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1
Content available remote Study on groundwater table distribution of loess tableland in the South Jingyang tableland, Northwest China
Wu Jiaqi, Dai Fuchu, Liu Pan, Huang Zhiquan, Wu Daixuan
Acta Geophysica
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2024
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Vol. 72, no. 1
257--272
EN
The South Jingyang tableland, located in Shaanxi Province, China, is composed of loess deposits. Agricultural irrigation in this area has caused the continuous rise of the groundwater table, which triggered a series of loess landslides. Although extensive research has been conducted on the effect of rising groundwater tables on induced loess landslides, less is known about the groundwater table distribution. This study jointly investigated the distribution of the groundwater table in the South Jingyang tableland using boreholes, electrical resistivity tomography (ERT), and laboratory tests. A qualitative analysis between the apparent resistivity and the water content was estimated at different depths, using the borehole data and ERT inversion data. A quantitative relationship between the resistivity and the water content was established in the laboratory tests. A combination of the qualitative analysis of the inversion results and the quantitative relationship was used to determine the distribution of the groundwater table. The groundwater table is present at the depth where the water content of the loess is 30% (liquid limit of loess) and the resistivity is 28.35 Q m. The groundwater table elevation in the southern study area is higher than that of the northern study area and decreases in a stepwise gradient from south to north. This study not only identified the distribution of the groundwater table in the South Jingyang tableland, but also makes up for the limitation and inaccuracy in determining groundwater tables by relying solely on the borehole information or inversion results of ERT. The ascertained distribution characteristics of the groundwater table provides a basis for the analysis of the formation mechanism of landslides.
2
Content available remote Processing for near-source potential resistivity based on the parallel electrical method
Hu Zean, Wu Rongxin, Cao Jianfu, Wang Xiaoyu
Acta Geophysica
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2022
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Vol. 70, no 6
2705--2714
EN
The parallel electrical method (PEM), which enhances the field-work efficiency by offering flexible data acquisition and processing a large amount of geoelectric field data, is mainly used to extract the high-density resistivity data. However, the method is unable to process nearly 75% of the data and hampered by noise interference. We proposed a novel method to calculate the apparent resistivity based on the near-source potential in the PEM system using the following algorithms: (1) selecting the measurement electrode nearest to the power source as the reference and keeping the AM interval as an invariant electrode distance, (2) calculating the potential difference between the measurement electrode and the first near-source electrode, (3) stepwise calculating the potential differences between other measurement electrodes and their corresponding near-source electrodes, and (4) calculating all the apparent resistivities at different positions. We further verified the effectiveness of near-source potential method to calculate resistivity based on the theoretical calculation and identified that it has higher calculation accuracy and stability. Compared to the maximum Pole-dipole deviation of 59.4%, the maximum deviation of the near-source potential resistivity is only 2.2%. The field experimental results showed that the near-source potential resistivity method performs well in the stratified geoelectric model and effectively improves the longitudinal resolution and the signal-to-noise ratio of deep apparent resistivity of the parallel direct-current resistivity method.
3
Content available remote Effective denoising of magnetotelluric (MT) data using a combined wavelet method
Ling Zhenbao, Wang Peiyuan, Wan Yunxia, Li Tonglin
Acta Geophysica
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2019
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Vol. 67, no. 3
813--824
EN
Noise interference, especially from human noise, seriously affects the quality of magnetotelluric (MT) data. Strong human noise distorts the apparent resistivity curve, known as the near-source effect, causing poor reliability of MT data inversion. Based on analyzing the frequency characteristics of human noise resulting from the surrounding environment, a new waveletbased denoising method is proposed for both synthetic and real MT data in this paper. The new technique combines multiresolution analysis with a wavelet threshold algorithm based on Bayes estimation and has a remarkable effect on denoising at all band frequencies. The multi-resolution analysis method was employed to reduce long-period noise, and a wavelet threshold algorithm was used to eliminate strong high-frequency noise. In this research, the improved algorithm was assessed via simulated experiments and field measurements with regard to the reduction in human noises. This study demonstrates that the new denoising technique can increase the signal-to-noise ratio by at least 112% and provides an extensive analysis method for mineral resource exploration.
4
Content available remote Modeling Electrical Field Distribution in Layered Geological Rock Formations with a Borehole Using the Coulomb Charges Method
Cichy A., Ossowski A.
Acta Geophysica
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2015
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Vol. 63, no. 5
1244--1255
EN
The Coulomb charges method is used to model apparent resistivity measurements carried out in layered geological formations with a borehole using various devices. It is characterized by a high level of effectiveness and accuracy. The results are compared with the theoretical solutions for a homogenous medium with the borehole and invaded zone for point current source lateral devices. The relative error was less than 2% for different values of the range of the invaded zone and resistivity of invaded and true resistivity of formation.
5
Content available Time-domain electromagnetic method in geological research
Klityński W., Stefaniuk M., Miecznik J.
Annales Societatis Geologorum Poloniae
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2014
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Vol. 84, No. 1
71--79
EN
This paper describes the theoretical basis of the time-domain electromagnetic method (TDEM) and its application in geological research. The paper discusses the behavior of the electromagnetic field of pulse current sources (magnetic dipole) in the near zone. The problem of the penetration depth of the TDEM method is discussed, in the context of its application in geological research and to gas exploration in the Rudka Gas Field, located in the Carpathian Foredeep. The TDEM sounding curves from the Rudka Field and the influence of noise on the penetration depth are discussed. A 1-D Occam inversion of TDEM soundings was obtained for a profile across the borehole A-10. An integrated (TDEM and borehole) interpretation of the data was made and a 2-D resistivity cross-section was obtained along the profile, as a contribution to geological reconnaissance for gas prospecting.
6
Content available remote Increased resolution of subsurface parameters from 1D magnetotelluric modeling
Shireesha M., Harinarayana T.
Acta Geophysica
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2013
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Vol. 61, no. 3
569--582
EN
In electric and electromagnetic techniques, it is well known that the principle of equivalence poses a problem in the interpretation of subsurface layers. This means the inversion problem can provide the conductivity-thickness product more confidently than the individual parameters - conductivity and thickness - separately. The principle of equivalence corresponds to the middle layer in a three-layer earth structure. In order to resolve this problem, we have touched upon the different formulae of apparent resistivity proposed by earlier workers considering the real and imaginary parts of the impedance tensor and designed a new formula to compute apparent resistivity for different models. We observed that the application of our new formula for apparent resistivity using the combination of real and imaginary parts of the impedance has a better resolution as compared to earlier conventional formulae of apparent resistivity. These results have been demonstrated through both forward and inverse modeling schemes.
7
Content available remote Near surface geophysical surveys with a high frequency mutual impedance measuring system
Beziuk G.
Acta Geophysica
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2012
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Vol. 60, no. 1
140-156
EN
Relationships for the mutual impedance of horizontal coplanar, vertical coplanar, perpendicular and vertical coaxial loop antenna arrays operating at a high frequency, located over a layered conducting medium, have been derived. The relationships take into account the occurrence of displacement currents in both the free space and the conducting medium. The relations can be used for calculating the earth parameters in geophysical prospecting. The use of high frequency antenna array makes it possible to determine distributions of apparent resistivity as well as apparent electrical permittivity. The potential application of the measurements of mutual impedance of loop antennas operating at a high frequency to the geophysical examinations is presented. The measuring system developed for this purpose is described. A simple method of interpreting the mutual impedance measurements to obtain the traces of apparent resistivity and apparent permittivity of the investigated earth is given.
8
Content available Sztuczne sieci neuronowe dla uzupełnienia danych w geofizyce otworowej - wybrane przykłady
Jarzyna J., Opyrchał A., Mozgowoj D.
Geologia / Akademia Górniczo-Hutnicza im. Stanisława Staszica w Krakowie
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2007
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T. 33, z. 4/1
81-102
PL
Wykorzystano sztuczne sieci neuronowe do odtwarzania profilowań geofizyki otworowej. Na podstawie dostępnych profilowań geofizyki wiertniczej odtworzono czas interwałowy rejestrowany przy profilowaniu akustycznym, gęstość objętościową będącą wynikiem profilowania gamma-gamma oraz oporność pozorną. Wybrano perceptrony i wsteczną propagację błędu jako metodę nauczania. Najbardziej skuteczne przy odtwarzaniu parametrów okazały się perceptrony z kilkoma neuronami na wejściu i przynajmniej kilkoma neuronami w warstwie ukrytej. Miarą poprawności wyników dostarczanych przez sieci były podstawowe statystyki obliczane dla odtworzonych parametrów w porównaniu z wynikami pomiarów lub estymacji. Najtrudniejsze do opracowania okazały się interwały, w których skały miały skomplikowaną litologię i zróżnicowaną przynależność stratygraficzną. Wyniki wykorzystano dla potrzeb interpretacji sejsmicznej, grawimetrycznej i magnetotellurycznej.
EN
Artificial Neural Networks were used for reconstruction of well logs. Transit interval time recorded during acoustic log, bulk density as a result of gamma-gamma log and apparent resistivity were reconstructed on the basis of available logs. Perceptrons were chosen and the back propagation method was applied as the most effective training algorithm. Perceptrons with a few input neurons and at least a few neurons in a hidden layer turned out in reconstructing parameters. Basic statistics calculated for the reconstructed parameters compared to the measured or estimated ones acted as measures of correctness of solutions delivered by the networks. Intervals containing rocks of complex lithology and diversified stratigraphy turned out to be the most difficult to interpret. Results were applied in seismics, gravimetric and magnetotelluric interpretations.
9
Content available remote Problem wprowadzania poprawek do pomierzonych wartości przewodności pozornej przy niskoczęstotliwościowych dipolowych profilowaniach indukcyjnych
Antoniuk J.
Geologia / Akademia Górniczo-Hutnicza im. Stanisława Staszica w Krakowie
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2001
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T. 27, z. 2--4
625-639
PL
Przeprowadzono analizę warunków asymptotycznych, przy których wyprowadzone zostały wzory na przewodność pozorną sigma/a, dla niskoczęstotliwościowych pomiarów indukcyjnych. Określono wielkości błędów wynikających z niespełnienia tych warunków w kontekście pomiarów wykonywanych aparaturą EM34-3 Geonics. Przedstawiono koncepcję i sposób wprowadzania poprawek do pomierzonych przewodności pozornych. Podano przykłady korygowania wartości sigma/a otrzymywanych zarówno z pomiarów terenowych, jak i uzyskiwanych z modelowań matematycznych
EN
Asymptotic conditions, under which equations for apparent conductivity sigma/a were defined, were analysed for low-frequency induction measurements. A quantitative evaluation of errors obtained in measurements using the ground conductivity meter EM34-3 Geonics, when such conditions have not been satisfied, was made. An approach for introducing corrections to measured values of apparent conductivity is proposed. Examples of correcting sigma/a values obtained both from field measurements and mathematical modelling are given
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