Audio-frequency Magnetotelluric Survey Research Articles

Shallow resistivity structure around the 2018 craters of Mt. Motoshirane of Kusatsu-Shirane Volcano, Japan, revealed by audio-frequency magnetotellurics

In 2018, a phreatic eruption occurred at Mt. Motoshirane, a pyroclastic cone group of the Kusatsu-Shirane Volcano in central Japan. The eruption was abrupt, and no signs of volcanic activity have been observed in recent years, in contrast to the other major pyroclastic cone group, Mt. Shirane, which hosts the active crater lake and has endured repeated phreatic eruptions. To understand the mechanism of the eruption at Mt. Motoshirane, information on the shallow hydrothermal system, which is thought to be the source region of phreatic eruptions, is required; however, few studies have been conducted on this particular cone group. In this study, we conducted an audio-frequency magnetotelluric survey in 2020 to reveal the shallow resistivity structure around the 2018 craters. A three-dimensional resistivity structure model showed generally two layers, with high resistivities at shallow depths overlain by low resistivities underneath. The boundary between the layers corresponded to the top boundary of the Neogene basement rocks. These low resistivities were not found beneath the 2018 craters; therefore, part of the Neogene basement rocks could have been lost by the eruption beneath the 2018 craters. This is consistent with the geochemical study on the mineral assemblage of the erupted deposits, which suggested that the explosions reached the depth of the basement.Graphical

Audio Magnetotellurics Study of the Geoelectric Structure across the Zhugongtang Giant Lead–Zinc Deposit, NW Guizhou Province, China

Non-invasive geophysical exploration methods a play key role in the exploration of ore deposits. In the present study, the audio-frequency magnetotelluric (AMT) method was applied to metallic mineral exploration. The metallic mineral deposit targeted was the recently discovered super large lead–zinc deposit of the Zhugongtang mining area of Hezhang County in the northwestern Guizhou province in China. The main objectives of this study were to estimate the geoelectric strike and generate geoelectric models that estimate both the depth and distribution of resistivity structures across the deposit. To achieve the objectives, we deployed sixty-one (61) AMT survey sites with an interstation separation of 20 m on a 1280 m survey track perpendicular to the geological strike across the Zhugongtang deposit. We operated in fifty-three (53) frequencies in the range 1–10,400 Hz to record the resistivity distribution of subsurface to a depth of more than 1200 m. The results from the AMT data computations estimated the geoelectric strike that varies between NE285° and NE315°. This range of strikes suggested that structures across the deposit are oriented in the NW–SE direction. Obtained two-dimensional (2D) models elucidated a remarkably low resistivity body (<15 Ωm) at an elevation of less than 1600 m above sea level (>0.50 km depth), thus extending to great depth and were interpreted as lead–zinc mineralization. Furthermore, low resistivity (<63 Ωm) features were imaged both in superficial and deeper depths and interpreted as shale, sandstone, claystone, and silty mudstone units. Dolomite and limestone lithologies were found widely distributed with high resistivity (>1000 Ωm). Bioclastic limestone and dolomite limestone were inferred and characterized by moderate-high resistivity (>250 Ωm) and were not widely distributed. A unit of basalts was found with moderate resistivity (>63 Ωm). In addition, it was also found that regions with high number of faults tend to have low resistivity values compared to regions with a low fault number. In summary, this case study presents the results of applying an AMT approach to explore the conductivity characteristics of structures across the Zhugongtang deposit. The findings may contribute to the literature about this deposit.

Properties and Engineering Applications of a New Goaf Grouting Filling Material

In the treatment of goafs in traffic engineering, technical problems such as those related to large-volume grouting and the precise control of material properties are often encountered. To address these issues, we developed a new composite material comprising cement-fly ash-modified sodium silicate (C-FA-MS). The setting time, fluidity, unconfined compressive strength, and microstructure were varied for different proportions of cement-sodium silicate (C-S) slurry, cement-fly ash-sodium silicate (C-FA-S) slurry, and C-FA-MS slurry, and their performances were compared and analysed. The experimental results showed that the initial setting time of the slurry was the shortest when both the original sodium silicate volume ratio ( V S ) and modified sodium silicate volume ratio ( V MS ) were 0.2. The final setting time of the C-S and C-FA-S slurries tended to decrease but then increased with decrease in V S , while that of the C-FA-MS slurry increased with lower V MS . The fluidity of the C-FA-S and C-FA-MS slurries decreased with decrease in V S or V MS at different fly ash admixture ratios. The consolidation compressive strength of C-S increased with decreasing V S , while that of C-FA-S showed a considerable increase only when V S decreased from 0.4 to 0.2. Meanwhile, the compressive strength of the C-FA-MS concretions first increased and then decreased with decrease in V MS . Microstructural analysis revealed that there were more cracks in the C-S agglomerate, the fly ash in the C-FA-S agglomerate reduced the relative density of the skeletal structure, and the stronger cross-linking in the C-FA-MS agglomerate improved the strength of the agglomerate. Under the condition of unit grouting volume, the cost of the C-FA-MS slurry was approximately 44.7% and 31.3% lower than that of the C-S and C-FA-S slurries, respectively. The new C-FA-MS material was applied for the treatment of the goaf in the Wu Sizhuang coal mine. Core drilling detection and audiofrequency magnetotelluric survey revealed that the goaf was sufficiently filled.

Возможности аудиомагнитотеллурических зондирований с бесконтактными электрическими линиями при изучении геоэлектрического строения полуострова Рыбачий (Мурманская область)

Возможности аудиомагнитотеллурических зондирований с бесконтактными электрическими линиями при изучении геоэлектрического строения полуострова Рыбачий (Мурманская область)

Возможности аудиомагнитотеллурических зондирований с бесконтактными электрическими линиями при изучении геоэлектрического строения полуострова Рыбачий (Мурманская область).

Возможности аудиомагнитотеллурических зондирований с бесконтактными электрическими линиями при изучении геоэлектрического строения полуострова Рыбачий (Мурманская область).

Relationship between volcanic activity and shallow hydrothermal system at Meakandake volcano, Japan, inferred from geomagnetic and audio-frequency magnetotelluric measurements

Hydrothermal activity at Meakandake volcano, Japan, from 2004 to 2014 was investigated by using long-term geomagnetic field observations and audio-frequency magnetotelluric (AMT) surveys. The total intensity of the geomagnetic field has been measured around the summit crater Ponmachineshiri since 1992 by Kakioka Magnetic Observatory. We reanalyzed an 11-year dataset of the geomagnetic total intensity distribution and used it to estimate the thermomagnetic source models responsible for the surface geomagnetic changes during four time periods (2004–2006, 2006–2008, 2008–2009 and 2013–2014). The modeled sources suggest that the first two periods correspond to a cooling phase after a phreatic eruption in 1998, the third one to a heating phase associated with a phreatic eruption in 2008, and the last one to a heating phase accompanying minor internal activity in 2013. All of the thermomagnetic sources were beneath a location on the south side of Ponmachineshiri crater. In addition, we conducted AMT surveys in 2013 and 2014 at Meakandake and constructed a two-dimensional model of the electrical resistivity structure across the volcano. Combined, the resistivity information and thermomagnetic models revealed that the demagnetization source associated with the 2008 eruptive activity, causing a change in magnetic moment about 30 to 50 times greater than the other sources, was located about 1000m beneath Ponmachineshiri crater, within or below a zone of high conductivity (a few ohm meters), whereas the other three sources were near each other and above this zone. We interpret the conductive zone as either a hydrothermal reservoir or an impermeable clay-rich layer acting as a seal above the hydrothermal reservoir. Along with other geophysical observations, our models suggest that the 2008 phreatic eruption was triggered by a rapid influx of heat into the hydrothermal reservoir through fluid-rich fractures developed during recent seismic swarms. The hydrothermal reservoir remained hot after the 2008 eruption, and heat was sporadically transported upward through its low permeability ceiling.

P1-33 比抵抗構造から推定される立山地獄谷の熱水系(ポスターセッション)

P1-33 比抵抗構造から推定される立山地獄谷の熱水系(ポスターセッション)

An integrated magnetotelluric and gamma exploration of groundwater in fractured granite for small-scale freshwater supply: a case study from the Boshan region, Shandong Province, China

Exploration and use of groundwater in hard rocks is a difficult but yet a significant issue, especially when the local surface water source is polluted. In this study, integrated geophysical and nuclear methods are used to delineate groundwater-bearing zones in a granite terrain in the Boshan region, Shandong Province, China. Structural analysis showed that fractures extend in a NW–SE direction, which is the same as the direction of river valley. A controlled source audio-frequency magnetotelluric survey conducted along the river valley indicated several different low resistivity anomalies or potential water-bearing fracture zones. They are seated at depth ranges down to 400 m from the surface along the river valley. The locations of deep water-bearing zones were further confirmed by higher gamma radiation measurements found near the river valley. A fault with a NW–SE strike near the river was deduced as an anomalous water-bearing zone. Two tube wells were drilled targeting at the water-bearing zone that has achieved single-well water yield greater than 360 m3/day. The well water was sampled for water chemistry and stable isotope analyses. Shallow groundwater samples show similar water chemistry and isotopic feature to river water, indicating connection between the two. Deep groundwater from the water-bearing fracture zones is not hydraulically connected with shallow groundwater, ensuring safety of the deep groundwater in the granite from being affected by the polluted shallow groundwater or river water. Integrated geophysical and gamma exploration methods helped to identify two disconnected groundwater systems, offering an alternative freshwater source for the region if shallow groundwater or surface water is polluted.

Resistivity structure and geochemistry of the Jigokudani Valley hydrothermal system, Mt. Tateyama, Japan

This study clarifies the hydrothermal system of Jigokudani Valley near Mt. Tateyama volcano in Japan by using a combination of audio-frequency magnetotelluric (AMT) survey and hot-spring water analysis in order to assess the potential of future phreatic eruptions in the area. Repeated phreatic eruptions in the area about 40,000years ago produced the current valley morphology, which is now an active solfatara field dotted with hot springs and fumaroles indicative of a well-developed hydrothermal system. The three-dimensional (3D) resistivity structure of the hydrothermal system was modeled by using the results of an AMT survey conducted at 25 locations across the valley in 2013–2014. The model suggests the presence of a near-surface highly conductive layer of <50m in thickness across the entire valley, which is interpreted as a cap rock layer. Immediately below the cap rock is a relatively resistive body interpreted as a gas reservoir. Field measurements of temperature, pH, and electrical conductivity (EC) were taken at various hot springs across the valley, and 12 samples of hot-spring waters were analyzed for major ion chemistry and H2O isotopic ratios. All hot-spring waters had low pH and could be categorized into three types on the basis of the Cl−/SO42− concentration ratio, with all falling largely on a mixing line between magmatic fluids and local meteoric water (LMW). The geochemical analysis suggests that the hydrothermal system includes a two-phase zone of vapor–liquid. A comparison of the resistivity structure and the geochemically inferred structure suggests that a hydrothermal reservoir is present at a depth of approximately 500m, from which hot-spring water differentiates into the three observed types. The two-phase zone appears to be located immediately beneath the cap rock structure. These findings suggest that the hydrothermal system of Jigokudani Valley exhibits a number of factors that could trigger a future phreatic eruption.

Imaging the hydrothermal system beneath the Jigokudani valley, Tateyama volcano, Japan: implications for structures controlling repeated phreatic eruptions from an audio-frequency magnetotelluric survey

This study focuses on the results of an audio-frequency magnetotelluric (AMT) survey across the Jigokudani valley, Tateyama volcano, Japan, to investigate the spatial relationship between the distribution of electrical resistivity and geothermal activity and to elucidate the geologic controls on both its phreatic eruption history and recent increase in phreatic activity. The AMT data were collected at eight locations across the Jigokudani valley in September 2013, with high quality data obtained from most sites, enabling the identification of an underground 2D resistivity structure from the transverse magnetic (TM) mode data. The data obtained during this study provided evidence of a large conductive region beneath the surface of the Jigokudani valley that is underlain by a resistive layer at depths below 500 m. The resistive layer is cut by a relatively conductive region that extends subvertically toward the shallow conductor. The shallow conductive region is divided into an uppermost slightly conductive section that is thought to be a lacustrine sediment layer of an extinct crater lake containing hydrothermal fluids and a lower section containing a mix of volcanic gases and hydrothermal fluids. The low permeability of the clay zone means that the uppermost clayey sediments allow the accumulation of gases in the lower section of the conductive region, suggesting the existence of a cap structure. The deep resistive layer likely consists of units similar to the granitic rocks that are widely exposed throughout the Jigokudani valley. We suggest that the relatively conductive zone that separates these granitic rocks represents a high-temperature volcanic gas conduit, given that the most active fumarole in the Jigokudani valley lies directly along the trajectory of this path.

Seismic reflection constraints on upper crustal structures in the volcanic-covered central Nechako basin, British Columbia1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

In 2008, a Vibroseis seismic reflection survey was acquired by Geoscience BC across the eastern part of the volcanic-covered Nechako basin in central British Columbia, where Cretaceous sedimentary rocks have been exhumed along a NNW trend. Good signal penetration through the volcanic cover is indicated by lower crustal reflections at 8–12 s, which were recorded by the entire seismic survey. Comparison of the 2008 seismic survey with data from a previous survey indicates that the lack of reflectivity in the earlier surveys is generally representative of the subsurface geology. The seismic data show that ∼1700 and ∼2900 m thick sub-basins are present at the northern and southern ends of this trend, but the intervening Cretaceous rocks are discontinuous and relatively thin. The creation of a passive-roof duplex by Campanian or later low-angle thrusting is inferred within the thickest Cretaceous strata, but elsewhere faulting is likely related to Eocene extension or transtension. Seismic reflections are also recorded from folded volcanic stratigraphy, the base of the surface volcanic rocks, an underlying volcaniclastic stratigraphy, and intrusions projecting into a Quaternary volcanic cone. Seismic interpretation is complemented by coincident audiofrequency magnetotelluric surveys, from which faulting is inferred at offsets in a regional conductor. No regionally extensive stratigraphy can be identified within the seismic data, and the central Nechako basin appears to be a complex network of small, deformed sub-basins, rather than a single large basin.

A heating process of Kuchi-erabu-jima volcano, Japan, as inferred from geomagnetic field variations and electrical structure

Since August 2000, we have recorded the total intensity of the geomagnetic field at the summit area of Kuchi-erabu-jima volcano, where phreatic eruptions have repeatedly occurred. A time series analysis has shown that the variations in the geomagnetic field since 2001 have a strong relationship to an increase in volcanic activity. These variations indicate thermal demagnetization of the subsurface around the presently active crater. The demagnetization source for the early variations, until summer 2002, was estimated at about 200 m below sea level. For the variations since 2003, the source was modeled on the basis of the expansion of a uniformly magnetized ellipsoid. The modeling result showed that the source is located at 300 m above sea level beneath the crater. We carried out an audio-frequency magnetotelluric survey with the aim of obtaining a relation between the demagnetization source and the shallow structure of the volcano. A two-dimensional inversion applied to the data detected two good conductors, a shallow thin one which is restricted to a region around the summit area, while the other extends over the edifice at depths between 200 and 800 m. These conductors are regarded as clay-rich layers with low permeability, which were assumed to be generated through hydrothermal alteration. The demagnetization source for the early variations was possibly located at the lower part of the deep conductor and the source after 2003 lies between the two conductors, where groundwater is considered to be abundant. Based on these results, as well as on seismological, geodetic, and geochemical information, we propose a heating process of the Kuchi-erabu-jima volcano. In the initial stage, high-temperature volcanic gases supplied from the deep-seated magma remained temporarily at the level around the lower part of the less permeable deep conductor since the ascent path had not yet been established. Then, when the pathway developed as a result of repeated earthquakes, it became possible for a massive flux of volcanic gases to ascend through the conductor. The high temperature gases reached the aquifer located above the conductor and the heat was efficiently transported to the surrounding rocks through the groundwater. As a consequence, an abrupt increase of the gas flux and diffusion of the heat through the aquifer occurred and the high-temperature zone expanded. Since the high-temperature zone is located beneath another conductor, which acts as caprock, we assume that the energy of the phreatic explosion is accumulated there.

Resistivity and density modelling in the 1938 Kutcharo earthquake source area along a large caldera boundary

Abstract We present the crustal structure around the fault zone pertaining to the 1938 Kutcharo earthquake (M 6.0), northern Japan, to consider why large earthquakes have occurred around calderas. The study was based on gravity anomalies and magnetotelluric and direct-current (DC) electrical-resistivity survey data. The density structure obtained from gravity anomalies indicated that the fault plane corresponded to the main depression boundary of the Kutcharo caldera. The resistivity section, based on audio-frequency magnetotelluric surveys, indicated that the estimated fault plane was located along the boundary of resistivity blocks, which also corresponded to the depression boundary. A detailed resistivity section in the ruptured zone revealed by a DC electrical-resistivity survey showed a discontinuity of layers, implying cumulative fault displacements. These results indicate that the 1938 earthquake was an abrupt slip along the main depression boundary of the Kutcharo caldera. The most likely hypothesis pertains to fluid intrusion along the depression boundary. However, additional seismic and geodetic studies are required to identify other feasible earthquake mechanisms.

A preparation zone for volcanic explosions beneath Naka-dake crater, Aso volcano, as inferred from magnetotelluric surveys

The 1st crater of Naka-dake, Aso volcano, is one of the most active craters in Japan, and known to have a characteristic cycle of activity that consists of the formation of a crater lake, drying-up of the lake water, and finally a Strombolian-type eruption. Recent observations indicate an increase in eruptive activity including a decrease in the level of the lake water, mud eruptions, and red hot glows on the crater wall. Temporal variations in the geomagnetic field observed around the craters of Naka-dake also indicate that thermal demagnetization of the subsurface rocks has been occurring in shallow subsurface areas around the 1st crater. Volcanic explosions act to release the energy transferred from magma or volcanic fluids. Measurement of the subsurface electrical resistivity is a promising method in investigating the shallow structure of the volcanic edifices, where energy from various sources accumulates, and in investigating the behaviors of magma and volcanic fluids. We carried out audio-frequency magnetotelluric surveys around the craters of Naka-dake in 2004 and 2005 to determine the detailed electrical structure down to a depth of around 1 km. The main objective of this study is to identify the specific subsurface structure that acts to store energy as a preparation zone for volcanic eruption. Two-dimensional inversions were applied to four profiles across the craters, revealing a strongly conductive zone at several hundred meters depth beneath the 1st crater and surrounding area. In contrast, we found no such remarkable conductor at shallow depths beneath the 4th crater, which has been inactive for 70 years, finding instead a relatively resistive body. The distribution of the rotational invariant of the magnetotelluric impedance tensor is consistent with the inversion results. This unusual shallow structure probably reflects the existence of a supply path of high-temperature volcanic gases to the crater bottom. We propose that the upper part of the conductor identified beneath the 1st crater is mainly composed of hydrothermally altered zone that acts both as a cap to upwelling fluids supplied from deep-level magma and as a floor to infiltrating fluid from the crater lake. The relatively resistive body found beneath the 4th crater represents consolidated magma. These results suggest that the shallow conductor beneath the active crater is closely related to a component of the mechanism that controls volcanic activity within Naka-dake.

Audio frequency magneto-telluric survey of Norikura Volcano in central Japan

Norikura Volcano has not been active during the last 10,000 years in spite of the activity of the surrounding volcanic mountains. To study past volcanic activities, geological studies were carried out extensively. However, quite a few geophysical investigations were conducted to contribute to volcanology. Our objective is to detect the present subsurface structure of Norikura Volcano and to define volcanic stratifications. In the vicinity of Norikura Volcano, geothermal fields are still active. Subsurface volcanic rocks in this area have been exposed to geothermal activity and altered. To comprehend volcanic stratifications of Norikura and geothermal activity, we conducted audio frequency magneto-telluric (AMT) surveys around Norikura Volcano. AMT survey is useful in clearly defining the resistivity structure related to volcanic regions. The AMT data were acquired over a frequency range 10 Hz–10 kHz. Decomposition analysis was applied to the tensor impedance data. Subsequently, apparent resistivity and phase data were inverted using a two-dimensional magneto-telluric (MT) inversion and a model of Norikura was derived. The final model manifests that the surface resistors are in agreement with andesite lava or dacite lava. As for the deeper structure, a horizontal conductor is situated above resistive basements. The alteration of the conductor was weak, while basement rocks were strongly altered and/or heated through the thermal activity. The existence of these layers seems to indicate the degree of thermal activity of Norikura Volcano.

Enhancement of Signal-to-noise Ratio in Natural-source Transient Magnetotelluric Data with Wavelet Transform

—For audio-frequency magnetotelluric surveys where the signals are lightning-stroke transients, the conventional Fourier transform method often fails to produce a high quality impedance tensor. An alternative approach is to use the wavelet transform method which is capable of localizing target information simultaneously in both the temporal and frequency domains. Unlike Fourier analysis that yields an average amplitude and phase, the wavelet transform produces an instantaneous estimate of the amplitude and phase of a signal. In this paper a complex well-localized wavelet, the Morlet wavelet, has been used to transform and analyze audio-frequency magnetotelluric data. With the Morlet wavelet, the magnetotelluric impedance tensor can be computed directly in the wavelet transform domain. The lightning-stroke transients are easily identified on the dilation-translation plane. Choosing those wavelet transform values where the signals are located, a higher signal-to-noise ratio estimation of the impedance tensor can be obtained. In a test using real data, the wavelet transform showed a significant improvement in the signal-to-noise ratio over the conventional Fourier transform.

On: “Results of a controlled‐source audiofrequency magnetotelluric survey at the Puhimau thermal area, Kilauea Volcano, Hawaii” by L. C. Bartel and R. D. Jacobson (GEOPHYSICS, 52, 665–677, May 1987).

Bartel and Jacobson try to correct apparent resistivity values from CSAMT measurements, where the plane‐wave approximation is not valid. They want to convert CSAMT curves to passive MT curves (Cagniard curves), which can be interpreted by 2-D modeling. The measured CSAMT [Formula: see text] value is projected onto the [Formula: see text] axis in parallel with the half‐space curves of a horizontal electrical dipole in the given distance between receiver and transmitter. The example looks convincing, and it may be supposed to be a simple near‐field correction.

On: “Results of a controlled‐source audiofrequency magnetotelluric survey at the Puhimau thermal area, Kilauea Volcano, Hawaii, by L. C. Bartel and R. D. Jacobson (GEOPHYSICS, 52, 665–677, May 1987)

A growing number of papers being published on the CSAMT-MT curve transformation, which — as the authors state — allows a simpler magnetotelluric interpretation of the corrected CSAMT curves. The concept of near‐field corrections is based on electromagnetic relations over a homogeneous earth, and the effects of subsurface layers or lateral inhomogeneities are usually neglected. Bartel and Jacobson (1987) especially suppress the bounds of the near‐field correction: After presenting several near‐field correction curves over a homogeneous earth in their Figure 2 (which includes an idealistic demarcation line instead of a gradual change between near‐field and far‐field regions), they simply add that “…for a layered earth a similar demarcation occurs between the far‐ and near‐field regimes.” Further, the problem of lateral inhomogeneities is not mentioned in the paper. Such a description might lead to an oversimplification. I should like here to underline both limitations.