DIGITAL.CSIChttps://digital.csic.esThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Sun, 20 Jun 2021 04:21:01 GMT2021-06-20T04:21:01Z50201- On the effect of non stationary (synthetic) sources in the magnetotelluric methodhttp://hdl.handle.net/10261/89062Title: On the effect of non stationary (synthetic) sources in the magnetotelluric method
Authors: Neukirch, Maik; García, Xavier
Abstract: A new, non stationary scheme for the statistical magnetotelluric (MT) transfer function estimation is used to assess the effects of non stationary noise in MT data processing. The scheme uses Empirical Mode Decomposition (EMD) to process spectral data and is referred to as Empirical-mode-decomposition-based Magneto-Telluric processing (EMT). We compare EMT with BIRRP by Chave and Thomson [2004], a traditional and efficient processing code based on the Fourier Transform. Two tests are performed, first, synthetic, non stationary data is constructed from two non stationary sources to demonstrates the inability of a Fourier based method to deal with non stationary sources. Then, secondly, these sources are used as a noise source. The non stationary noise is added only to the electric fields and leaves the magnetic and remote channels completely unaffected. Therefore, we can show that the computation of the spectra via Fourier Transform fails, because uncorrelated stationary noise in the spectra should be cleaned by the remote referencing technique, which is applied in the test. Since any uncorrelated (random) non stationary noise acts as any random stationary noise and does not affect the measurements other than decreasing the confidence in the results (larger error bars), this test shows that the mere fact that the added noise is non stationary affects the estimated results by a Fourier Transform based method or even makes it impossible to extract reasonable transfer functions, whereas the EMT algorithm is able to deal with the non stationarity and allows a more precise estimation to a lower signal-to-noise ratio. In summary, we show that non stationary sources can heavily impact on traditional MT processing routines which rely on the Fourier Transform but that this effect can be diminished by relying on a purely non stationary analysis. The non stationary source is specifically designed to disturb the Fourier Transform and to break its assumptions, however the results provide an insight in how bad real non stationary noise can affect MT measurements and encourage to verify these findings on a real world problem with data that is suspected to contain, in particular, non stationary noise, e.g. data that is acquired close to train lines, mining shafts and elevators or electric fences
Description: European Geosciences Union General Assembly 2013, 7-12 April, Vienna, Austria
http://hdl.handle.net/10261/89062
- Non Stationary Magnetotelluric Data Processinghttp://hdl.handle.net/10261/109944Title: Non Stationary Magnetotelluric Data Processing
Authors: Neukirch, Maik
Abstract: [EN] Studies have proven that the desired signal for Magnetotellurics (MT) in the electromagnetic (EM) field can be regarded as ’quasi stationary’ (i.e. sufficiently stationary to apply a windowed Fourier transform). However, measured time series often contain environmental noise. Hence, they may not fulfill the stationarity requirement for the application of the Fourier Transform (FT) and therefore may lead to false or unreliable results under methods that rely on the FT. In light of paucity of algorithms of MT data processing in the presence of non stationary noise, it is the goal of this thesis to elaborate a robust, non stationary algorithm, which can compete with sophisticated, state-of-the-art algorithms in terms of accuracy and precision. In addition, I proof mathematically the algorithm’s viability and validate its superiority to other codes processing non stationary, synthetic and real MT data. [...]; [DE] Studien zeigen, dass die für Magnetotelurische (MT) Messungen wichtigen elektromagnetischen (EM) Quellen als quasistationär angesehen werden können, sodass eine Fourier Transformation mit geeigneter Fensterfunktion angewendet werden kann. Die gemessenen Zeitreihen enthalten jedoch nebst dem Signal auch Rauschen. Dieses erfüllt nicht notwendigerweise die zwingende Bedingung der Stationarität für die Fourieranalyse und kann daher zu falschen oder unzuverlässigen Ergebnissen f¨uhren. Existierende Lösungen für das Verarbeiten von MT Messreihen mit nichtstationärem Verhalten sind unzureichend entwickelt, da das Signal selbst als stationär angenommen werden kann. In dieser Doktorarbeit wurde ein statistisch robustes, nichtstation äres Verfahren entwickelt, welches mit hoch entwickelten, renommierten Algorithmen, die auf die Fourieranalyse aufbauen, in Zuverlässigkeit und Präzision vergleichbar ist. Zusätzlich wird die mathematische Grundlage für das verwendete Verfahren bewiesen und dessen Überlegenheit im Vergleich zu alternativen Algorithmen anhand echter und synthetischer MT Messreihen, mit und ohne nichtstationären Eigenschaften, bestätigt. [...]
Description: Memoria de tesis doctoral presentada por Mail Neukirch para optar al grado de Doctor en ciencias por la Universitat de Barcelona (UB), realizada bajo la dirección del Dr. Xavier Antoni García Martínez del Institut de Ciències del Mar (ICM-CSIC) y del Dr. Juanjo Ledo de la Universitat de Barcelona (UB).-- 163 pages
Thu, 29 Jan 2015 07:44:55 GMThttp://hdl.handle.net/10261/1099442015-01-29T07:44:55Z
- EMT - Empirical mode decomposition based Magneto-Telluric processing: Using a non stationary method to compute instantaneous spectral datahttp://hdl.handle.net/10261/94553Title: EMT - Empirical mode decomposition based Magneto-Telluric processing: Using a non stationary method to compute instantaneous spectral data
Authors: Neukirch, Maik; García, Xavier
Abstract: Non stationary electromagnetic noise affects the computation of Fourier Spectra and, along, the estimation of the magnetotelluric (MT) transfer functions (TF). We provide a TF estimation scheme based on an emerging non linear, non stationary time series analysis tool, called Empirical Mode Decomposition (EMD) in order to be better suited in the presence of non stationary effects. In contrast to previous works, we argue the necessity of a multivariate analysis, highlight the possibility to use instantaneous parameters and define the homogenization of frequency discrepancies between data channels. Our scheme uses the robust statistical estimation of transfer functions based on principal component regression and can be used with and without aid by remote reference station(s)
Description: 21st EM Induction Workshop, 25-31 July 2012, Darwin, Australia
Mon, 31 Mar 2014 11:40:11 GMThttp://hdl.handle.net/10261/945532014-03-31T11:40:11Z
- GarField - A MT Survey of the Crustal Structure of the Intraplate Volcanic Region La Garrotxa, Northwestern Spainhttp://hdl.handle.net/10261/94529Title: GarField - A MT Survey of the Crustal Structure of the Intraplate Volcanic Region La Garrotxa, Northwestern Spain
Authors: Neukirch, Maik; García, Xavier; Galiana, Savitri; Roux, E.
Abstract: The Olot Volcanic Zone (OVZ) is located between the southern boundary of the Pyrenees, the eastern side of the Ebro Basin and the northern end of the Catalan Transverse System. The structural pattern of the zone is dominated by the E-W thrusts and folds intersecting NW-SE faults, which are related to the Alpine compressive deformation. A system of NE-SW extensional faults resulted from the Neogene distensive episode, which affected the whole eastern flank of the Iberian Peninsula (Gallart et al. 1990). The volcanism ranges in age between 10,000 and 110,000 years, and up to 9 different lava flows have been described. This complex fault system in such a highly resistive (basaltic and basaltic) background marks an interesting setting for an MT experiment since faults are often related to conductive (water bearing) zones. Because of the high background resistivity, we expect a deep magnetotelluric penetration and the goal is to see how deep the fault system reaches into the crust and whether or not it reaches the mantle. If there are faults reaching through the entire crust, this may well explain some of the regional volcanic activity. We collected data on a 30 km profile with a station spacing roughly about 2.5 km. The SW-NE orientation of the profile is perpendicular to and cuts through three NE-SW orientated fault axes. The broadband data is sampled at 128 Hz and gathered for two days with one to three stations measuring at the same time. The processed data shows clear 3D features and/or anisotropy at most sites but resembles well the high background resistivity up to tens of kOhm. The 2D inversion shows features, which are likely to be artifacts, and underscores the assumption of present 3D effects and/or anisotropy due to the fault system
Description: European Geosciences Union General Assembly 22-27 April 2012, Vienna, Austria.-- 1 page
Mon, 31 Mar 2014 10:16:30 GMThttp://hdl.handle.net/10261/945292014-03-31T10:16:30Z
- EMT - Empirical-mode-decomposition-based Magneto-Telluric Processinghttp://hdl.handle.net/10261/94517Title: EMT - Empirical-mode-decomposition-based Magneto-Telluric Processing
Authors: Neukirch, Maik; García, Xavier
Abstract: We present a new Magneto-Telluric (MT) data processing scheme based on an emerging non linear, non stationary time series analysis tool, called the Empirical Mode Decomposition (EMD) or Hilbert-Huang Transform (HHT), to transform data into a non-stationary frequency domain and a robust principal component regression to estimate the most likely MT transfer functions from the data with the 2- confidence intervals computed by a bootstrap algorithm. Optionally, data quality can be controlled by a physical coherence and a signal power filter. MT sources are assumed to be quasi stationary and therefore a (windowed) Fourier Transform is often ap- plied to transform the time series into the frequency domain in which Transfer Functions (TF) are defined between the electromagnetic field components. This assumption can break down in the presence of noise or when the sources are non stationary, and then TF estimates can become unreliable when obtained through a stationary transform like the Fourier transform. Our TF estimation scheme naturally deals with non stationarity without introducing artifacts and, therefore, potentially can distinguish quasi-stationary sources and non-stationary noise. In contrast to previous works on using HHT for MT processing, we argue the necessity of a multivariate EMD to model the MT problem physically correctly and highlight the resulting possibility to use instantaneous parameters as independent and identically distributed variables. Furthermore, we define a homogenization between data channels of frequency discrepancies due to non stationarity and noise. The TF estimation in the frequency domain bases on a robust principal component analysis in order to find two source polarizations. These two principal components are used as predictor to regress robustly the data channels within a bootstrap algorithm to estimate the Earth¿s Transfer function with 2-¿ confidence interval supplied by the measured data.The scheme can be used with and without aid by any number of remote reference stations. The performance of this scheme will be demonstrated on MT data and compared with BIRRP, a widely used MT processing software by Alan Chave
Description: European Geosciences Union General Assembly 22-27 April 2012, Vienna, Austria.-- 1 page
Mon, 31 Mar 2014 09:31:07 GMThttp://hdl.handle.net/10261/945172014-03-31T09:31:07Z
- Frequency Shift in the Convolution of Non Stationary Time Serieshttp://hdl.handle.net/10261/94780Title: Frequency Shift in the Convolution of Non Stationary Time Series
Authors: Neukirch, Maik; García, Xavier
Abstract: The Hilbert-Huang Transform (HHT, [1]) is a novel tool to analyze non stationary time series and describe them with their instantaneous, spectral information. HHT decomposes a time series into a number of zeromean, oscillatory modes (Intrinsic Mode Functions, IMF) in order to ensure existence of an interpretable analytic signal of each IMF. Then, it is possible to express the analytic signal in terms of time series of the instantaneous parameters: amplitude, phase and frequency. Therefore, each IMF resides in the timefrequency domain and is described by amplitude and phase as a function of time. In [2] we show that the convolution of IMFs with any spectral response function translates into a complex multiplication of the IMF with that response function. Since the convolution of a (non) stationary time series with a spectral response function in the time domain can be transformed into a basic algebraic formulation, in this work we focus on the repercussions of a convolution of non stationary signals by analyzing the instantaneous parameters and their time derivations of the resulting convolved signal. Most notably, we find that there can be a frequency shift in the resulting signal with respect to the original signal depending on the degree of non stationarity. This finding may be important for non stationary time series, which are filtered by a system response for technical reasons, as it is often the case for physical measurements. However, the work is in a preliminary state and only carried out in the theory
Description: International Workshop in Recent Advances in Time Series Analysis (RATS), 9-12 June 2012, Protaras, Cyprus
Wed, 02 Apr 2014 11:08:06 GMThttp://hdl.handle.net/10261/947802014-04-02T11:08:06Z
- The Amplitude Phase Decomposition for the Magnetotel- luric Impedance Tensor and Galvanic Electric Distortionhttp://hdl.handle.net/10261/162050Title: The Amplitude Phase Decomposition for the Magnetotel- luric Impedance Tensor and Galvanic Electric Distortion
Authors: Neukirch, Maik; Rudolf, Daniel; García, Xavier
Abstract: The introduction of the phase tensor marked a major breakthrough in understanding of, analysing of and dealing with galvanic distortion of the electric field in the Magnetotelluric method. The phase tensor itself can be used for (distortion free) dimensionality analysis, if applicable distortion analysis and even to invert for subsurface models. However, impedance amplitude information is not stored in the phase tensor, therefore the impedance corrected by distortion analysis (or alternative remedies) may yield better results. We formulate an impedance tensor decomposition into the known phase tensor and an amplitude tensor that is shown to be complementary and independent of the phase tensor. The rotational invariant amplitude tensor contains galvanic and inductive amplitudes of which the latter are physically related to the inductive phase information present in the phase tensor. We show, that for the special cases of 1D and 2D subsurfaces, the geometric amplitude tensor parameter (strike and skew) converge to phase tensor parameter and the singular values are the amplitudes of the impedance in TE and TM mode. Further, the physical similarity between inductive phase and amplitude is used to approximate the galvanic amplitude for the general subsurface, which leads to the qualitative interpretation of 3D galvanic distortion: (i) the (purely) galvanic part of the subsurface (as sensed at a given period) may have a changing impact on the impedance (over a period range) and (ii) only the purely galvanic response of the lowest available period should be termed galvanic distortion. The approximation of the galvanic amplitude (and therewith galvanic distortion), though not accurate, offers a new perspective on galvanic distortion, which breaks with the general belief of the need to assume 1D or 2D regional structure for the impedance. The amplitude tensor itself is complementary to the phase tensor containing integrated (galvanic and inductive) subsurface information, it is illustrated and compared to the phase tensor on an example
Description: European Geosciences Union General Assembly 2016 (EGU2016), 17-22 April 2016, Vienna, Austria.-- 1 page
Mon, 12 Mar 2018 12:49:58 GMThttp://hdl.handle.net/10261/1620502018-03-12T12:49:58Z
- On Non Stationary CSEM Source Waveform Design to Reduce Bias Due to Towed Transmittershttp://hdl.handle.net/10261/139683Title: On Non Stationary CSEM Source Waveform Design to Reduce Bias Due to Towed Transmitters
Authors: Neukirch, Maik; García, Xavier
Abstract: Controlled source electromagnetic (CSEM) methods are sensitive to the subsurface conductivity structure and thus had led to its use in resource exploration. Since the frequency for peak sensitivity and the exact location of an exploration target is normally unknown prior exploration, it is desirable to acquire the received source signal for a broad range of frequencies and in a wide area. Investigations in both directions have been driven by optimising properties of the Fourier transform in order to enhance the frequency range and the detection of the signal for larger source - receiver separations, even though typical issues are intrinsically non stationary such as a floating EM source transmitter on a moving vessel. We demonstrate on a complete synthetic example that processing of chirped waveform (CWF) signals delivers reliable results and offers improvements like the possibility of longer transmission durations for moving transmitters. With this particular feat CWF can provide longer periods (for more depth penetration) and more data points per period (for more accuracy/robustness in the transfer function estimation) compared to e.g. a squared wave or a broad band ternary waveform
Description: 77th Earth Science for Energy and Environment (EAGE) Conference & Exhibition 2015, 1-4 June 2015, Madrid, Spain.-- 5 pages, 3 figures, 1 table
Wed, 02 Nov 2016 07:46:54 GMThttp://hdl.handle.net/10261/1396832016-11-02T07:46:54Z
- 3D seismic-velocity structure of Campo de Dalías basin (SE Spain) from diffuse-field modelling of the ambient noise wavefield and estimation of its seismic responsehttp://hdl.handle.net/10261/203105Title: 3D seismic-velocity structure of Campo de Dalías basin (SE Spain) from diffuse-field modelling of the ambient noise wavefield and estimation of its seismic response
Authors: García-Jerez, Antonio; Luzón, Francisco; Sánchez-Sesma, Francisco José; Seivane. Helena; Neukirch, Maik; Molina, Luis; Navarro, Manuel; Villaseñor, Antonio; Navarro, Francisco; Posadas, Antonio M.; Piña-Flores, José
Abstract: We use passive seismic methods to obtain a model of the seismic-velocity structure of Campo de Dalías, a large
(300 km2) coastal plain in the southeastern mountain front of the Betic Cordillera (SE Spain).
To this aim, H/V spectral ratios of ambient noise were computed from broadband records at 310 sites, approximately
on the vertexes of a 1000 x 1000 m square grid. Two approaches based on power spectral densities
and the Hilbert-Huang transform have been compared. Most of the H/V ratios show clear peaks in a wide
frequency range (down to 0.25 Hz) associated with up to 1000 meters of soft sedimentary rocks (mainly
Miocene marls) overlying the stiffer basement. Broader peaks appear in the foothills of Sierra de Gádor mountains.
The spectral ratios were inverted in the range 0.15 – 15 Hz to obtain horizontal layered models of the local
structure from which a 3D model was interpolated. Forward computations of H/V(f) relied on the Diffuse Field
Approximation (DFA), which considers the surface- and body-wave components of the wavefield. Additional
constraints were provided by 10 surface-wave dispersion curves derived from array measurements of ambient
noise, 95 km of Vp profiles from multichannel seismic reflection, and by lithological data at deep boreholes. These
constraints reduce the trade-off between Vp and Vs and between thicknesses and velocities. The inversion procedure
combined Monte Carlo sampling with subsequent model refinements using the downhill-simplex algorithm.
The stability of the H/V has been assessed using 30 months of continuous records at three Güralp-3ESPDC broad
band stations inside the basin. Even though remarkable stability exists in the band used for inversion, common
seasonal variations appear for frequencies below 0.15 Hz.
The resulting model describes well the main structural features in this area such as the El Ejido Synform
and the Guardias Viejas Antiform, with ENE–WSW-trend, and provides reliable ranges for the seismic wave
velocities of the main geological units. Considering these alignments, a 2.5 D version of the model has been used
to estimate, with moderate computational requirements, the response of the basin to the incidence of plane P, S
and Rayleigh waves. The wavefield was computed at 3098 virtual stations using the Indirect Boundary Elements
method (IBEM) for several incidence angles and azimuths. It presents complex amplification patterns depending
on frequency, position and illumination. These results increase knowledge on wave propagation and site effects in
the basin, which is located in a seismically active region.
Fri, 06 Mar 2020 14:44:56 GMThttp://hdl.handle.net/10261/2031052020-03-06T14:44:56Z
- Shallow Vs Structure of the Mauleon Basin (Western Pyrenees) by Joint Inversion of Horizontal-to-Vertical Spectral Ratios and RayleighWave Group Velocities from the large-N Maupasacq Experimenthttp://hdl.handle.net/10261/203464Title: Shallow Vs Structure of the Mauleon Basin (Western Pyrenees) by Joint Inversion of Horizontal-to-Vertical Spectral Ratios and RayleighWave Group Velocities from the large-N Maupasacq Experiment
Authors: Neukirch, Maik; García-Jerez, Antonio; Villaseñor, Antonio; Stehly, L.; Boue, P.; Chevrot, S.; Sylvander, Matthieu; Diaz, J.; Ruiz Fernández, Mario; Luzón, Francisco; Collin, Magali; Calassou, Sylvain; Polychronopoulou, K.; Martakis, N.; Bitri, A.
Abstract: Horizontal-to-Vertical Spectral Ratios (H/V) and Rayleigh group velocity dispersion curves can be used to
estimate the shallow S-wave velocity (Vs) structure. Knowing the Vs structure is important for geophysical data
interpretation either in order to better constrain data inversions for P-wave velocity (Vp) structures such as travel
time tomography or full waveform inversions, or to directly study the Vs structure for geo-engineering purposes
(e.g. ground motion prediction). The main purpose of this study is to investigate the potential and performance of
the combination of H/V and surface wave dispersion data for a dense passive seismic array. The joint inversion of
H/V and dispersion data for 1D Vs structure allows to characterize the uppermost crust and near surface, where
the H/V data (0.03 to 10 s) is most sensitive while the dispersion data (1 to 30 s) constrains the deeper model
which would, otherwise, add complexity to the H/V data inversion and impede its convergence.
During this large-scale experiment, 197 three-component short-period stations were continuously operated
for 6 months (April to October 2017) covering an area of approximately 1500 km2 with a site spacing of
approximately 3 km. The recovered 1D models reach to a depth of 500 to 1000 m. Due to the wide site spacing
compared to the model depth, combining these 1D models to a 3D model by interpolation is computationally more
economic than attempting a real 3D inversion with this data. The illustrated 3D model can be used to constrain the
fine structure of the upper crust for subsequent inversions that attempt to recover deeper models with inherently
less resolution such as travel time tomography, or full waveform inversion that requires a very good starting
model. Due to the sensitivity of H/V data to density, we also show a preliminary 3D density model of the area that
could (and, in the future, will) be used in combination with available gravity measurements.
Description: EGU General Assembly 2019, in Viena, Austria, 7–12 April 2019
Tue, 10 Mar 2020 13:51:02 GMThttp://hdl.handle.net/10261/2034642020-03-10T13:51:02Z
- Nonstationary time series convolution: on the relation between the Hilbert–Huang and Fourier transformhttp://hdl.handle.net/10261/89832Title: Nonstationary time series convolution: on the relation between the Hilbert–Huang and Fourier transform
Authors: Neukirch, Maik; García, Xavier
Abstract: The Hilbert-Huang Transform (HHT) decomposes time series into intrinsic mode functions (IMF) in time-frequency domain. We show that time slices of IMFs equal time slices of Fourier series, where the instantaneous parameters of the IMF define the parameters amplitude and phase of the Fourier series. This leads to the formulation of the theorem that nonstationary convolution of an IMF with a general time domain response function translates into a multiplication of the IMF with the respective spectral domain response function which is explicitly permitted to vary over time. We conclude and show on a real world application that a de-trended signal's IMFs can be convolved independently and then be used for further time-frequency analysis. Finally, a discussion is opened on parallels in HHT and the Fourier transform with respect to the time-frequency domain
Description: 13 pages, 4 figures
Thu, 23 Jan 2014 10:11:04 GMThttp://hdl.handle.net/10261/898322014-01-23T10:11:04Z
- Non Stationary, Broad-band Waveforms for CSEM - An Analysis with Synthetic Datahttp://hdl.handle.net/10261/128122Title: Non Stationary, Broad-band Waveforms for CSEM - An Analysis with Synthetic Data
Authors: Neukirch, Maik; García, Xavier
Abstract: Controlled source electromagnetic (CSEM) methods are sensitive to the subsurface conductivity struc- ture and thus had led to its use in resource exploration. Since the frequency for peak sensitivity and the exact location of an exploration target is normally unknown prior exploration, it is desirable to ac- quire the transfer functions for a broad range of frequencies and in a wide area. Investigations in both directions have been driven by optimising properties of the Fourier transform in order to enhance the frequency range and the source-receiver distances. Recent research on non-stationary (NS) time series analysis tools significantly enhanced processing of NS time series. This work assesses the possibility of NS source waveforms by presenting a chirp source that is highly customisable in amplitude and fre- quency range in order to accommodate any frequency range in combination with virtually any amplitude for each frequency (e.g. in order to counter attenuation by decreasing power from low frequencies to increase high frequency power assuming constant energy supply). A numeric example illustrates the NS waveform and that robust NS time series processing may lead reliably to the transfer functions of a 1D conductivity model. Lastly, the advantages of a freely customisable waveform design are discussed
Description: 76th EAGE Conference & Exhibition 2014, Experience the Energy, 16-19 June 2014, Amsterdam, Netherlands.-- 3 pages, 1 figure
http://hdl.handle.net/10261/128122
- Nonstationary magnetotelluric data processing with instantaneous parameterhttp://hdl.handle.net/10261/109064Title: Nonstationary magnetotelluric data processing with instantaneous parameter
Authors: Neukirch, Maik; García, Xavier
Abstract: Nonstationarity in electromagnetic data affects the computation of Fourier spectra and therefore the traditional estimation of the magnetotelluric (MT) transfer functions (TF). We provide a TF estimation scheme based on an emerging nonlinear, nonstationary time series analysis tool, called empirical mode decomposition (EMD) and show that this technique can handle nonstationary effects with which traditional methods encounter difficulties. In contrast to previous works that employ EMD for MT data processing, we argue the advantages of a multivariate decomposition, highlight the possibility to use instantaneous parameters, and define the homogenization of frequency discrepancies between data channels. Our scheme uses the robust statistical estimation of transfer functions based on robust principal component analysis and a robust iteratively reweighted least squares regression with a Huber weight function. The scheme can be applied with and without aid of any number of available remote reference stations. Uncertainties are estimated by iterating the complete robust regression, including the robust weight computation, with a bootstrap routine. We apply our scheme to synthetic and real data (Southern Africa) with and without nonstationary character and compare different processing techniques to the one presented here. As a conclusion, nonstationary noise can heavily affect Fourier-based MT data processing but the presented nonstationary approach is nonetheless able to extract the impedances
Description: 21 pages, 13 figures, 1 appendix
Fri, 02 Jan 2015 08:32:47 GMThttp://hdl.handle.net/10261/1090642015-01-02T08:32:47Z
- Inverting Capacitive Resistivity (Line Electrode) Measurements with Direct Current Inversion Programshttp://hdl.handle.net/10261/115614Title: Inverting Capacitive Resistivity (Line Electrode) Measurements with Direct Current Inversion Programs
Authors: Neukirch, Maik; Klitzsch, Norbert
Abstract: The capacitive resistivity (CR) method is a time- and labor-saving alternative to traditional direct current (DC) resistivity methods. The line electrode variant of CR suffers from the absence of data inversion programs as available for the DC resistivity method. Direct current inversion programs were applied to determine the resistivity distribution from CR measurements using an approximately equivalent four-point dipole-dipole configuration. We optimized configurations to minimize the systematic error applying DC inversion programs to CR, using data based on the comparison of the two-dimensional sensitivities of the proposed DC approximations. The optimal four-point dipole-dipole geometry has a dipole length of 80%. © Soil Science Society of America
Description: 11 pages, 11 figures
Fri, 22 May 2015 07:15:54 GMThttp://hdl.handle.net/10261/1156142015-05-22T07:15:54Z
- The Magnetotelluric Amplitude Tensor as Compliment to the Phase Tensor for Mapping, Inversion and Distortion Analysishttp://hdl.handle.net/10261/172067Title: The Magnetotelluric Amplitude Tensor as Compliment to the Phase Tensor for Mapping, Inversion and Distortion Analysis
Authors: Neukirch, Maik; Rudolf, Daniel; García, Xavier
Abstract: The introduction of the phase tensor marked a major breakthrough in understanding galvanic electric distortion for Magnetotellurics. It is used for dimensionality analysis, distortion analysis and even for inversion. However, impedance amplitude information is neglected which may result in information loss. We formulate an impedance tensor decomposition into known phase tensor and new, complementary and independent amplitude tensor. The amplitude tensor contains galvanic and inductive amplitudes of which the latter are physically related to the inductive information of the phase tensor. The physical similarity between inductive phase and amplitude can be used to approximate the galvanic amplitude for the general subsurface, which leads to the qualitative interpretation of 3D galvanic distortion: (i) the (purely) galvanic part of the subsurface (as sensed at a given period) may have a changing impact on the impedance (over a period range) and (ii) only the purely galvanic response of the lowest period should be termed galvanic distortion. The approximation of the galvanic amplitude offers a new perspective on galvanic distortion, which breaks with the general belief of the need to assume 1D or 2D regional structure for the impedance. The amplitude tensor is illustrated and compared to the phase tensor on one example
Description: 78th European Association of Geoscientists & Engineers (EAGE) Conference & Exhibition, 30 May - 2 June 2016, Vienna, Austria.-- 5 pages, MT3D workshop datasets are publicly available at: http://mtnet.dias.ie/workshops/em_workshops.html
Fri, 09 Nov 2018 13:32:04 GMThttp://hdl.handle.net/10261/1720672018-11-09T13:32:04Z
- Amplitude-phase decomposition of the magnetotelluric impedance tensorhttp://hdl.handle.net/10261/190639Title: Amplitude-phase decomposition of the magnetotelluric impedance tensor
Authors: Neukirch, Maik; Rudolf, Daniel; García, Xavier; Galiana, Savitri
Abstract: The introduction of the phase tensor marked a breakthrough in the understanding and analysis of electric galvanic distortion effects. It has been used for (distortion-free) dimensionality analysis, distortion analysis, mapping, and subsurface model inversion. However, the phase tensor can only represent half of the information contained in a complete impedance data set. Nevertheless, to avoid uncertainty due to galvanic distortion effects, practitioners often choose to discard half of the measured data and concentrate interpretation efforts on the phase tensor part. Our work assesses the information loss due to pure phase tensor interpretation of a complete impedance data set. To achieve this, a new MT impedance tensor decomposition into the known phase tensor and a newly defined amplitude tensor is motivated and established. In addition, the existence and uniqueness of the amplitude tensor is proven. Synthetic data are used to illustrate the amplitude tensor information content compared with the phase tensor. Although the phase tensor only describes the inductive effects within the subsurface, the amplitude tensor holds information about inductive and galvanic effects that can help to identify conductivity or thickness of (conductive) anomalies more accurately than the phase tensor. Furthermore, the amplitude and phase tensors sense anomalies at different periods, and thus the combination of both provides a means to evaluate and differentiate anomaly top depths in the event of data unavailability at extended period ranges, e.g., due to severe noise
Description: 10 pages, 5 figures, appendixes.-- A MATLAB executable to compute and plot the amplitude and phase tensor parameters is available at https://www.dropbox.com/sh/61g7vbl2z8dtpey/AACmbGBsbpTGzW9nrdrc1u37a?dl=0 for academic use. At the same location, parameter maps of the models discussed in this work are available for a finer and longer period range for the interested reader as well as the data files for the first example.-- Data associated with this research are available and can be obtained by contacting the corresponding author
Fri, 13 Sep 2019 08:02:52 GMThttp://hdl.handle.net/10261/1906392019-09-13T08:02:52Z
- Horizontal-to-Vertical Spectral Ratio of Ambient Vibration Obtained with Hilbert–Huang Transformhttp://hdl.handle.net/10261/241166Title: Horizontal-to-Vertical Spectral Ratio of Ambient Vibration Obtained with Hilbert–Huang Transform
Authors: Neukirch, Maik; García-Jerez, A.; Villaseñor, Antonio; Luzón, F.; Ruiz Fernández, Mario; Molina, Luis
Abstract: The Horizontal-to-Vertical Spectral Ratio (HVSR) of ambient vibration measurements is a common tool to explore near surface shear wave velocity (Vs) structure. HVSR is often applied for earthquake risk assessments and civil engineering projects. Ambient vibration signal originates from the combination of a multitude of natural and man-made sources. Ambient vibration sources can be any ground motion inducing phenomena, e.g., ocean waves, wind, industrial activity or road traffic, where each source does not need to be strictly stationary even during short times. Typically, the Fast Fourier Transform (FFT) is applied to obtain spectral information from the measured time series in order to estimate the HVSR, even though possible non-stationarity may bias the spectra and HVSR estimates. This problem can be alleviated by employing the Hilbert–Huang Transform (HHT) instead of FFT. Comparing 1D inversion results for FFT and HHT-based HVSR estimates from data measured at a well studied, urban, permanent station, we find that HHT-based inversion models may yield a lower data misfit χ2 by up to a factor of 25, a more appropriate Vs model according to available well-log lithology, and higher confidence in the achieved model. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
Wed, 19 May 2021 09:54:09 GMThttp://hdl.handle.net/10261/2411662021-05-19T09:54:09Z
- Lithospheric thinning under the Araripe Basin (NE Brazil) from a long-period magnetotelluric survey: Constraints for tectonic inversionhttp://hdl.handle.net/10261/174628Title: Lithospheric thinning under the Araripe Basin (NE Brazil) from a long-period magnetotelluric survey: Constraints for tectonic inversion
Authors: García, Xavier; Julià, Jordi; Nemocon, Ana Milena; Neukirch, Maik
Abstract: The lithospheric architecture of the western Borborema Province and northern São Francisco craton of NE Brazil has been investigated through analysis of long-period magnetotelluric data acquired along a 700 km long survey, using 12 instruments. The survey samples several tectonic terrains in the Province and penetrates into the adjacent São Francisco craton after crossing the Araripe Basin, an aborted rift basin filled with Mesozoic sediments that peak at ∼1000 m above mean sea level. High conductivities are observed at shallow depths under the main Precambrian shear zones that pervade the Province – consistent with tectonic reactivation – and as a small patch embedded within the high resistivities that characterize the São Francisco craton. High conductivities (∼25 Ωm) are also observed below 120 km depth between the Patos and Pernambuco lineaments – right under the Araripe Basin – flanked by resistive (>120 Ωm) material immediately to the north and south. This deep, highly conductive body is found consistent with the presence of melt and aqueous fluids, and is interpreted as shallow asthenospheric mantle bounded by thicker lithosphere. We propose that extensional stresses in the Mesozoic stretched and thinned the lithosphere under the Araripe Basin, causing passive upwelling of asthenospheric material and lateral flow of the overlying lithosphere, and resulting in thickening of the lithosphere under the flanks and uplift of the Araripe Basin. We also hypothesize that thermal weakening of the lithospheric mantle – perhaps sustained by channeling of asthenospheric flows under the basin – would have caused regional stresses to concentrate in the brittle upper crust and contribute to basin inversion. We thus propose that a combination of localized horizontal stresses and vertical buoyancy from underlying asthenospheric material are ultimately responsible for the actual topography of the Araripe Basin. © 2019 International Association for Gondwana Research.
Thu, 24 Jan 2019 08:35:55 GMThttp://hdl.handle.net/10261/1746282019-01-24T08:35:55Z
- Appraisal of magnetotelluric galvanic electric distortion by optimizing amplitude and phase tensor relationshttp://hdl.handle.net/10261/223112Title: Appraisal of magnetotelluric galvanic electric distortion by optimizing amplitude and phase tensor relations
Authors: Neukirch, Maik; Galiana, Savitri; García, Xavier
Abstract: The introduction of the phase tensor marked a major breakthrough in the analysis and treatment of electric field galvanic distortion in the magnetotellurics method. Recently, the phase tensor formulation has been extended to a complete impedance tensor decomposition by introducing the complementary amplitude tensor, and both tensors can be further parameterized to represent geometric properties such as dimensionality, strike angle, and macroscopic anisotropy. Both tensors are characteristic for the electromagnetic induction phenomenon in the conductive subsurface with its specific geometric structure. The central hypothesis is that this coupling should result in similarities in both tensor¿s geometric parameters, skew, strike, and anisotropy. A synthetic example illustrates that the undistorted amplitude tensor parameters are more similar to the phase tensor than increasingly distorted ones and provides empiric evidence for the predictability of the proposed hypothesis. Conclusions drawn are reverse engineered to produce an objective function that minimizes when amplitude and phase tensor parameter dissimilarity is, along with any present distortion, minimal. A genetic algorithm with such an objective function is used to systematically seek the distortion parameters necessary to correct any affected amplitude tensor and, thus, impedance data. The successful correction of a large synthetic impedance data set with random distortion further supports the central hypothesis and serves as comparison to the state-of-the-art. The classic BC87 data set sites lit007/lit008 and lit901/lit902 have been noted by various authors to contain significant distortion and a 3D regional response, thus invalidating current distortion analysis methods and eluding geologic interpretation. Correction of the BC87 responses based on the present hypothesis conforms to the regional geology
Description: 20 pages, 5 figures, 1 table.-- Miensopust for providing the distortion parameters for the 3D dataset from the MT3D-2 workshop. All MT3D workshop datasets are publicly available at http://www.complete-mt-solutions.com/mtnet/workshops/em_workshops.html#3DMTINV
Wed, 18 Nov 2020 11:00:06 GMThttp://hdl.handle.net/10261/2231122020-11-18T11:00:06Z
- Statistics on the Performance of Instrument Types and the Significance of HVSR data for Shallow Vs HVSR/DC Joint Inversions - A Result from the Large-N Maupasacq Experiment (Southern France)http://hdl.handle.net/10261/214679Title: Statistics on the Performance of Instrument Types and the Significance of HVSR data for Shallow Vs HVSR/DC Joint Inversions - A Result from the Large-N Maupasacq Experiment (Southern France)
Authors: Neukirch, Maik; García-Jerez, Antonio; Villaseñor, Antonio; Stehly, L.; Boue, P.; Chevrot, S.; Sylvander, Matthieu; Diaz, J.; Ruiz, Mario; Luzón, Francisco; Collin, Magali; Calassou, Sylvain; Polychronopoulou, K.; Martakis, N.; Bitri, A.
Abstract: Horizontal-to-Vertical Spectral Ratios (HVSR) and Rayleigh group velocity dispersion curves (DC) can be used to estimate the shallow S-wave velocity (Vs) structure. Knowing the shallow Vs structure is important for geophysical data interpretation either in order to better constrain data inversions for P-wave velocity (Vp) structures such as travel time tomography or full waveform inversions, or to directly study the Vs structure for geo-engineering purposes (e.g. ground motion prediction). The purpose of this study is to appraise in particular how much information HVSR can add in a large N experiment and how different instrumentation types affect this.
During the Maupasacq large-scale experiment, 197 three-component short-period stations, 190 geophone nodes and 54 broadband seismometers were continuously operated in Southern France for 6 months (April to October 2017) covering an area of approximately 1500 km2 with a site spacing of approximately 1 to 3 km. On the obtained HVSR and DC data, a statistical Joint inversion is performed for the shallow Vs structure. The results indicate that the addition of HVSR data to the DC inversion reduces the variance of the recovered shallow Vs model and improves the convergence to a smaller data misfit. While broadband and short period instruments delivered similar results, geophone nodes performed significantly worse due to their much higher cut off frequency.
Description: European Geosciences Union (EGU) General Assembly 2020, 4-8 May 2020.
Wed, 17 Jun 2020 18:27:00 GMThttp://hdl.handle.net/10261/2146792020-06-17T18:27:00Z