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Journal of Applied Geophysics
Elsevier
Journal of Applied Geophysics

Elsevier

0926-9851

Journal of Applied Geophysics/Journal Journal of Applied GeophysicsAHCIISTPSCIEI
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    Simultaneous inversion of spectral IP data with frequency constraints

    Kim, BitnaraeSon, Jeong-SulYu, HuieunJang, Hanna...
    13页
    查看更多>>摘要：Spectral-induced polarization (SIP) measures the apparent complex resistivities (CRs) of electrical current sources at frequencies of less than 10 kHz. The measured apparent CRs are usually inverted to recover the intrinsic subsurface CRs, which depend on the source frequencies. The Cole-Cole (CC) model is commonly used to describe the dispersion characteristics of the intrinsic CRs; electrical resistivity, chargeability, and the time and relaxation constants can be derived using this model. For quantitative estimation of these parameters, a reliable inversion method for multifrequency SIP datasets is required. Conventionally, SIP data at each frequency are independently and sequentially inverted. This does not consider the relationships between SIP datasets of different frequencies. To consider such relationships, here, we simultaneously invert multi-frequency SIP datasets by introducing frequency-smoothness constraints between adjacent frequencies. We numerically verified the utility of the approach in terms of both amplitude and phase recovery; the method yielded more reliable relaxation models (e.g., CC models) than conventional inversion. For field verification, the method was applied to SIP data of several frequencies obtained in South Korea.
      原文链接:
    • NSTL
    • Elsevier

    Three-dimensional numerical modeling of direct current resistivity methods in mixed space-wavenumber domain

    Dai, ShikunLing, JiaxuanZhang, YingLi, Kun...
    10页
    查看更多>>摘要：At present, in order to obtain highly accurate solution for complex (geo-electric) conditions, it is necessary to divide the model precisely in the numerical modeling of DC resistivity method, which results in greatly increases the amount of computation and memory. To solve this problem, we propose a 3D numerical simulation method in a mixed space-wavenumber domain. The method transforms the partial differential equation of DC abnormal potential into many independent 1D differential equations with different wavenumbers by performing a 2D Fourier transform along the horizontal direction. It decomposes a large-scale 3D numerical modeling problem into many 1D numerical modeling problems, which greatly reduces the computation and memory requirements of modeling, and each 1D differential equation is independent, so it has high parallelism. The vertical direction is reserved as the spatial domain, which is convenient for the subdivision, taking into account the calculation accuracy and efficiency. A finite-element algorithm combined with a chasing method is used to solve the 1D differential equations with different wavenumbers, and a contraction operator based on electromagnetic integral equation method is used to the iteration of the algorithm for the first time. Taking full advantage of the rapidity of Fourier transform, the high efficiency of solving 1D differential equations and the high parallelism of 1D problems with different wavenumbers, it can greatly reduce the amount of calculation and storage, thus greatly improves the efficiency of the algorithm. The accuracy and convergence of the algorithm are verified by using the model of a sphere in half space. Based on the model with five cubes in half space, the efficiency of the algorithm is verified by comparing with the academic Gimli package based on the finite element method. In addition, a parallel algorithm based on OpenMP is used to verify the parallelism of the proposed algorithm. The examples results show that, the 3D numerical modeling method of DC resistivity in the mixed space-wavenumber domain has the characteristics of high efficiency, high precision and highly parallel. This method provides a new way for 3D numerical simulation of DC resistivity method.
      原文链接:
    • NSTL
    • Elsevier

    3D large-scale transient electromagnetic modeling using a Shift-and-Invert Krylov subspace method

    Lu, KailiangZhou, JianmeiLi, XiuFan, Ya'nan...
    11页
    查看更多>>摘要：In this paper, the combination of the Shift-and-Invert Krylov (SAI-Krylov) subspace method and the preconditioned conjugate gradient method (PCG) solves the problem of 3D large-scale time-domain transient electromagnetic (TEM) forward modeling. Firstly, the Galerkin method is used to obtain the spatial discretization governing equation of the electric field from the governing equation of the electric field. The electric field can be expressed in the form of a matrix exponential function using the model reduction method, according to the homogeneous ODE. The TEM response of small and medium-scale models can be solved quickly by using the SAI-Krylov technique, selecting the optimal shift gamma(opt) , and combining with the direct method solver. The orthogonal basis of the SAI-Krylov subspace is difficult to build by the direct method due to the high coefficient matrix order of the large-scale TEM forward modeling problem, but it can be solved successfully using the PCG method. Since the optimal shift gamma opt in the SAI-Krylov method applies to the direct method solver, the computation efficiency is quite low if directly applied to the PCG method. In conclusion, it is necessary to re-select the optimal shift gamma(opt) when solving the orthogonal basis of SAI-Krylov subspace using the PCG method. In this paper, the optimal shift gamma(opt) is selected based on the relationship between the condition number of the coefficient matrix and shift gamma to reduce the conditional number of the coefficient matrix and minimize the number of iterations in the calculation process. Furthermore, the tolerance epsilon and relaxation factors omega in the PCG method are chosen based on the 5% error threshold. By reselecting the shift gamma, the calculation time is reduced by 2/3. Finally, the validity and applicability of the proposed method are verified by the homogeneous half-space model with different resistivity, the 3D vertical contact zone model, and the large-scale complex sulfide ore body model.
      原文链接:
    • NSTL
    • Elsevier

    1-D inversion of frequency-domain airborne electromagnetic data using the open-source aempy toolbox

    Kiyan, DuyguRath, VolkerMuller, Mark R.Ture, Mohammednur D....
    18页
    查看更多>>摘要：Over the past decade, Geological Survey Ireland has acquired, and continues to acquire, frequency and time domain airborne electromagnetic data as part of the Tellus project. While the project advances towards national coverage, we have developed an open-source and freely available pre-inversion processing and onedimensional inverse modelling toolbox, aempy, for numerical modelling of the existing and future electromagnetic data sets. aempy is a flexible tool facilitating the quantitative interpretation and optimal use of the data sets by the broader scientific community, supporting the ethos and intent in the free public release of the Tellus airborne data sets. Coded in Python, the toolbox is freely and readily adaptable for specific educational and research objectives and for application to data acquired using other electromagnetic systems. The inversion algorithms in the toolbox are currently implemented in 1-D, assuming that the Earth is horizontally stratified and has laterally-constant layer model parameters, e.g., conductivity and thickness. In its current state, modules and functions of aempy work on processed frequency-domain airborne electromagnetic data. In this paper, we introduce the concept, structure and capacities of aempy and present model results derived from the use of different filtering and inversion techniques implemented in the toolbox, as well as an analysis of the influence of the flight altitude parameter on resulting subsurface inversion models.
      原文链接:
    • NSTL
    • Elsevier

    Comparing induced polarization effect on semi-airborne and airborne transient electromagnetic data: A numerical study

    Li, HaiLi, KeyingChen, Wen
    11页
    查看更多>>摘要：In a transient electromagnetic (TEM) survey, when a chargeable body presents, its induced polarization (IP) will generate an additional electromagnetic field that further superposes on the TEM data. If the added fields are not considered properly when designing a TEM survey or inverting the TEM datasets, erroneous results may be obtained. Thus, we compare the IP effects on the airborne and semi-airborne TEM methods in this study. One big difference between the two methods is that the semi-airborne method mainly uses the ground-based galvanic source and the airborne method utilizes the air-based inductive source. To fully illustrate this difference, we calculated the IP-TEM coupled responses at a time range of 10(-6) s to 10 s. Despite the commonly known reversal of signs in the transient, we found that in some cases of airborne TEM response, the sign will reverse twice. The relative errors caused by the IP effect are further demonstrated. A minimum peak at the early time that is closely related to the IP parameters is revealed, indicating that the IP fields will drop rapidly at the early time and then decay slowly. Finally, a misfit space analysis is performed, showing that both methods can well resolve the resistivity and the IP parameters of a conductive target, while it is difficult to recover the IP parameters of a resistivity target.
      原文链接:
    • NSTL
    • Elsevier

    4D inversion of resistivity monitoring data with adaptive time domain regularization

    Cho, In-KyJeong, Da-Bhin
    14页
    查看更多>>摘要：The time-lapse or 4D inversion aims to image significant underground changes and efficiently suppress inversion artefacts. However, the inverted model changes are frequently over-smoothed along the time-axis. Particularly in cases where models have poor resolution, even significant model changes are severely blurred or blunted in the reconstructed image. In this study, we developed a new 4D inversion algorithm that automatically determines time domain (TD) regularization based on model changes and the resolution to achieve a more accurate interpretation of resistivity monitoring data. By assigning a smaller TD constraint to model parameters with poor resolution, model changes at depth can be imaged if the models are significantly changed over time. Moreover, TD regularization was adaptively adjusted according to the TD error roughness, which can effectively control model smoothness. We conducted inversion experiments on synthetic and field data to test the performance of the proposed inversion algorithm. The inversion results for synthetic data demonstrated that the 4D inversion with adaptive TD regularization yields the model change that is most similar to the true change, among various options of TD regularization. The adaptive 4D inversion confirmed its capability to clearly image widespread and locally confined model changes. Model changes with poor resolution could likewise be imaged. Finally, the applicability of the proposed 4D inversion algorithm was demonstrated with real resistivity monitoring data collected for leakage detection at an embankment dam in Korea.
      原文链接:
    • NSTL
    • Elsevier

    Determining the static Young's modulus and Poisson's ratio, and compressive strength of the friable Erin Formation rocks using P-wave velocity

    Harrypersad-Daniel, A. M.Blake, O. O.Ramsook, R.
    12页
    查看更多>>摘要：It is important to understand the mechanical properties (elastic properties and strength) of rocks as these properties are used to model the rock's deformation in civil and underground engineering projects. Determining these properties for friable rocks are very difficult and scarce due to the rock's poor cementation making it challenging to retrieve samples from outcrop and boreholes, and difficult to prepare specimens for laboratory testing. To improve on the lack of mechanical properties of friable rocks, this study aims to measure the mechanical properties and P-and S-wave velocities and determine if empirical relationships exist between them. Experiments were carried out at effective pressures up to 130 MPa under dry and saturated conditions on friable sandstone and thin-bed shale lithofacies from the Erin Formation. The static Young's modulus of the rocks is less than 8 MPa and the unconfined strength ranged from 1.0 to 2.2 MPa, which indicate that the rock is very weak. The confined compressive strength is relatively high due to large strain accumulation (average of 20% strain), which resulted in significant strain hardening. At low effective pressures, velocity measurements were highly attenuated, resulting in limited S-wave velocity measurements. Empirical relationships established between the mechanical properties and P-wave velocities were predominantly weak to strong. Moderate and strong linear relationships were established between the unconfined compressive strength and static elastic properties and between the tensile strength and static Poisson's ratio. The results of this study could be carefully applied to velocity data that are readily available from well logs to estimate the mechanical properties of friable rocks.
      原文链接:
    • NSTL
    • Elsevier

    Pedoenvironmental variations assessment using magnetic susceptibility in Lut Watershed, Central Iran

    Rasooli, NajmehMahmoodabadi, MajidBoroujeni, Isa EsfandiarpourFarpoor, Mohammad Hady...
    15页
    查看更多>>摘要：Mass specific magnetic susceptibility (x) is an efficient technique to describe environmental processes and pedogenic variation patterns in soils. This study aimed to investigate the pedoenvironmental changes, including soil forming factors and processes, in Lut Watershed using the x technique. For this purpose, three climatic sites, including site 1 (xeric/mesic soil moisture/temperature regimes), site 2 (weak aridic/mesic, weak aridic/ thermic, and typic aridic/thermic regimes), and site 3 (extreme aridic/hyperthermic regimes), with different lithology and geomorphology were selected. Twenty-four pedons out of 50 which showed the maximum variations were selected as the representative pedons. Soil samples were analyzed for physicochemical properties, different iron forms, and x measured in two frequencies 0.46 and 4.6 kHz. Results indicated that the minimum and maximum x values were 20.6 x 10(-8) m(3) kg(-1) and 1031 x 10(-8) m(3) kg(-1), respectively. Mean x values in soils formed on different rocks were in the order of sedimentary rocks < sedimentary-metamorphic rocks < felsic igneous rocks < intermediate igneous rocks < mafic igneous rocks. The frequency-dependent magnetic susceptibility percentage (xfd%) ranged from 0 to 3.38%. The vertical distribution of x along the studied pedons showed the concomitant impact of climate, lithology, relief, drainage, soil attributes, lithological discontinuity, paleoclimate, redistribution processes, and soil evolution. The negative significant correlation among x, xfd, and free Fe oxides (Fed) in site 1 with igneous rocks revealed the probable conversion of primary ferrimagnetic minerals to the minerals with lower magnetic properties during pedogenesis. Moreover, the positive significant correlation (r = 0.33 and p < 0.01) between soil evolution Feo/Fed index (ratio of soluble acid oxalate and free Fe oxides) and x values in site 2 indicated that x decreased during soil evolution. Due to aridity and the lack of pedogenic processes, detrital Fe-bearing minerals mainly affected x values in site 3. Overall, the x technique is a potential pedoenvironmental indicator for a better understanding of soil forming factors and processes in Lut Watershed.
      原文链接:
    • NSTL
    • Elsevier