magnetics and time-domain electromagnetics proved to be the most definitive and economical for detecting and mapping the deposit.

Marionoak–a Very Deep Conductive Target

At the Marionoak property in Tasmania, a large deep conductor has served as a good site for testing and comparing deep penetration EM systems such as UTEM, Controlled Source Audio Magnetotellurics (CSAMT) and Sirotem. All of the systems detect the conductor, different problems being encountered when attempting an accurate interpretation of any of them. Up to the time of writing this abstract, a final interpretation had not been tested by drilling.

Experimental Controlled Source Audiomagnetotelluric Measurements on The Elura Orebody, NSW

With the permission of EZ Coy. A'asia Ltd., personnel of the Minerals Department of Esso Australia Limited conducted audiomagnetotelluric measurements along one grid line over the Elura ore body. The surveying was experimental in nature and involved the use of prototype equipment. The work was accomplished during January 15 to 17, 1977.