Shallow Low Resistance Research Articles

Three-dimensional interpretation of magnetotelluric data at Fogo Volcano, Azores Islands

The resistivity structure of Fogo volcano and the seismically active Congro region of São Miguel Island has been determined by 3-D inversion from 44 magnetotelluric soundings to yield new insights into the internal architecture of this volcanic island. Following comprehensive testing of processing codes to yield optimum magnetotelluric responses from the collected time-series, a robust electrical resistivity model was obtained. Sensitivity analysis of various features from the inversion process was used to determine their reliability, and aid geological interpretation. The magnetotelluric data imaged, and provided new structural insights into the Ribeira Grande geothermal system on the northern flank of Fogo volcano, where a shallow low resistivity (1 - 5Ωm) region has strong correlation with borehole data, and is shown to be an excellent proxy for mapping temperature and clay alteration mineralogy. Beneath the central edifice of Fogo volcano and also throughout the Congro region, the geology is very resistive, however the new magnetotelluric observations do not yield any further constraints on the origin of the seismicity that poses threats to the local populations residing on this hazardous island.

Integrated approach for geothermal exploration: Case study from Salar area (Afyonkarahisar, Turkey)

The intense investment demand in the geothermal sector in Afyonkarahisar province in recent years has enabled the utilization of geothermal waters such as district heating and greenhouse heating, electricity generation, and spa facilities and accelerated the exploration of new geothermal areas in the region. In this study, the Salar (Afyonkarahisar) region's geothermal potential was investigated using the mineralogy and geochemistry of hydrothermal alteration, hydrogeochemistry, and resistivity models obtained from magnetotelluric data. The Salar region is located within the Afyon-Akşehir Graben (AAG) and 10 km south of Afyonkarahisar province. The most important manifestations of the geothermal system are the geothermal water at temperatures of 25 °C and 31 °C obtained from the boreholes and hydrothermal alteration in Salar. Afyon volcanoclastics are reservoir rocks. Smectite and illite are the most important clay minerals in the hydrothermal alteration zones. The transformation from volcanic glass and alkali feldspar to smectite and illite reflects neutral-alkaline alteration conditions in felsic rocks. The clay minerals' stable isotopes (δD and δ18O) indicate hypogene conditions. Discharge temperature, electrical conductivity and pH of Salar region geothermal waters vary from 25 to 31 °C, 320–357 µs/cm, and 6.8, respectively. The Salar geothermal waters are Ca-(Na)-HCO3 type chemically. The electric resistivity models reveal shallow low resistivity (10 < ρ < 80 Ωm) layer related to the alluvium, Gebeceler formation, and Afyon volcanoclastics and deeper high resistivity (80 <ρ < 200 Ωm and ρ > 200 Ωm) layer based on Deresinek and Değirmendere formation respectively. The difference in electrical resistivity arises from the geothermal waters and hydrothermal alteration zones, influenced by the AAG tectonics.The stable isotopes (δD and δ18O) and alpha cristobalite geothermometer calculations indicate that the condition of the temperature in the active and fossil geothermal systems in the Salar does not change, and the condition of the temperature is between 44 °C and 112 °C.

3D seismic imaging of the Nesjavellir geothermal field, SW-Iceland

We present a detailed seismic imaging of the harnessed Nesjavellir geothermal area, SW-Iceland, which is one of several geothermal fields on the flanks of the Hengill volcano. We map thevP,vS, andvPvSratio using seismic data recorded in 2016–2020 and compare them with both a resistivity model of the same area and the rock temperature as measured in boreholes. The results show that the shallower crust (depth less than 1 km) is characterized by lowvPandvS, and highvPvSratio (around 1.9). Shallow low resistivity values at similar depths in the same area have been interpreted as the smectite clay cap of the system. At depths between 1 and 3 km the observed lowvPvSratio of 1.64–1.70 correlates with high resistivity values. In this area, characterized by temperatures larger than 240°C, the seismicity appears to be sparse and located close to the production wells. This seismicity has been interpreted as induced by the production in combination with naturally occurring earthquakes. At depths greater than 4 km, highvPvSratio of 1.9 correlates well with low resistivity values. In the valley of Nesjavellir, a deep-seated conductive body, domes up at about 4.500 m b.sl. and coincides spatially with a significant highvPvSratio anomaly (&amp;gt;1.9). Above these anomalies an elevated temperature is registered according to borehole temperature data. This is proposed here to be caused by hot 600°C–900°C cooling intrusives, close to the brittle ductile transition—probably the heat source(s) of the geothermal field above. These anomalies are at the same location as the last fissure eruption in Hengill almost 2,000 years ago. The NNE-SSW trending, deeper seismic cluster at 3–6 km depth is located at the edge of this highvPvSanomaly. The heat source of the Nesjavellir geothermal field is most likely connected to this most recent volcanism as reflected by the deep-seated low resistivity body and highvPvSratio, located beneath the deep fault that connects the flow path of the high temperature geothermal fluid, resulting in an actively producing reservoir.

Magnetotelluric imaging of the shallow-seated magma reservoir beneath the Karadağ stratovolcano, Central Anatolia, Turkey

The central Anatolian Volcanic Province encompasses many volcanoes elongated in a northeast-southwest direction. Karadağ is one of the dormant Quaternary stratovolcanoes of the region at the southern end of the province. We acquired broadband magnetotelluric (MT) data in the period range 0.0001–251 s at 124 locations to obtain a three-dimensional electrical resistivity model to identify the associated magmatic-hydrothermal system beneath the Karadağ stratovolcano. The resistivity model provides evidence for low resistivity anomalies that include (1) a near-surface localised low resistivity (~ 20 Ωm) zone situated directly beneath the crater centre that is interpreted to be lacustrine sediments of an extinct crater lake, (2) a shallow low resistivity (~ 10 Ωm) zone of probable hydrothermal alteration in the eastern part of the stratovolcano and (3) a deep and large zone of low resistivity (<10 Ωm) attributed to a shallow-seated magma reservoir at a depth of 3 to 7 km beneath the volcanic edifice. A few hundred meters thick resistive surface layer corresponds to the Karadağ adakitic volcanics (i.e., andesites and dacites), and a resistive basement representing Precambrian to Mesozoic high- and medium-grade metamorphic rocks on the flanks is observed in the resistivity model.

Crustal Electrical Structure of the Zhaheba Complex Imaged by Magnetotelluric Data and Its Tectonic Implications

A Magnetotelluric profile stretching northward from the Wulungu Depression (on the northern margin of the Junggar Basin) to the Dulate arc (crossing the Zhaheba–Aermantai ophiolite belt) was carried out in an attempt to probe the crustal structure and properties of the East Junggar, NW China. Along the profile, the inversion model was used to determine the electrical structure of the crust and uppermost mantle. The results revealed that the crust of the eastern Junggar Basin is composed of the shallow low resistivity layer and underlying high resistivity bodies. There is a crustal detachment in the basement: the upper layer is a Hercynian folded basement and the lower is a Precambrian basement. The Zhaheba complex is characterized by relatively high resistivity, with a thickness of ~5 km, the bottom controlled by the Zhaheba–Aermantai fault. The crust of the Yemaquan arc is composed of the residual continental crust, characterized by stable resistance. The exposed intrusive rocks are characterized by irregular resistors. The crust of the Dulate arc is characterized by relatively low resistivity. The shallow low resistivity layers represent the Zhaheba depression composed of the Devonian-Permian volcanic and sedimentary rocks. The crustal conductive anomalies are related to the magmatism and mechanism of metal deposits in the post-collision period.

The effect of active extensional tectonics on the structural controls and heat transport mechanism in the Menderes Massif geothermal province: Inferred from three-dimensional electrical resistivity structure of the Kurşunlu geothermal field (Gediz Graben, western Anatolia)

The current study in the Menderes Massif geothermal province aims to assert a crustal-scale geoelectrical evidence showing the role of extensional tectonics on the flow of geothermal fluids through both high-angle and low-angle normal (detachment) faults and on the heat transport mechanism in the convection dominated-amagmatic geothermal play systems, on the basis of three-dimensional electrical resistivity structure of the Kurşunlu geothermal field, Gediz Graben, western Anatolia. The electrical resistivity structure was derived from the three-dimensional inversion of the magnetotelluric data acquired from 74 sites in the period range from 0.001 s to 1000s. The resistivity model brings out two different types of reservoirs, namely (i) a shallow low resistivity reservoir (secondary reservoir) corresponding to the Kurşunlu hot spring in the sedimentary layer of the Gediz Graben and (ii) a deep more resistive reservoir (primary reservoir) beneath a prominent highly conductive (≤ 10 ohm-m) hydrothermal clay alteration zone. The circulation of geothermal fluids in reservoirs is dominantly controlled by high-angle normal faults including the main graben-bounding fault and low-angle Gediz detachment fault. The possible heat source for the geothermal system has been attributed to high heat arising from upwelling asthenosphere through the mantle window in the northward subducting and retreating African slab, which has been accompanied by crustal extension resulting in a thin crust and shallow mantle, and the deeply penetrating main graben-bounding fault acting as a conduit through which heat is transported from the deeper parts of the crust to near surface probably controls heat transport in the area.

Electrical Constraints on the channel flow underneath the northeastern Tibetan plateau: Results of the Longriba-Minjiang magnetotelluric sounding profile

The Minjiang Fault belt on the eastern edge of the Tibetan Plateau is the boundary between the Motianling block and the Songpan-Ganzi fold belt. It has recently been a focus of many geological and geophysical studies. In this paper, a NW-SE trending magnetotelluric (MT) sounding profile which crossed the Minjiang Fault and adjacent regions was used to study the deep structure of this region. The dimensionality analysis results indicate that the structure of the research area is 3-D. This is because most of the Swift and Bahr skew values are larger than 0.2 in the long period data. In contrast, impedance tensor decomposition results show a poorly-defined strike angle of N20°E with the 90° ambiguity removed with regional geological information. A similar strike angle was given by phase tensor analysis as well. Thus, both 2-D and 3-D approaches were applied to produce resistivity models. The resistivity model can be divided into three parts which are the Ruoergai basin, the Maerkang block and the Minshan block from west to east. These three parts are separated by the Longriba Fault in the west and the Minjiang Fault in the east. The Ruoergai block and Maerkang block are characterized by highly resistive upper crust and less resistive middle to lower crust. The low resistivity layer is located beneath the Minjiang Fault and connects with the shallow low resistivity body in the upper crust of the Minshan block. The shallow conductor beneath the Minshan block may be caused by fluid and partial melt. In contrast, the widespread low resistivity middle to lower crust beneath the Minshan block, is interpreted as partial melt and can be related to the northeastward channel flow of the weakened Tibetan crust. The channel flow is interpreted to travel through the Longriba Fault in the eastern margin of the Tibetan plateau and was blocked by the rigid Motianling block east of the Minshan block. This blockage caused the upwelling of partial melt in the middle and lower crust, leading to crustal deformation in the weak Minshan uplift zone. As a result, the Minshan uplift was formed as a result of an eastward-thrusting nappe, whereas the left-lateral, strike-slip movement of the Minjiang Faults and Huya fracture accelerates the uplift of the Minshan block. The other portion of the channel flow is blocked by the Motianling block and turns to the southeast, resulting in the Maerkang block moving to the south; this phenomenon corresponds to the dextral strike-slip movement of the Longriba Fault and the sinistral strike-slip movement of the Minjiang Fault.

Structure of the Tongariro Volcanic system: Insights from magnetotelluric imaging

The dynamics of magma reservoirs (the main repositories for eruptible magma) play a fundamental role in the style and behaviour of volcanic systems. A key first step in understanding these systems is to identify their location and size accurately. We present results from a broadband magnetotelluric study of the Tongariro Volcanic system and discuss how the results fit within current petrological models. The Tongariro Volcanic system is a composite andesitic cone complex, located at the southern end of the Taupo Volcanic Zone in the central North Island of New Zealand. We use data from 136 broadband magnetotelluric soundings within a 25×35 km area covering the volcanic system to construct a 3D image of the magmatic system of the Tongariro Volcanic Complex including Mount Ngauruhoe. The structure of the Tongariro magmatic system has been determined from 3D forward and inverse modelling of the magnetotelluric data and allowed for an estimation of the melt fraction present within the system. 3D inverse modelling of the magnetotelluric data shows: a well-developed shallow low resistivity zone outlining the geothermal system; a zone of even lower resistivity representing a shallow crustal magma accumulation zone located at a depth of ∼4–12 km offset to the east of the Tongariro vent system; and a zone with a slightly higher resistivity connecting these two components of the magmatic system providing the path for magmatic fluids from the deeper source region to reach the surface during eruptive events.

Exploration of the Near-surface Structure of Mud Volcanoes using Electromagnetic Techniques: A Case Study from Perekishkul, Azerbaijan

The general aim of this study is, for the first time, to investigate the inner structure of mud volcanoes in Azerbaijan using several near-surface electromagnetic methods. The central-loop transient electromagnetic (TEM) method and the radiomagnetotelluric (RMT) method were applied on mud volcanoes near Perekishkul, Azerbaijan. These methods complement each other in terms of depth of investigation. In particular, the RMT method generally resolves the very shallow resistivity structures (&lt;10 m) directly beneath the mud volcanoes, whereas the TEM method provides a resolution of up to 100- to 150-m depth. The obtained data sets were processed and interpreted individually. The TEM data were interpreted by conventional 1-D inversion algorithms, i.e., Occam's and Marquardt inversion, and by a quasi-2-D laterally constrained inversion. The RMT data were interpreted by 2-D inversion. Subsequently, a combined model regarding the sediments and corresponding resistivities for the area around the mud volcanoes was derived. Overall, a resistivity range of 1–20 Ω-m was observed. The resistivity models obtained from TEM support the assumption of a three-layer subsurface for the resolved depth range. Also, a lateral resistivity variation caused by the mud volcanoes was observed. Furthermore, the RMT results indicate the presence of shallow low resistivity structures, which can be interpreted as mud reservoirs.

Volcanic structure of the southern sector of Mt. Etna after the 2001 and 2002 eruptions defined by magnetotelluric measurements

Two magnetotelluric (MT) surveys were carried out on the Mt. Etna volcano after two of the most intense eruptions of the last 30 years which took place in summer 2001 and winter 2002–2003. Surveying was pursued for two main reasons. First, we sought to contribute to the definition of the first-order structure and physico-chemical state (temperature, fluids, melts) of a volcano that has been extensively explored and monitored by means of various geophysical methods, but where only few electrical and electromagnetic surveys have been performed. Secondly, we acquired MT data in the same sites in the two different surveys with the aim of monitoring the possible changes of the first-order structure, since conditions are expected to vary on an active volcano such as Etna, and are supposed to be linked to the eruptive events. Soundings have been acquired in an E-W 10 km-long profile across the southern flank of Mt. Etna, at a distance of almost 6 km south from the Central Crater. The first survey was carried out three months after the 2001 eruption. Inverse models define a pronounced (4 km thickness) low resistivity section at a depth of about 1 km b.s.l. to the west. To the east, a low resistivity section is still present, but appears deeper, thinner and more resistive, and a shallow low resistivity anomaly also exists. The shallow anomaly to the east is tentatively correlated with altered and clayey volcanic units and/or temporary groundwater storage. The deep anomalies are interpreted as being due to melt storage at shallow depths which was not exhausted during the eruption. This would be confirmed by the abundance of lava erupted within one year from the end of the survey. The few good sites retrieved in the second survey, carried out a few weeks after the eruption of 2002–2003, confirm the picture defined in the first survey, and provide a better definition of the bottom of the deep anomaly located in the sedimentary basement.

Shallow Silicided Junctions for VLSI Cmos Transistors by Furnace and Rapid Thermal Processing

AbstractAs a result of device scaling very shallow low resistance diffusions are required for VLSI CMOS fabrication. This paper describes a technique for their formation using silicon implantation for preamorphisation, counterdoping arsenic implantation and overall boron fluoride implantation for the sources and drains of the n- and p-channel transistors. Platinum silicidation has been used to reduce diffusion and polysilicon sheet resistances to 8Q/square. Activation of the shallow diffusions has been achieved either by furnace annealing (FA) or rapid thermal annealing (RTA) in the range 900°C to 1100 °C. Materials results are discussed including TTEM, SIMS and SR profiling. The suitability of the technique for VLSI CMOS applications is demonstrated by the fabrication of sub-micron transistors. With larger wafer diameters (&gt;5') the FA conditions considered are not practicable owing to ramped diffusion effects which lead to deeper junctions. Hence RTA is necessary: optimum conditions found were 1100 °C for 10 seconds when device performance equivalent to or better than FA can be achieved.