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Extended leak-off test (XLOT) is one of the in-situ tests, routinely conducted to evaluate integrity of the cased and cemented wellbores during deep borehole drilling, as well as in situ hydraulic properties at a casing shoe depth.We introduce results of the XLOT conducted in a large diameter borehole, which is drilled for installation of deep borehole based geophysical monitoring system to monitor micro-earthquakes and fault behavior of major linearments in the subsurface. The borehole was planned to secure a final diameter of 200 mm (or more) at a depth of ~1 km deep, with 12" diameter wellbore to intermediate depths, and 7-7/8" (~200 mm) to the bottom hole depths.We drilled first the 12" diameter borehole to approximately 504 m deep and installed API standard 8-5/8" casing, then cemented the annulus between the casing and bedrock. Then we carried out the XLOT, for several purposes such as confirming casing and cementing integrity, as well as estimating in-situ stress and hydraulic conductivity at the casing shoe depth. To that end, we drilled 4 m length interval to directly inject water and pressurize into the rock mass using the upper API casings. During the XLOT, we recorded flow rates and interval pressures in real time. Based on the logs, we tried to analyze hydraulic conductivity of the test interval, and compare the results with previously reported hydraulic properties measured in other ways.

A three-dimensional (3D) magnetotelluric (MT) survey was carried out to delineate subsurface structures and possible fractures for geothermal development around Ulleung Island, Korea. Good-quality ...

This paper presents a touch sensing analog front end (AFE) for capacitive touch-screen integrated into an ultra-thin display. Reduced distance between the touch screen and display causes large capacitive coupling, resulting in increased parasitic capacitance and reduced touch sensitivity. Display noise interference is worse due to the large coupling capacitance. Hence, it is a challenge to design an AFE capable of accurate and energy efficient sensing of a touch input in the integrated touch-screen panel. An adiabatic multi-driving method based on charge recycling is proposed to provide power-efficient stimulation of the touch-screen panel. Furthermore, in order to cancel out the display noise interference through the large parasitic capacitance, a fully differential touch sensing module is incorporated in the AFE. A correlated noise sampling (NS) is employed in the decoder stage for the multi-driving demodulation process. To further improve power efficiency, the sensing module is multiplexed in four ways while achieving an optimal conversion time per sample. The proposed AFE was implemented in a 180-nm CMOS process. The fabricated AFE shows 57.0-dB signal-to-noise ratio (SNR) at 120 fps while consuming 17.8 mW. Compared with power consumption of 19.9 mW expected with a conventional signal generation, the proposed adiabatic signal generator dissipates only 7.1 mW, exhibiting a power reduction of 64% due to the adiabatic driving method.

The combustion instability characteristics in a model dump combustor with an exhaust nozzle were experimentally investigated. The first objective was to understand the effects of operating conditions and geometric conditions on combustion instability. The second objective was to examine more generalised parameters that affect the onset of combustion instability. Three different premixed gases consisting of air and hydrocarbon fuels (C2H4, C2H6, C3H8) were burnt in the dump combustor at various inlet velocity, equivalence ratio and combustion chamber length. Dynamic pressure transducer and photomultiplier tube with a bandpass filter were used to measure pressure fluctuation and CH* chemiluminescence data. Peak frequencies and their maximum power spectral densities of pressure fluctuations at same equivalence ratios showed different trends for each fuel. However, the dynamic combustion characteristics of pressure fluctuations displayed consistent results under similar characteristics chemistry times regardless of the used hydrocarbon fuels. The results showed that characteristic chemistry time and characteristic convection time influenced combustion instabilities. It was found that the convective-acoustic combustion instability could be prevented by increasing the characteristic chemistry time and characteristic convection time.

Effects of orifice geometry on liquid column trajectories of liquid jets in subsonic crossflows were experimentally studied. One circular and four elliptical plain-orifice injectors with different aspect ratios of 1/3 to 3, were designed and utilized. By changing the injection pressure drop from 1 bar to 6 bar with an increment of 1 bar, and thus the liquid–air momentum flux ratio from 15 to 106, back-lit spray photographs of liquid jets in crossflows were taken and then analyzed using an in-house code to detect the upper boundary and centerline of the liquid jet before the breakup point. Analytical liquid column trajectories for circular and elliptical liquid jets were derived through simple assumptions and compared with present experimental data. It was found that the aspect ratio affected the difference between the analytical drag coefficient and the experimental one. Novel empirical equations for liquid column trajectories of circular and elliptical liquid jets in crossflows were suggested as a function of equivalent orifice diameter, liquid–air momentum flux ratio and aspect ratio. The results showed that the present empirical equation for the circular orifice injector could be reasonably applied to the elliptical orifice injectors when the aspect ratio was below one.

This brief presents an asynchronous successive approximation register (SAR) analog-to-digital converter (ADC) for energy-harvesting body sensor node system-on-chips applications. To improve the power efficiencies of analog and digital blocks independently, a dual-supply voltage scheme is adopted, and a clock-to-Q delay compensator is proposed to realize the timing-calibration-free asynchronous operation with scalable supply voltages. For a maximum sampling rate of 20 kS/s, the SNDR is increased by 3 dB and the power consumption of the SAR logic is decreased by 37%, improving the FoM $_{\text{W}}$ by 26% with the analog and digital supply voltages of 0.6 V and 0.35 V, compared with the single supply of 0.4 V. To obtain a high effective number of bits (ENOB), the ADC exploits a current-biased dynamic comparator and a capacitive DAC with a modified partial common-centroid layout. With a DNL of +0.18/−0.2 LSB and an INL of +0.26/−0.25 LSB, the ADC achieves an ENOB of 9.64 bits at a sampling rate of 100 kS/s, exhibiting power consumption of 562 nW. The prototype ADC is fabricated in 0.18- ${\mu }\text{m}$ CMOS technology, occupying an area of 0.0468 mm2.

The first enhanced geothermal system (EGS) project in South Korea was initiated in Pohang in 2010, and the induced microseismicity has been regarded as a key component. This study presents a regional case study of geothermal energy development in South Korea focusing on a comprehensive protocol that addresses induced microseismicity. The protocol essentially follows the seven steps suggested by the U. S. Department of Energy with necessary site-specific adjustments and improvements. The site selection procedure, outreach program, establishment of a local seismic network, and the methodology of establishing a traffic light system are introduced along with an analysis of induced microseismicity from the first hydraulic stimulation campaign. The equation for estimating vibration due to blasting was converted to an equation that relates peak ground velocity (PGV) to magnitude by using conversion equations between the amount of explosives, the magnitude of the earthquake and the radiated energy. A site-specific traffic light system is suggested based on the equation that relates PGV to magnitude, the site-specific parameters of which were determined by natural earthquakes that occurred within 50 km of the Pohang EGS site. The traffic light system was applied to the first hydraulic stimulation that occurred in January and February of 2016. The obtained induced seismic data were roughly in agreement with the suggested empirical equation.

The productivity of a geothermal reservoir, which is a function of the pore-space and fluid-flow path of the reservoir, varies since the properties of the reservoir changes with geothermal reservoir production. Because the variation in the reservoir properties causes changes in electrical resistivity, time-lapse (TL) three-dimensional (3D) magnetotelluric (MT) methods can be applied to monitor the productivity variation of a geothermal reservoir thanks to not only its sensitivity to the electrical resistivity but also its deep depth of survey penetration. For an accurate interpretation of TL MT-data sets, a four-dimensional (4D) MT inversion algorithm has been developed to simultaneously invert all vintage data considering time-coupling between vintages. However, the changes in electrical resistivity of deep geothermal reservoirs are usually small generating minimum variation in TL MT responses. Maximizing the sensitivity of inversion to the changes in resistivity is critical in the success of 4D MT inversion. Thus, we further developed a focused 4D MT inversion method by considering not only the location of a reservoir but also the distribution of newly-generated fractures during the production. For the evaluation of the 4D MT algorithm, we tested our 4D inversion algorithms using synthetic TL MT-data sets.

With the increasing computational demands of neural networks, many hardware accelerators for the neural networks have been proposed. Such existing neural network accelerators often focus on popular neural network types such as convolutional neural networks (CNNs) and recurrent neural networks (RNNs); however, not much attention has been paid to attention mechanisms, an emerging neural network primitive that enables neural networks to retrieve most relevant information from a knowledge-base, external memory, or past states. The attention mechanism is widely adopted by many state-of-the-art neural networks for computer vision, natural language processing, and machine translation, and accounts for a large portion of total execution time. We observe today's practice of implementing this mechanism using matrix-vector multiplication is suboptimal as the attention mechanism is semantically a content-based search where a large portion of computations ends up not being used. Based on this observation, we design and architect A3, which accelerates attention mechanisms in neural networks with algorithmic approximation and hardware specialization. Our proposed accelerator achieves multiple orders of magnitude improvement in energy efficiency (performance/watt) as well as substantial speedup over the state-of-the-art conventional hardware., Comment: To be published in 2020 IEEE International Symposium on High Performance Computer Architecture (HPCA)

This paper presents an always-on mutual-capacitive touchscreen controller (TSC) with a reconfigurable power consumption of 0.53–13.4 mW, frame rate of 1–120 Hz, and an SNR of 40.0-46.2 dB to support not only a normal sensing mode but also a low-power (LP) and an ultra-low-power (ULP) modes. For the LP and ULP modes, the power-frame rate scalability is realized by a frame rate controller, which turns off analog front-end (AFE) transmitter (TX) and receiver (RX) periodically. Moreover, the TSC improves an out-of-band noise attenuation by utilizing a sixth-order current-mode band-pass filter and second-order incremental hybrid delta-sigma (ΔΣ) modulator, providing an SNR up to 45.8 dB when the display on. The prototype TSC is fabricated in an 80-nm high-voltage CMOS technology with an active area of 4.873 mm2.

The first hydraulic stimulation for enhanced geothermal system (EGS) development in Korea had been conducted in the PX-2 well of 4,348 m depth in Pohang EGS site from January 29 th to February 20 th , 2016. Treatment histories of injection rate, wellhead pressure and corresponding induced microseismicity data were obtained from the stimulation test upon 140 m long open hole section at the well bottom. Wellhead pressure was up to 89 MPa and considerable level of flow rate was attempted up to 47 L/sec. Microseismicity observation showed a trend of lager and more frequent seismicity occurrence in shut-in phase than in injection phase. The injectivity index during the stimulation periods had increased as 2.7 times in January 30 th at the wellhead pressure of 73 MPa. Postulating the existence of a major fracture zone intersecting the open hole section, the transmissivity and the corresponding equivalent aperture of the fracture were evaluated. Required breakdown pressures by hydrofracturing and hydroshearing mechanisms were estimated based on the various scenarios on the in-situ stress condition, major fracture zone orientation and shear failure criteria.

This letter presents an event-driven digital low-dropout regulator (DLDO) with an adaptive linear/binary two-step search achieving a fast transient response. A two-dimensional (2-D) circular shifting register (CSR) offers an adaptive linear-search regulation. When a large voltage droop occurs, the CSR activates a fast-tracking mode that provides immediate recovery from the droop. Once the linear search by the CSR is completed, a subrange successive-approximation register (Sub-SAR) conducts the binary-search regulation. The full-scale current range of the Sub-SAR is adaptively scaled by referencing the CSR, which reduces the number of searching steps and improves undershoot or overshoot caused by the binary-search operation. Ring amplifier based 1.5b continuous-time (CT) comparators and the asynchronous controllers realize the event-driven operation that breaks a tradeoff between transient response and sampling clock frequency. The proposed DLDO was fabricated in a 40 nm CMOS process. The DLDO can operate in an input voltage V IN range from 0.6 to 1.2 V. When a load current step of 104.2 mA/1 ns was applied at a V IN of 1.0 V, a droop-recovery time and a settling time were measured as 6 and 15 ns, respectively.

This paper presents a noise-shaping SAR ADC for IoT sensor applications. The ADC exploits a 2nd-order passive noise-shaping loop without a quiescent current. To reduce harmonic distortion induced by mismatches between MSBs, thermometer-coded 3-bit MSBs are implemented with a simple shift register-based dynamic element matching (DEM) technique. Furthermore, a programmable majority-voting (PMV) technique for LSB decision is applied in order to relax noise requirement of a comparator. With the DEM and PMV, SFDR, SNDR and SNR are enhanced by 9.8, 4.7 and 1.9 dB at a 1.0 V supply, respectively. For 50 kHz BW, the modulator dissipates 74.5 μW from a 1.0 V supply and achieves a peak SNDR of 72 dB, a peak SNR of 72.2 dB and a DR of 73.8 dB. The prototype modulator is fabricated in 28 nm CMOS technology, occupying an area of 0.0575 mm2.

Numerous stimulation tests have been performed on Enhanced Geothermal System (EGS) or Hot Dry Rock (HDR) projects during the past three decades, however, there is much room for improvement in our knowledge and understanding of the mechanisms of stimulation. This paper investigated the hydraulic stimulation tests carried out on seven EGS or HDR projects where massive volume of fluid was injected into the long open section of the well with interval of tens to hundreds of meters in the crystalline formation. The key characteristic test and performance parameters were defined and collected through extensive survey of stimulation results. Attempts were made to carry out comparative analysis on reservoir conditions, test parameters and test observations. The analysis and discussion suggest that 1) the reservoir stress regime impacts the growth of stimulated region and the reverse faulting stress regime can be favorable for the layout of multiple well system as it may lead to a horizontally or sub-horizontally oriented stimulated zone; 2) the injection pressure for activating shear slip and the associated onset of seismicity is mainly field stress controlled; 3) there is strong dependency of injectivity on injection pressure and a high pressure makes a better hydraulic injectivity during stimulation and consequently afterwards for circulation; 4) the stimulated region and number of induced seismic events are mainly injection volume controlled and the potential strategy to reduce seismic risks is either to extend stimulation in time or to separate stimulation in space; and 5) the differential stress condition is one of the necessary factors to raise a large magnitude event (LME) and the difference of maximum injection pressure achieved over that at onset of seismicity is an important additional factor to induce LMEs.

While geothermal energy provides the only base-load power among renewable energy sources, its development has been carried out predominantly in volcanic area. EGS (Enhanced Geothermal System) is a ubiquitous technology that can allow the geothermal power generation virtually in any area. This manuscript introduces the current state-of-the-art of EGS development in the world and presents the hydraulic stimulation technology and associated microseismicity which are key technical component in EGS. Finally this paper suggests the key research areas required in Korea for further development of EGS. Key words Geothermal energy, EGS (Enhanced Geothermal System), Hydraulic stimulation, Hydraulic fracturing, Hydrauilc shearing초 록 지열에너지는 기저부하를 제공하는 신재생에너지이나 현재까지 화산지대에만 대부분의 지열발전이 이루어져 왔다. 인공저류층 지열시스템 (Enhanced Geothermal System, EGS)는 비화산지대의 지열발전을 가능하게 할 개념으로 알려져 있으며 수리자극 (hydraulic stimulation)이 핵심기술이다. 본 논문은 EGS지열발전의 개발 현황을 소개하고 , 수압파쇄와 수리전단이 주 메커니즘인 수리자극의 핵심원리 , 설계변수 및 수리자극에 수반되어 발생하는 미소진동의 원리 및 관측기술을 소개한다 . 한국에서의 EGS 지열발전을 위하여 필요한 과제를 소개하여 향후 기술개발의 방향을 제시한다 .핵심어 지열에너지, 인공 저류층 지열시스템 , 수리자극, 수압파쇄, 수리전단

Various experiences on enhanced geothermal system (EGS) has been accumulated from the Soultz project through various scientific experiments and research activities for more than 20 years since it started in the year of 1984 until the 1.5 MW Organic Rankine Cycle (ORC) binary power plant has been built up in Soultz-sous-Forets area, France. They have been applied to Cooper basin in Australia, Landau and Insheim in Germany and so forth. This report summaries the experiences from Soultz in the aspect of artificial reservoir creation, expecting to be helpful for reducing any trial and errors or unnecessary expenses in ongoing Korean EGS project in Pohang area, where the geological features are similar to Soultz area. Key words Soultz project, Geothermal power generation, Enhanced geothermal system (EGS), Pohang초 록 1984년 프랑스 Soultz-sous-Forets 지역에서 시작된 Soultz 프로젝트는 2007년 11월에 1.5 MW급 Organic Rankine Cycle (ORC) binary 발전 설비가 완성되기까지 20여년에 걸쳐 인공 지열저류층 생성기술 (EGS; Enhaced Geothermal System) 개발을 위한 다양한 실험과 연구가 이루어져 , 그 축적된 기술은 호주의 Cooper basin, 독일의 Landau, Insheim 등의 지열발전 프로젝트에 활용되고 있다 . 우리나라의 포항에서도 현재 지열발전 프로젝트가 진행되고 있는 시점에서 이 보고에서는 지열저류층 생성 측면에서 Soultz 에서의 경험을 살펴보았다 . 포항의 경우, 지질학적인 측면에서 Soultz의 환경과 유사한 부분이 많으며 따라서 , Soultz에서의 경험과 know-how를 잘 연구하고 개선하여 적용한다면 불필요한 시행착오나 시간적 , 경제적인 낭비요소를 배제할 수 있을 것이며 , 이 보고가 그에 보탬이 되기를 기대한다 .핵심어 Soultz 프로젝트, 지열발전, 인공 지열저류층 생성기술 , 포항

Three-dimensional (3D) magnetotelluric (MT) surveys have been performed in Jeju, the largest volcanic island in Korea to figure out any possible structures or potential anomaly for remnant deep geothermal resources. Various approaches have been applied to interpret MT data observed in Jeju. MT dataset shows generally simple stratigraphy of four layers, though contains the severe static and the sea-effects. In our previous works, the induction vectors and 3D inversion results have commonly indicated the existence of a conductive anomaly in central parts of the island, beneath Mt. Halla. The 3D inversion dealt the static shifts as inversion parameters. The Jeju MT dataset, however, still contains the effect of conductive sea water surrounding the island. The sea-effect on MT impedance can be represented as a distortion tensor and excluded from the Jeju MT dataset by an iterative sea-effect correction. In this study, 3D inversion incorporating static shift parameterization was conducted using MT dataset corrected using 1D resistivity model obtained from the iterative scheme. Reasonably reconstructed images are obtained through the 3D inversion and using the MT dataset with sea-effect correction. The inversion result still shows the conductive anomaly in a similar depth. RMS misfits converged to a lower value than that of inversion using MT data before the sea-effect correction. From the fact, it is highly possible that the conductive anomaly is not an artifact but a real underground structure. Further investigation about the anomaly including exploration drilling is needed to see if it is from a fracture containing conductive sea water or related to the old volcanic activities.

Magnetotelluric(MT) soundings were done in Central Victoria, Australia, to investigate the electrical conductivity structure of the Bendigo and Melbourne structural zones. We reprocess and reinterpret the MT data, which were acquired along the same path as the seismic transect in 2006 by GeoScience Victoria (GSV). Deep crustal faulting in this region is considered to be responsible for significant mineralization and magmatic processes. The resulting two dimensional (2D) MT conductivity model is consistent with the seismic interpretation in that it clearly shows known boundaries as well, as additionally reveals four clearly isolated resistors, which could be hardly imaged by reflection seismic. With the result previously published, these resistors can be interpreted as granite or granodiorite intruded in the Devonian, or alteration associated with fluid migration. The 2-D electric conductivity model also agrees with seismic interpretations, which shows that internal faults in the Bendigo Zone are thrust fault systems with listric geometries that are connected in the mid-crust, and clarifies that the Bendigo Zone and Melbourne Zone experienced different tectonic evolution during the Benambran Orogeny in the Silurian. The model also supports the conjecture that the Melbourne Zone formed as a foreland basin to the uplifted Bendigo Zone.

We estimated geothermal power generation potential in Korea through Enhanced Geothermal System (EGS) technology following the recently proposed protocol which was endorsed by international organizations. Input thermal and physical data for estimation are density, specific heat and thermal conductivity measurements from 1,516 outcrop samples, 180 heat production, 352 heat flow, and 52 mean surface temperature data. Inland area was digitized into 34,742 grids of size and temperature distribution and available heat were calculated for 1 km depth interval from 3 km down to 10 km. Thus estimated theoretical potential reached 6,975 GW which is 92 times total generation capacity of Korea in 2010. Technical potential down to 6.5 km and considering land accessibility, thermal recovery ratio of 0.14 and temperature drawdown factor of was 19.6 GW. If we disregard temperature drawdown factor, which can be considered in estimating economic potential, the technical potential increases up to 56 GW.

A magnetotelluric (MT) survey has been performed to delineate deeply extended fracture systems at the geothermal field in Seokmo Island, Korea. To assist interpretation of the MT data, geological surveying and well logging of existing wells were also performed. The surface geology of the island shows Cretaceous and Jurassic granite in the north and Precambrian schist in the south. The geothermal regime has been found along the boundary between the schist and Cretaceous granite. Because of the deep circulation along the fracture system, geothermal gradient of the target area exceeds 45°C/km, which is much higher than the average geothermal gradient in Korea. 2D and 3D inversions of MT data clearly showed a very conductive anomaly, which is interpreted as a fracture system bearing saline water that extends at least down to 1.5 km depth and is inclined eastwards. After drilling down to the depth of 1280 m, more than 4000 tons/day of geothermal water overflowed with temperature higher than 70°C. This water showed very similar chemical composition and temperature to those from another existing well, so that they can be considered to have the same origin; i.e. from the same fracture system. A new geothermal project for combined heat and power generation was launched in 2009 in Seokmo Island, based on the survey. Additional geophysical investigations including MT surveys to cover a wider area, seismic reflection surveys, borehole surveys, and well logging of more than 20 existing boreholes will be conducted.

A numerical model for the simulation of temperature changes in a borehole heat exchanger (BHE) with fluid circulating through U-tubes is developed. The model can calculate the thermal power transferred from heat pumps to BHEs while considering the nonlinear relationship between temperature of the circulating fluid and the thermal power. The use of the developed model enables the design of a geothermal heat pump (GHP) system with the view of pursuing efficiency and financial benefit. The developed model is validated by comparing two measurement datasets with their respective simulation results. The numerical evaluation of a real GHP system with 28 BHEs and 79 heat pumps involved consideration of a base case and modified cases. In all cases, the temperatures of the circulating fluid at the BHE inlet and outlet, heat pump efficiency, and the heating power and electric power of heat pumps were obtained. The estimated cost of electricity in the year 2030 is 0.146 US$/kW. The most cost-effective system in this case is for there to be 4, 6, and 6 BHEs on the first, second, and third floors, respectively.

Magnetotelluric (MT) surveys were conducted in Pohang, Korea, for low-temperature geothermal exploration in 2002 and 2003. Pohang is located in the southeastern part of the Korean Peninsula and close to the East Sea. In the interpretation of MT data from a coastal environment, sea effects must be correctly included because seawater is a strong conductor. We first constructed a five-layered earth model with a realistic coastline and bathymetry to investigate sea effects on MT data measured in Pohang. This model clearly shows that the Pohang data are significantly influenced by sea water at frequencies blow 1 Hz at the whole measurement sites. Next, we utilized a three-dimensional inversion algorithm based on the Gauss–Newton approach to produce a reliable resistivity model. Seawater is excluded from the inversion domain to fix the resistivity, while included in the modeling domain to simulate sea effects on MT responses. Blocks for the sub-seafloor are included in unknown parameters since they are sufficiently close to the survey area to affect MT responses in Pohang. Static shifts are also considered in inversion for more accurate interpretation. The rms data misfit is smoothly reduced from 11.2 to 1.87 after 7 iterations. The resulting resistivity model shows a pattern of low–high–low resistivity with depth. The model is compatible with resistivity logs obtained from four boreholes in the survey area, and can explain major geological features in Pohang.

Magnetotelluric (MT) surveys have been performed along five lines in the Jeju volcanic island located at South Sea of Korea. The purposes of the MT surveys are to see if there still remains thermal regime and if there exist deeply extended fractures or an aquifer system beneath the mid-mountain region of Mt. Halla in Jeju. By performing Audio-frequency MT (AMT) surveys together with MT surveys at each site and by operating far-remote reference site of approximately 480 km apart, we could get a broadband (10 − 4 –10 4 Hz) MT dataset of totally 108 sites with very good quality. 3D inversion of MT data from 88 measurement sites gives a reasonable estimation of the subsurface structures beneath the island. An induction vector analysis and 3D inversion results commonly indicate the existence of a conductive anomaly extended down to a few-kilometer depth beneath the central part of the island, which can be related to ancient volcanic activities. Because the island is surrounded by electrically conductive sea and has steep topographic variation, measured MT data in terms of impedance is seriously biased by distortion of electric fields and is suffered from the static shifts. The Jeju MT dataset is valuable for the research purpose in the sense that the dataset contains so many problems, some of which can frequently be observed in every field data, and that the geology of Jeju Island is relatively simple.

This paper presents a comparison of four different three-dimensional (3D) magnetotelluric (MT) modelling algorithms in terms of accuracy and computation time. Three of them use the finite difference method while the last one uses the edge finite-element method. The modelling algorithms are analysed with respect to governing equations, boundary conditions, solvers, preconditioners and static divergence correction. The accuracy of the algorithms is examined by simulating MT responses for two different models, both of which have anomalous bodies in simple background media. Although the simulated responses generally show good agreement, small discrepancies are observed over the anomalous bodies. These discrepancies are caused by the difference between the electric- and magnetic-field formulations used by the algorithms. The three algorithms using the finite difference method are adequate for 3D inversion of MT data since they produce solutions in a short computation time. Although the algorithm using the edge finite-element method requires longer computation time than the others, it can provide more accurate solutions and model surface topography.

SUMMARY Magnetotelluric (MT) surveys have been conducted to investigate the volcanic structure in Jeju Island in the South Sea of Korea. The island is oval in shape, 31 km wide and 73 km long, with a peak elevation of 1950 m at Mt. Halla, located in the centre of the island. MT data were obtained in a wide range of frequencies in a mid-mountain region and may be affected by distortion of electric fields due to topographic and bathymetric effects. The purpose of this paper is to investigate the topographic and bathymetric effects on MT data for a four-layered model having the topographic and bathymetric undulation of Jeju Island, using an edge finite-element method. The model effectively reproduces a general feature of MT sounding curves observed on the island. The topographic effect appears only at high frequencies, whereas the sea effect causes a split of apparent resistivity curves in different polarizations at low frequencies. Since the island has a gentle slope of less than 10° except in the close vicinity of Mt. Halla, it has a similar effect of a change in thickness in the top layer. In contrast, the sea effect can be observed as quasi-2-D rather than 3-D along the major axis of the island and generates a parallel shift of the log-apparent resistivity curve at low frequencies.

SUMMARY A method for accurately approximating sensitivities is introduced for the efficient 3-D inversion of static-shifted magnetotelluric (MT) data. Approximate sensitivities are derived by replacing adjoint secondary electric fields with those computed in the previous iteration. These sensitivities can reduce the computation time, without significant loss of accuracy when constructing a full sensitivity matrix for 3-D inversion, based on the Gauss–Newton method. Additional reduction of computational cost can be attained by modifying the inversion scheme to run on a parallel computing platform. The effectiveness of approximate sensitivities is tested by inverting both synthetic and field data obtained in Pohang, Korea, and Bajawa, Indonesia. The accuracy of approximate sensitivities is validated by sensitivity analysis of synthetic data. To make the inversion of static-shifted MT data more stable, a weighting coefficient for static-shift parameters is added to the objective function and is updated at each iteration. Approximate sensitivities are calculated much faster than exact sensitivities, and are accurate enough to drive an iterative inversion algorithm.

Two different time series data sets, shallow ground temperatures of 58 synoptic stations of the Korea Meteorological Administration (KMA) and groundwater temperatures of 67 wells of the National Groundwater Monitoring Network (NGMN), were analyzed to estimate the apparent thermal diffusivity by using the analytical solution of the one-dimensional heat conduction equation. The KMA temperature data measured at 1–5 m depths illustrated values of the phase delay and the amplitude decay coincident with their theoretical relationship, indicating that the conductive heat transport should prevail over the nonconductive processes. On the contrary, some of the estimates from temperatures at a depth of 0.5 m were away from the theoretical values. It is most likely that the deviation would be caused by the effects of latent heat associated with freezing and thawing of the near ground surface. In contrast to KMA data, results obtained from the NGMN data highly deviated from the theoretical ones, and thereafter yielded unacceptably high values of thermal diffusivities as compared to the representative values of soils and rocks. Implication of the discrepancy between two data sets was discussed in conjunction with perturbation of the conductive heat transport by free convection of water and air occurring in large diameter wells as well as the convective heat transport by groundwater flow.

SUMMARY Topographic effects due to irregular surface terrain may prevent accurate interpretation of magnetotelluric (MT) data. Three-dimensional (3-D) topographic effects have been investigated for a trapezoidal hill model using an edge finite-element method. The 3-D topography generates significant MT anomalies, and has both galvanic and inductive effects in any polarization. This paper presents two different correction algorithms, which are applied to the impedance tensor and to both electric and magnetic fields, respectively, to reduce topographic effects on MT data. The correction procedures using a homogeneous background resistivity derived from a simple averaging method effectively decrease distortions caused by surface topography, and improve the quality of subsurface interpretation. Nonlinear least-squares inversion of topography-corrected data successfully recovers most of structures including a conductive or resistive dyke.

Magnetotelluric soundings were obtained along two traverse lines to the north and west of the Century mine in northwest Queensland. The survey was designed to cross the Termite Range Fault, a major structure on the Lawn Hill Platform, and to provide insights into the crustal-scale architecture that may have controlled the location of this world-class zinc deposit. The projected surface trace of the Termite Range Fault is coincident with a major change in resistivity character that extends to a significant depth. A relatively flat-lying, stacked series of resistive/conductive layers occurs on the northeastern side of the fault , while on the southwestern side the resistive/conductive layers are much less evident. The major contrast in resistivity is interpreted as due to a steep northeast-dipping Termite Range Fault that may extend to 20 km depth. To the southwest of the Termite Range Fault, a second major fault, the Riversleigh Lineament, is inferred from geology and gravity data, although there is no cor...

We have developed an inversion algorithm for loop-loop electromagnetic (EM) data, based on the localised non- linear or extended Born approximation to the solution of the 2.5D integral equation describing an EM scattering problem. Source and receiver configuration may be horizontal co-planar (HCP) or vertical co-planar (VCP). Both multi-frequency and multi-separation data can be incorporated. Our inversion code runs on a PC platform without heavy computational load. For the sake of stable and high-resolution performance of the inversion, we implemented an algorithm determining an optimum spatially varying Lagrangian multiplier as a function of sensitivity distribution, through parameter resolution matrix and Backus-Gilbert spread function analysis. Considering that the different source-receiver orientation characteristics cause inconsistent sensitivities to the resistivity structure in simultaneous inversion of HCP and VCP data, which affects the stability and resolution of the inversion result, we adapted a weighting scheme based on the variances of misfits between the measured and calculated datasets. The accuracy of the modelling code that we have developed has been proven over the frequency, conductivity, and geometric ranges typically used in a loop-loop EM system through comparison with 2.5D finite-element modelling results. We first applied the inversion to synthetic data, from a model with resistive as well as conductive inhomogeneities embedded in a homogeneous half-space, to validate its performance. Applying the inversion to field data and comparing the result with that of dc resistivity data, we conclude that the newly developed algorithm provides a reasonable image of the subsurface.

Ground subsidence occurred in the centre of a small city in South Korea. In order to investigate the cause of the geological hazards and to estimate the ground safety, we carried out integrated geophysical surveys comprising two-dimensional (2D) resistivity, controlled source magnetotelluric (CSMT), magnetic, ground penetrating radar, geophysical well logging and crosshole electrical resistivity tomography (ERT) surveys. Since the target area is located in the downtown area, surface geophysical methods could not be applied systematically. To understand regional geology and to facilitate the interpretation of detailed geophysical surveys in the target area, 2D resistivity, CSMT and magnetic surveys were conducted outside the downtown area. From these results, we could define the regional structure and successively infer the geologic condition in the city centre as well. Among the geophysical techniques applied for the detailed investigation in the main target area, crosshole ERT and geophysical well logging played the most important role. For the efficient ERT field work in the busiest quarter of the city, we devised a new electrode array, modified pole–dipole array, and proved that the proposed array is efficient particularly in the area where installing a remote electrode is nearly impossible. The distribution of cavities and weak zones was interpreted by careful examination of the resistivity tomograms and geophysical logging results. Based on the distribution of cavities interpreted in a 3D manner, numerical analyses of rock engineering were further carried out and geologic hazard maps were presented. Through this comprehensive approach comprising geophysics and rock engineering, shallow limestone cavities were found to be the main cause of the ground subsidence and the excessive pumping of groundwater might trigger or accelerate the geological hazard. Reinforcement works have been carried out based on the results of these geophysical and rock engineering investigations.

A series of events occurred at an earth-filled dam which raised a serious question about the safety of the dam. An integrated geophysical survey, including dc resistivity, controlled source magnetotelluric (CSMT), ground penetrating radar (GPR), and seismic refraction and tomography methods, has been conducted on the dam. The purpose of the survey was to provide basic data through imaging the internal structure of the embankment for a precise safety and stability examination of the dam. One of the essential reasons applying various kinds of geophysical methods is to view the physical properties of the internal structure of the dam and to compare the results each other. The second reason is to accomplish two survey objectives in the safety examination of the dam: imaging of the internal condition of the dam and investigation of the basement structures beneath the dam and its vicinity. All the applied geophysical investigation showed their own characteristic responses to anomalous zones. The zones delineated by each method agree with each other, confirming that these anomalies are directly related to the weak zones that cause the problems in the dam. The CSMT results indicated the dam was partly located on a fracture zone. The 3-D resistivity imaging provided an overall image and the progressive nature of the weak zone development in the dam. The GPR images implied that some disturbances might take place in the internal part and thus deform the shallow layered structure. Comparing the results of the seismic tomography and 3-D resistivity images led to the conclusion that the clay core not only contains materials other than clay, such as gravels and rock fragments, but also an abnormally large amount of water which have caused a serious dam stability problem.