MT Data Research Articles

Input and output files of ModEM 3-D inversion of MT data in Nanling

Input and output files of ModEM 3-D inversion of MT data in Nanling

In vitro characterization of the serotonin biosynthesis pathway by CEST MRI.

The diagnosis of monoamine-related psychiatric disorders is based on the phenomenological evaluation of symptoms and behavior by trained clinicians. The CEST technique can be sensitive to monoamines such as serotonin. This study quantifies the CEST properties of the compounds in the serotonin biosynthesis pathway with the goal of developing noninvasive techniques aimed at advancing the diagnostic assessment of serotonin dysfunction. Saturation transfer-weighted images of L-tryptophan, 5-hydroxytryptophan, serotonin, 5-hydroxyindoleacetic acid, and melatonin phantoms were acquired over a range of saturation amplitudes and frequency offsets along with observed T1 , T2 , and B1 efficiency maps at physiological temperature and pH of 5.5, 6.7, and 7.4. The CEST and MT data were fitted to a three-pool Bloch-McConnell model of exchange to estimate the model parameters. At apH of 5.5, tryptophan, 5-hydroxytryptophan and serotonin exhibited significant CEST contrast at resonance frequency offset, Δω between 2.64 ppm and 2.71 ppm, and magnetization transfer ratio asymmetry amplitudes up to 20% per 30 mM. At apH of 7.4, all molecules exhibited significant CEST contrast between 5.11 ppm and 5.47 ppm, and magnetization transfer ratio asymmetry amplitudes up to 9.5%per 30 mM. At apH of 6.7, all studied compounds except melatonin exhibited a CEST peak from each of the preceding two pHs. At apH of 5.5, tryptophan, 5-hydroxytryptophan, and serotonin CEST contrast originates from the side chain, whereas at apH of 7.4, CEST contrast is due to the chemical exchange between water and the NH proton on the indole ring. The data in this study could be used to inform future investigations aimed at detecting and measuring in vivo serotonin.

Array analysis of magnetic and electric field observatories in China: estimation of magnetotelluric impedances at very long periods

SUMMARY Ten years (2008–2017) of continuous measurements from 40 electric and 36 magnetic sites collected in China for earthquake prediction research represent a unique EM array data set, which can be used to explore the challenging problem of very long-period MT data acquisition, to study source characteristics, and ultimately to learn about electric conductivity of Earth's mantle beneath East Asia. In this study, we focus on basic noise and signal characteristics in this data set, and on estimation of the MT impedances. We report a novel method to fix the numerous timing errors in the electric data caused by limitations in instrumentation and data acquisition. Then, we use multivariate array analysis to study signal and noise characteristics for periods from 250 s to 3.5 × 105 s (4 d). Signal-to-noise ratios (SNR) are above 30 dB in magnetic fields for the first two dominant modes, which correspond roughly to N–S and E–W quasi-uniform sources. SNRs for electric fields are lower, especially at very long periods, and especially for N–S electric components. There are clear peaks in signal strength at the daily variation (DV) periods, but source structure becomes more complex, and significant biases in MT impedance tensors are more often seen at these periods. The MT quasi-impedance, computed using the closest magnetic site for each electric site, is estimated by robust remote reference techniques (RR) and by using linear combinations of PCA (principal component analysis) modes that best approximate a uniform or plane-wave source (PW). For almost all sites, smooth impedances are obtained for periods up to 104 s using either approach. This result, and a more detailed analysis of impedances estimates obtained with shorter-wavelength (gradient) sources extracted from the array, suggests that source effects in MT impedances are minimal for periods below 104 s, at least at the latitude of China. At many sites curves can be extended a decade further, to 105 s, but here results are improved by carefully omitting DV bands and (at a few sites) with the PW approach. For longer periods (>105 s) SNR is very low in electric field channels at most sites, making estimation challenging. However, at a few sites, even some near big cities (e.g. including a site within 40 km of Beijing) smooth impedance components related to N–S magnetic sources (Zxx and Zyx) are obtained to periods to 3.5 × 105 s (4 d). This result suggests that cultural noise may not be the main impediment to collecting very long-period MT data.

Quality over quantity: on workflow and model space exploration of 3D inversion of MT data

3D inversions of magnetotelluric data are now almost standard, with computational power now allowing an inversion to be performed in a matter of days (or hours) rather than weeks. However, when compared to 2D inversions, these are still very computationally demanding. As a result, 3D inversions are generally not subjected to as rigorous testing as a 1D or 2D inversion would be, which has implications when these models are used for geological interpretation. In this study, we explore the parameter space for inversion of continent-scale datasets. The generalisations made regarding the effects of each parameter should also be scalable to smaller surveys and will enable MT practitioners to optimise their results. We have performed testing on a subset of the South Australian component of the eventual Australia-wide AusLAMP (Australian Lithospheric Architecture Magnetotelluric Project). The subset was inverted with different parameters, model setup and data subsets. Specifically, results from testing of the model covariance, the resistivity of the prior model, the inclusion of 'known' information into the prior model, the model cell size, the data components inverted for and the damping parameter lambda were all investigated. In our testing of the 3D inversion software, ModEM3DMT, we found that the resistivity of the starting/prior model had significant effect on the final model. Careful selection of initial lambda value can aid in reducing computational time whilst having a negligible effect on the resultant model, whilst large covariance values and model cell sizes enhanced conductive features at depth.

Development of noise reduction method for MT data processing using Multi-channel singular spectrum analysis at Boso Peninsula in Japan

Development of noise reduction method for MT data processing using Multi-channel singular spectrum analysis at Boso Peninsula in Japan

Deep Geoelectric Structure and Its Relation to Seismotectonics of the Saurashtra Region, Western India

The Saurashtra Peninsula and its adjoining regions covered by Deccan Traps (DT) are one of the important parts of the Indian continental lithosphere with interesting geophysical anomalies, tectono-thermal evolution since the Mesozoic times. Knowledge on the deep structure beneath these formations is important for understanding the seismo-tectonics of the region. This region has gained importance after the occurrence of a major earthquake (7.9 Mw) north of Saurashtra, namely Bhuj earthquake during 2001. It is also observed that Saurashtra region has experienced several earthquake swarms limited to small regions. Accordingly, it is important to investigate the deep structure of the Saurashtra region from seismotectonics point of view. In our study, magnetotelluric results of the deep crustal structure along five NS oriented traverses are presented. The five traverses are—Halvad-Rohisa (HR), Sapar-Iswaria (SI), Mota Dahinsara-Bamagadh (MB), Jodiya-Jamkhandorna (JJ) and VavBeraja-Devda (VD). The total length of these 5 traverses is about 670 km. The derived deep geoelectric structure is also compared and correlated with gravity data to get more confidence on the derived results. The 2-D geoelectric section has delineated anomalous high conductivity structure at places extending from 20 km to about 40 km. From the spatial correlation, anomalous high conductive structure derived from MT data with intense localized seismic activity is an interesting observation. In the present study, the results of magnetotelluric studies along with other geophysical results are presented.

The First Study of 3-D Seismic Velocities Tomography Inversion to Delineate Reservoir Steam Zone at “FR” Geothermal Field West Java

Understanding reservoir conditions is one of the important aspects in fluid production monitoring activities in geothermal systems. The objective of this study to delineate the reservoir element especially the steam phase. It is the first study to requires mapping the existence and geometry of steam phase as an initial step to monitor reservoir conditions at “FR” Geothermal Field using the tomography travel time method. This study used seismogram data measured for 95 days and recorded by 11 seismometers. Arrivals time picking results get 213 microearthquake events with a minimum recorded by 3 recording seismometers. Hypocenter relocation is done by two methods in combination. The first use Joint Hypocenter Determination (JHD) and second use Double Difference relocation method. Then the travel time tomography inversion is using simulps12 software. The results of the hypocenter relocation distribution show one cluster around the main production well those are PPX well may be related to weak zones and reveal majority structure with NW-SE orientation. While in terms of hypocenter depth distributed clusters around the trajectory of production wells B and C from elevations +1 to 0 Km. The tomogram results show that the delineate steam zone at an elevation of +1 to 0 Km with strongly low perturbation VP, low VS and low VP/VS with arround 1.6 to 1.7 value. Integration with MT data show the steam zone has located at reservoir boundary zone with resistivity value of 30 to 60 Ohm meter. Integration with PPX temperature profiling shows the steam zone has convection heat transfers.

Experience of using three-dimensional inversion for the large volume of magnetotelluric data

When switching to 3D inversion of MT data, the requirement for computer technology is significantly increased. In this paper we will discuss a few examples of 3D inversion of electromagnetic geophysical field data with the usage of “Lomonosov” supercomputer and show its effectiveness on several geological objects. Each object is associated with a variety of problems: from search for shallow ore to regional hydrocarbon exploration. But all these objects contain a large volume of measurements obtaining qualitative results for which requires a huge amount of time. So that the use of 3D inversion with a high-performance computational complex makes it possible to obtain a qualitative result of solving a wide range of problems.

Structure‐Coupled 3‐D Imaging of Magnetotelluric and Wide‐Angle Seismic Reflection/Refraction Data With Interfaces

AbstractMagnetotelluric (MT) and wide‐angle seismic reflection/refraction surveys play a fundamental role in understanding the crustal rheology and lithospheric structure of the Earth. In recent years, the integration of the two methods in order to improve the robustness of the inversion has started to gain attention. We present a new approach for joint 3‐D inversion of MT and wide‐angle seismic reflection/refraction data to accurately determine crustal structures and Moho depth. Based on H‐κ stacking of teleseismic receiver functions, we estimate an initial reference Moho. This is used as input for the subsequent MT/seismic joint inversion, where the Moho interface is updated and crustal structures are added to the model. During the joint inversion process, structural similarity is facilitated through the cross‐gradient constraint. Synthetic model tests show an improvement of the inversion results over separate inversions. In particular, the tests based on two geologically realistic models demonstrate that the crustal structure and even the trade‐off between velocity and Moho interface can be sufficiently resolved by combined MT and seismic data sets when using the estimates from analysis of receiver functions. These results show that the new method can provide useful constraints on crustal structures including their geophysical properties and discontinuities.

Identification of "WS" geothermal field system by analyzing TE, TM, and TE-TM of 2D magnetotelluric inversion models

Magnetotelluric data has been carried out at the "WS" geothermal field to analyze the resistivity model resulting from 2D inversion of magnetotelluric data in TE, TM and TE-TM modes. Base on the three models produced, the mode is determined to produce the most representative model to assist in the interpretation of the "WS" geothermal system. There is a step of modes separation, namely TE (Tranverse Electric) and TM (Transverse Magnetic) modes in processing MT data. Each mode produces a 2D model with different conductivity properties. The analysis results of the three modes explain that TE mode is dominated by low resistivity with a range of values of 10-35 Ωm and medium resistivity with a value range of 35-250 Ωm and a vertical resistivity contrast. The TM mode describes the high resistivity in the Southwest and the center of the track with a value of more than 470 sehinggam resulting in lateral resistivity contrast. While the TE-TM mode produces a model that is not much different from TM mode, only the distribution of the resistivity value is a combination with TE mode. This mode describes the distribution of resistivity both vertically and laterally. Based on the analysis of the three modes, it can be concluded that the TE-TM mode is the mode that produces the most representative model. Interpretation model shows that from the TE-TM mode we have a low resistivity distribution (10-35 Ωm) represent a cap rock zone, reservoir rock with a medium resistivity distribution (35-380 Ωm), resistive zone with a high resistivity distribution (more than 380 Ωm), and the existence of the three of faults structures ro be a controller system of the "WS" geothermal.

Correlation of change in magnetotelluric parameters before and after Sikkim Himalayan earthquakes associated with stress variation

MT stations that were recorded before the moderate 5.3 Mw, 2006 Sikkim Himalaya earthquake had been reoccupied after 2007 (Mw 5.0) and 2011 (Mw 6.9) seismic activities. We analyzed the broadband MT data (0.001–1000 s) within the 60 km radius from epicenters location and within the focal depth range of 9–30 km. The observed change in apparent resistivity in the period range of 0.001–10 s has shown a considerable post-seismic decrease of the order 10–30%. The 2D geoelectric model constructed with post seismic acquired data also reveals the feature of a decrease in resistivity (C1). The relative change (decrease) of observed apparent resistivity ((Δρ/ρobs) after 2006, 2007 and 2011 earthquakes find a correlation with combined stress drop 11–18.2 MPa values estimated by Arms-Δσ approach. The correlation is further strengthened by a similar change in relative apparent resistivities ((Δρ/ρ2D_model) derived from 2D model with stress drop. Integrated stress drop values attribute to the accumulated apparent stresses (σa = 0.23 ∆σ) that reveals the area might be subjugated to 0.6–1.2 MPa stress before 2006, 1–1.5 MPa stress before 2007 and 1.7–2.9 MPa before 2011 earthquakes occurrence. Development of micro-cracks and generation of aqueous fluids is possible due to such stressed and slow rate of relentless thrust/shear environment within the area of crust. Therefore, resistivity could have been decreased with a fluid transition from compression to dilatancy zones at different depth levels before each earthquake and further decreased upon subsequent stress drops associated with strong ground motions. We interpreted that maximum stress might be dropped in dilatational zones oblique to the maximum compression stress directions along N200E, N-S and NNW-SSE before 2006, 2007 and 2011 earthquakes and resulted in a considerable decrease in apparent resistivity along E-W (TE-mode), i.e., ρyx component.

Integrated 2D joint inversion models of gravity, magnetic, and MT for geothermal potentials: a case study from Gujarat, India

Joint Inversion of different geophysical potentials promises to reduce chances of uncertainties which are more probable in case of independent inversions. In this paper, cross-gradient Joint Inversion was applied for gravity, magnetic, and MT data to understand the subsurface geothermal potential in Dholera and Unai, Gujarat, India. This paper narrates the understanding of individual inversion for gravity, magnetic, and MT data followed by joint inversion to delineate more clear understanding of geological and geothermal bodies in the subsurface. To formulate data space, a generalized cross-gradient inversion for data set and model set parameters are discussed. On the basis of the algorithm proposed, an integrated subsurface model for Dholera and Unai was prepared which depicts geological as well as lithological characteristics of the area. For the structural coupling, a Lagrange data multiplier was applied in an iterative manner to reach a global minima. Fluid flow models of Dholera and Unai were prepared where “J” type drilling is suggested up to a depth of 800 m and 700 m, respectively.

2D Interpretation Of Controlled Source Audio Magnetotelluric (CSAMT) Data Integrated With Borehole Data In Kamojang Geothermal Field West Java, Indonesia

CSAMT is the one of electromagnetic method which uses unnatural source grounded dipole with the range of frequency between 0.1Hz-10 kHz. In order to qualify the response measured is plan wave, then the corrected source effect. The objective of this study was to obtain that have been corrected for source to have the same characteristic as MT data. CSAMT data that has been corrected, subsequently inverted to get the exact model and validated to well log data in the form of temperature data. After interpretation found that the some of the line measurement at geothermal field Kamojang has two layers. There are seven line, line-1 until line-7, line-2 until line-7 have the characteristic of geothermal, the first layer has a resistivity of 3-15 Ω.m and the average of thickness of 400-1500 meters. The second layer has a resistivity value of 20-300 Ω.m that is more resistive than the first layer, except for line 1, there is no low resistivity in the first layer. The first layer is a layer of clay, identified with resistivity values ranging from 2,5 - 15 Ω.m with 900-1400m thickness with temperature data on KMJ-12 range 180-200°C, and the next layer is more resistive layer than the second layer is 18 Ω.m -100 Ω.m with temperature data at KMJ-53 at a depth of 710 meters is 240°C.

Electrical Resistivity Maps of the Australian Lower Crust

Crustal silicate rocks at sub-solidus temperatures normally have high electrical resistivities. However, although upper crust is typically > 103 .m, it is not unusual for lower crust to be < 102 .m, and in places < 100 .m. That lower crust (below 10-15 km) can be as electrically conducting as seawater is remarkable, and indicates a substantial and highly-connected mineral, melt or aqueous phase. To date, temporal and spatial mechanisms to give rise to the low resistivity are speculative and poorly constrained by observation and laboratory measurement. We present new maps of the Australian crust resistivity inferred from the regional EM responses. The project addresses the question as to whether the low resistivity is primary in the formation of the crust, or overprint due to melt and fluid migration from a deeper thermal source. A secondary question is how regions of low resistivity from an interconnected phase can be preserved through time-scales of billions of years. Observations are drawn from: the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP); 2D MT transects; and legacy MT and geomagnetic depth sounding (GDS) data. Our research demonstrates a strong spatial correlation of crustal resistivity with tectonic domains in Australia. Lowest resistivities are often imaged just below the rheological boundary between upper and lower crust at ~10-15 km. Below, low resistivity appears as a broad zone, tens or hundreds of kilometres wide, and tens of kilometres thick; above the boundary, regions of low-resistivity appear as narrower pathways. Such maps are correlated with long-wavelength Bouguer gravity data, suggesting a common origin that changes both density and resistivity.

Study on Testing Method of Non-polarized Electrode

The observation of geoelectric field in the field data acquisition of electrical prospecting is usually achieved by the use of non-polarized electrodes. The performance of non-polarized electrodes directly affects the quality of field data. The existing non polarized electrode is usually used to evaluate the initial range and the stability of the electrode. In order to make a further evaluation, an indoor and field test scheme is developed. In the laboratory, three parameters, such as the extreme stability, the source impedance and the background noise level, are tested. The field test is carried out from the following two aspects: the parallel consistency test and the MT test. The results show that the above parameters and the field test results show that the electrode can be used for field MT data acquisition. It provides a scientific and feasible test scheme for the full evaluation of the non-polarized electrode test.

Imaging subsurface northern Rahat Volcanic Field, Madinah city, Saudi Arabia, using Magnetotelluric study

Rahat volcanic field is one of the basaltic fields in Saudi Arabia that has three major geohazard events; the historical eruption (1256 CE), the fissure eruption (~4500–1500 BP), and seismic swarm (1999). These events were studied and evaluated using geophysical and geochemical studies as well as volcanological studies. Geophysical studies include gravity, seismic, and magnetotelluric surveys. In the current research, the magnetotelluric data will be analyzed using 3D inversion technique in order to image the subsurface resistivity setting of the study area.Results from 3D inversion of the MT data revealed four major resistivity structures. The first is a layer of intermediate resistivity (40–250 Ω-meters) which is thicker (~800 m) to the south-west and thinner to the eastern edge, mostly covered by exposed Precambrian basement. This layer is underlain by resistive (>1000 Ω-metres) granitic basement. Intruding into the resistive basement are two near-vertical conductive (<20 Ω-metres) structures. One is located immediately west of the historic eruption (1256 CE) centre in the north, at a depth of about 15 km. The other intrusive is on the southern end of the survey area, also at a depth of about 15 km. Two conductive “channels” trending NW-SE and NE-SW were observed at depths of about 18 km. The NE-SW aligned “channel” runs through the northern intrusive while the NW-SE trending “channel” runs in the middle of the study area and connects both intrusive. The conductive intrusive and “channels” may be attribute partial melts stored in the pre-existing structures within the lower crust. Although the youngest known trachytic eruptions from the study area are several hundred thousand years old, recent and ongoing seismicity strongly suggest there may be magmatic activities in the lower and probably upper crust.

Monitoring geothermal reservoir developments with the Controlled-Source Electro-Magnetic method — A calibration study on the Reykjanes geothermal field

Surface geophysical monitoring techniques are important tools for geothermal reservoir management as they provide unique information on the reservoir development away from boreholes. For magmatic environments, electromagnetic (EM) methods are attractive monitoring tools as they allow to characterize the reservoir and hence potentially monitor changes related to fluid injection/production. Indeed, the electrical resistivity of reservoir rocks is highly dependent on the volume, chemistry and phase of the in-situ geothermal brine (e.g. liquid, vapor, supercritical).Passive EM techniques (e.g. magnetotellurics or MT) are traditionally used for geothermal exploration and a few recent studies have demonstrated its potential for monitoring reservoir development. One of the main challenges is though the presence of cultural noise and/or variability of the Earth magnetic field that can obfuscate the EM signals of interest. We have investigated the benefits and drawbacks of active EM surveying (Controlled-Source EM or CSEM) to tackle this challenge, first with a synthetic study and subsequently with an actual time-lapse survey acquired in 2016 and 2017 over the Reykjanes geothermal field in Iceland before (baseline) and after (monitor) the thermal stimulation of the supercritical RN-15/IDDP-2 geothermal well.The synthetic study showed that for geothermal fields having a resistivity structure similar to the Reykjanes field (i.e. a conductive caprock overlying a more resistive higher temperature reservoir), CSEM and MT measurements can both detect resistivity changes within the deep resistive reservoir, provided that measurement errors are small. Variations in many survey parameters (e.g. errors in receiver position/orientation, differences in recording devices, variations of near surface conditions, external noise) can create significant time-lapse CSEM measurement errors. Our actual time-lapse survey showed that when similar CSEM equipment is used during the baseline and monitor surveys and systematically d-GPS positioned, the remaining key parameter controlling the survey repeatability is the level of external noise. Since the influence of external noise on CSEM data can be artificially reduced (e.g. by increasing the transmitter dipolar moment), it offers the possibility to adapt the survey design to increase the chance of detecting the time-lapse signals of interest. On the contrary, little control is possible on the MT signal to noise ratio and hence repeatability.The time-lapse EM survey acquired over the Reykjanes geothermal reservoir showed indeed that a high CSEM survey repeatability can be achieved with electric field measurements (within a few percent) but that time-lapse MT survey is a challenging task because of the high level of cultural noise in this industrialized environment. To assess the quality of our CSEM dataset, we inverted the data and confronted the resulting resistivity model with the resistivity logged in the RN-15/IDDP-2 well. We obtained a good match up to 2-3km depth, i.e. enough to image the caprock and the liquid-dominated reservoir but not deep enough to image the reservoir in supercritical conditions. To obtain such an image, we had to jointly invert legacy MT data with our CSEM data. On the monitoring aspects, the analysis of changes in electric fields did not allow to identify any CSEM signal related to the thermal stimulation of the RN-15/IDDP-2 well. One possible explanation is the weakness of the time-lapse CSEM signal compared the achieved CSEM survey repeatability as a result of a limited resistivity change over a limited volume within the reservoir. Future reservoir developments in the supercritical reservoir (e.g. hydraulic stimulation, long-term fluid circulation) will most likely generate stronger resistivity variations over a larger volume than during the thermal stimulation of the well. This calibration study provides the basic information for deciding when and how an EM monitor survey must be performed for the monitoring of the Reykjanes geothermal reservoir but also for the definition of the monitoring strategy of similar high-enthalpy geothermal reservoirs.

Three-dimensional Inversion of Electromagnetic Geophysical Data with Parallel Computational Code on Supercomputer Complex “Lomonosov”

Usage of 2D inversion of magnetotelluric data for real geological objects can cause distortion, but it is more often used in commercial projects, because of its effectiveness and great experience. Whereas in the case of 3D inversion is not such a great experience and there are a number of global problems. When switching to 3D inversion of MT data, the requirement for computer technology is significantly increased. In this paper we will discuss a few examples of 3D inversion of electromagnetic geophysical field data with the usage of Lomonosov supercomputer and show its effectiveness on several geological objects. Each object is associated with a variety of problems: from search for shallow ore to regional hydrocarbon exploration. But all these objects contain a large volume of measurements obtaining qualitative results for which requires a huge amount of time. So that the use of 3D inversion with a high-performance computational complex makes it possible to obtain a qualitative result of solving a wide range of problems.

Low duty-cycle pulsed irradiation reduces magnetization transfer and increases the inhomogeneous magnetization transfer effect

Intense off-resonant RF irradiation can lead to saturation of the macromolecular pool magnetization and enhance bound pool dipolar order responsible for the inhomogeneous magnetization transfer (ihMT) effect, but the intensity of RF power in human imaging studies is limited by safety constraints on RF heating. High RF intensities can still be achieved if applied in short pulses with low duty-cycle. Here we investigate the benefits of low duty-cycle irradiation for MT and ihMT studies with both theoretical and experimental methods.Solutions for pulsed irradiation of a two-pool model including dipolar order effects were implemented. Experiments were conducted at 3 T in the brain and through the calf of healthy human subjects. 2D echo planar images were acquired following a preparation of RF irradiation with a 2 s train of 5 ms pulses repeated from between 10 to 100 ms for duty-cycles (DCs) of 50% to 5%, and at varying offset frequencies, and time averaged RF powers. MT and ihMT data were measured in regions of interest within gray matter, white matter and muscle, and fit to the model.RF irradiation effects on signal intensity were reduced at 5% relative to 50% DCs. This reduced RF effect was much larger for single than dual frequency irradiation. 5% DC irradiation reduced single and dual frequency MT ratios but increased ihMT ratios up to 3 fold in brain tissues. Muscle ihMT increased by an even larger factor, depending on the frequency and applied power. The model predicted these changes with duty-cycle. The model fit the data well and constrained model parameters.Low duty-cycle pulsed irradiation reduces MT effects and markedly increases dipolar order effects. This approach is an attractive method to enhance ihMT signal-to-noise ratio and demonstrates a measurable ihMT effect in muscle tissue at 3 T under acceptable specific absorption rates. The effects of duty-cycle changes demonstrated in a separate MT/ihMT preparation provide a route for new applications in magnetization-prepared MRI sequences.

Reservoir characterization and basement estimates in the Papuan Fold belt (Papua New Guinea-PNG), from reanalysis of the PNG MT data set

The PNG MT data set covering a section of the Papuan Fold Belt (PFB) in Papua New Guinea (PNG), which is known for its extensive hydrocarbon resources, re-analyzed using advanced dimensionality technique and inversion algorithms. Although the phase tensor analysis retrieved a dominant 2D conductivity direction matching the regional tectonic trend, a mild 3D behavior with some local strong 3D effect is apparent. The 3D and 2D inversions carried out yielded resistivity features related to the broad stratigraphic column of the area, but with variations in the depth of occurrence and thicknesses of resistivity structures between the inversion models. The preferred 3D model shows an about 1 km thick resistive Miocene Darai limestone cover, thickening towards northeast of the profile, and supports a thick-skinned thrust model. The 3D resistivity section shows an additional differentiation within the underlying Mesozoic column, a highly conductive (∼1 Ωm) and moderately conductive (10–40 Ωm) layer, sandwiched between the Darai formation and crystalline basement. The upper highly conductive layer can be related to the Cretaceous Ieru shale formation, which acts as the regional cap rock in the PFB, and the below moderately conductive layer represents the stack of Mesozoic strata that possibly comprise of the Neocomian Toro sandstone reservoir rocks and underlying Jurassic source rock formations. Nevertheless, the resolution tests do not fully support the demarcation between the Ieru formation and underlying Mesozoics containing the reservoir and source formations, and demand additional constraints for confirmation.