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Abstract: Abstract In recent years, sparker source has gradually been applied for high-resolution seismic surveys. But the air gun is still the most commonly used seismic sources in marine seismic exploration. The seismic data frequency range of the air gun sources is below 200 Hz. On the contrary, the seismic data frequency range of the sparker sources is about 50–500 Hz. The low and high frequency components of the seismic data are both important for high resolution seismic exploration. Usually the energy produced by the air gun sources is stronger than that of the sparker sources, so the exploration depth of the air gun sources is bigger than that of the sparker sources. How to make full use of the two kinds of source to carry out high-resolution seismic exploration is a particularly meaningful work. Here the combined processing idea of the two kinds of sources’ towed streamer seismic data were presented and we designed a complete data processing workflow. The amplitude matching of air gun and sparker source seismic data is a very key technique in this combined processing. The main processing steps include conventional processing such as noise attenuation, amplitude compensation, wavelet processing, velocity analysis, pre-stack time migration and so on. But also, there are some special processing techniques such as the residual static corrections, CDP trim statics corrections and post-stack predictive deconvolution which are applied to the sparker source data. The results show that the migration section of the combined processing is better than those of the separately processing. PubDate: 2022-05-21
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Abstract: Abstract This paper presents interpretation examples of synthetic marine frequency domain CSEM responses from multiple reservoirs of known and realistic geologic models. The electric field responses were decomposed into constituent responses to examine individual reservoirs’ contributions to the EM field. Several realistic geological models of the reservoirs were created with all the structural complexities which are synonymous with the real-life scenario in the Niger Delta. The model has three reservoir layers, tagged 1st, 2nd and 3rd reservoir layers, in 2.5 km water depth at 0.25 Hz frequency. The result shows a clear Normalized Amplitude Ratio (NAR) for each resistive layer, with different detectability strengths of the layers. The most near-seafloor resistive layer, dubbed 1st reservoir layer, is marked by high NAR while the deeper resistive layer shows lower detectability. The value of NAR for each decomposed resistive layer reflects their depth of burial while the sum of individual NAR values correlates with the NAR obtained for the model containing all the resistors. It implies that the magnitude of NAR could be used to deduced the relative depth of burial of a resistive hydrocarbon layer as well as to detect the presence of multiple resistors. The decomposition of the resistive layers could be of help during real-time exploration surveys and suggest possible multiple hydrocarbon layers. The study also allows the understanding that the resultant response of two vertical reservoir layers is a summation of the strength of the individual layer, which differs from the seismic response of the same layers. PubDate: 2022-05-21
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Abstract: Abstract The complex underwater environment and sonar parameters make the captured acoustic side scan sonar imagery to suffer from depleted contrast, low brightness, speckle noise, and deteriorated contour. Though the electromagnetic waves are highly absorbed in water and sonar is exemplary considered, these issues will affect the performance of the imaging Side Scan Sonar (SSS). Hence, these images need effective enhancement to achieve a privileged visual effect. The paper proposes the Retinex based Contrast-Enhanced Edge Preserved (RCEEP) technique to enhance the low-quality SSS image. Initially, the degraded image is convolved with a smoothing filter to obtain an illumination map. After the noise suppression, the reflectance map is computed and the brightness factor is interpolated. To rid of the blurred edges, the amended unsharp mask filter is applied to obtain the sharp-contour and smoothens the speckle noise. Finally, the contrast factor is weighted with a masked image to retain the contrast-enhanced sharpened image. The qualitative and quantitative analysis is carried out on the acoustic imagery. To evaluate each of the image attributes, the considered quantitative parameters are Root Mean Square Error (RMSE), Peak Signal to Noise Ratio (PSNR), Contrast Enhancement based contrast-changed Image Quality (CEIQ), Natural Scene Statistics (NSS), and Perceptual Sharpness Index (PSI). It is observed that the proposed RCEEP methodology enhances even the features in the dark region and outperforms the other state-of-the-art enhancement techniques. PubDate: 2022-05-14
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Abstract: Abstract In estimating the geohazards posed by submarine landslides, the rheological properties of marine sediments are of significant importance for their postfailure dynamics. We report an experimental study of the rheological behavior of marine sediments taken from the Ulleung Basin, East Sea and their influence on numerical simulations of debris flow runout. Marine sediments exhibit a typical yield stress behavior, such as that of low-activity clays. For the materials examined, different yield stresses are observed depending on the shearing methods. Steady-state and oscillatory shear tests were conducted for different volumetric concentrations of sediment. According to the test results, the Bingham yield stresses under controlled shear stress and shear rate range from approximately 100 Pa to 1500 Pa, but the yield stresses under oscillatory shear loads range from approximately 25 Pa to 3500 Pa for a given sediment concentration. In the latter cases, the value obtained in the elastic region is approximately doubled. Experiments under steady-state and oscillatory shear loads can be helpful in determining the yield points in the elastic and viscous regions and in explaining changes in the structure of the soil sample due to shear. We apply the range of measured yield strength values to numerical simulations of debris flow runout using a Herschel-Bulkley model and find that only the lowest values of yield strength, despite the low sediment concentrations, could account for the observed runout and thickness distribution. We infer that significant wetting must occur during debris flow motion to attain the observed runout. PubDate: 2022-04-20
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Abstract: Abstract Back-arc rifting at the middle eastern continental margin of the Korean Peninsula took place in the early stage of separation of the SW Japan Arc from mainland Asia in the Cenozoic. As a result, the margin preserves structural elements of rifting seawards that include a narrow rift (Hupo Basin), a major syn-rift border fault (Hupo Fault), and an uplifted rift flank (Hupo Bank). We correlated current seismicity at the margin with the rifting structures and estimated the potential seismic hazards. The Hupo Basin, in its overall shape, is a half-graben created by back-tilting of the hanging wall toward the Hupo Fault. The trace of the Hupo Fault is composed of multiple segments arcuate seawards, suggesting nucleation of rift faults at multiple places. The Hupo Fault is vertical at a shallow depth, but hypocentral locations of earthquakes indicate that the Hupo Fault is listric down to the upper and lower crustal boundary. Stochastic estimations of the maximum possible magnitude of earthquakes at the margin encompassing the Hupo Basin and the Hupo Bank average to ML 5.86. The estimated b-value of 0.63 is lower than that in the Korean Peninsula, which may suggest the dominance of thrust faulting with a strike-slip (or oblique) component on the listric and lower dipping portions of the faults at the margin. PubDate: 2022-04-19
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Abstract: Abstract The southwestern continental margin of India reveals the presence of an anomalous bathymetric high feature located in the mid-continental slope region off Trivandrum. Based on the analysis of the available geophysical data and plate tectonic reconstruction, this feature was interpreted as a scar of India-Madagascar separation, with its conjugate identified on the Northern Madagascar Ridge. Although the conjugate nature of the Alleppey-Trivandrum Terrace Complex and the Northern Madagascar Ridge was postulated, this inference has not yet been evaluated by comparing their geophysical signatures and crustal structure. The present study is aimed to derive and compare the crustal configuration of these two features using an up-to-date compilation of the bathymetry, gravity and magnetic data, and by employing integrated forward modelling of gravity and magnetic anomalies. Our derived crustal models for the Alleppey-Trivandrum Terrace Complex and the Northern Madagascar Ridge suggest that both these features can be explained in terms of thinned continental crust intermingled with volcanic intrusives. The crustal thicknesses of the Northern Madagascar Ridge and Alleppey-Trivandrum Terrace Complex at their conjugate continent-ocean boundaries are ~ 17 km and both these features are associated with high amplitude magnetic anomalies whose genesis is attributed to the volcanism caused by the Marion hotspot activity. Therefore, based on our integrated interpretation of the geophysical data, we support the earlier interpretation on conjugate nature of the Northern Madagascar Ridge and the Alleppey-Trivandrum Terrace Complex that was proposed based on the fitting of shape and size of the bathymetric notch observed in the southeastern continental margin of Madagascar with a bathymetric protrusion observed in the southwestern continental margin of India in the India-Madagascar pre-drift scenario. These features remained as a single unit prior to ~ 88 Ma and subsequently got separated during the India-Madagascar breakup. PubDate: 2022-04-16
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Abstract: Abstract Vertical cable seismic (VCS) is a reflection seismic exploration technique, which is mainly used for structural imaging in high dip angle areas. Because the source and receiver points are usually not in the same plane, it is not possible to use conventional velocity analysis to obtain the velocity field. In most cases, the velocity field of the streamer data is processed in the same survey area as the VCS. Seismic interferometry is to obtain new seismic signals by cross-correlation or convolution operation of seismic signals received by different receivers. Therefore, we propose to apply seismic interferometry to VCS exploration. Compared with conventional VCS data processing, this method does not need towed streamer data and improves exploration efficiency. In this paper, the method is applied to model data and actual data of South China Sea respectively to obtain the stacking profile. The results show that this method is applied to VCS data, and the stacking profile is continuous in phase axis and clear in structure. PubDate: 2022-04-15
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Abstract: Abstract Three-dimensional (3D) sub-bottom profilers (SBPs), which extend the data acquisition range of traditional single-channel SBPs, have been developed and utilized in various forms. However, owing to the fixed arrangement of the source and receivers, there are limitations on changing the survey specifications. In this study, we developed and field-tested a 3D SBP system that can be efficiently used in various fields, including the identification of buried objects and imaging of geological structures. This modular 3D SBP can arbitrarily adjust the type of seismic source and the number and interval of receiving channels, as demonstrated using a field-test in the waters near Ulsan, Korea. Using CHIRP as a seismic source, raw data with a sweep signal for a 1420 × 320 m2 area was successfully acquired; a 3D seismic cube was created for the same area through subsequent 3D data processing based on 50 × 50 × 4.5 cm3 volumetric resolution. Using the 3D seismic cube, we could identify various geological features, including irregular seafloor, seafloor channels, sub-parallel stratification, and the top surface of free gas. The results confirm that this method can be utilized in geological study for various engineering purposes. PubDate: 2022-04-15
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Abstract: Abstract Continental shelves around the globe are hosts to vast reservoirs of offshore freshened groundwater. These systems show considerable complexity, often as a function of the geological heterogeneity. Data needed to characterise these systems are often sparse, and numerical models rely on generalized simplifications of the geological environment. In order to improve our understanding of these systems, it is necessary to implement modelling approaches that can produce large-scale geologically representative models using sparse data. We present an interdisciplinary stochastic modelling workflow incorporating borehole data, 2D depth-migrated seismic profiles, seismic attributes, and prior knowledge of the depositional setting. We generate a conditioned Gaussian field of porosity on the New Jersey shelf. We also perform a petrophysical conversion to a corresponding permeability distribution. The model dimensions are 134 km × 69 km × 1.7 km, with an adjustable resolution that can be adapted for process-based models of flow and solute transport. The integrated approach successfully translates small-scale porosity variations to a shelf-scale model that honors key characteristics of the New Jersey shelf wave-dominated depositional environment. The model was generated using open-source packages. All data and code to reproduce the complete workflow are provided along with this study so the model can be reproduced at any resolution for further studies of continental shelf processes offshore New Jersey. PubDate: 2022-04-12
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Abstract: Abstract We studied the Quaternary incised fills drilled at the northern Yangsan Fault having multiple deformation histories since Late Cretaceous or Paleogene to determine tectonic influence on development of incised valley and its sedimentation. Incised valley fills were deposited during and after the Last Glacial Maximum and are composed of fluvial lag, debris flow deposits interbedded with fluvial sediments, shallow marine sandy deposits, and fluvial sediments from bottom to top. These fills show lateral changes in sediment thickness from 44 to 11.5 m over a short distance of 230 m, implying sediment stacking in a deep and steeply inclined valley. Fluvial lag and debris flow deposits are common in the thalweg of a valley. Despite small drainage basin (195.9 km2), the development of deep incised valley is interpreted to have resulted from fluvial downcutting on erodible basement during sea level fall as a consequence of dense development and fault and fracture networks in the pre-Quaternary rocks caused by multiple movements of Yangsan Fault. With steep gradient, the damaged rocks led to frequent slope failure and forceful accumulation of debris flow deposits on the valley’s axis at the time. In addition, stacking of debris flow deposits resulted in decrease of longitudinal gradient of incised valley, promoting rapid transgression during sea level rise (9 to 7 ka). This resulted in insufficient time for the central basin mud to be accumulated, which explains why the studied fills lack central estuarine mud that is common in incised valleys fills deposited during transgression. PubDate: 2022-04-08
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Abstract: Abstract Analysis of high-resolution seismic profiles from the SE continental shelf of Korea reveals that the shelf sequence consists of four sedimentary units, namely, Pliocene deposits (III), Lower Pleistocene deposits (IIB), Upper Pleistocene deposits (IIA), and Holocene deposits (I), which were mainly controlled by regional tectonic activity and sea-level fluctuations. Vertically, with the exception of unit I, the units form a series of superimposed prograding wedges that thicken seaward. A structural high, including several faults and folds, occurs north of Tsushima Island, extending northeastward to the Dolgorae Thrust Belt at the southern margin of the Ulleung Basin. In this study, we deduced that the deformed zone was formed due to compressional deformation associated with back-arc closure that occurred after the extension of the East Sea. Most of the faults and folds, oriented in NE–SW or NNE–SSW trends, mainly developed in the lower two units (III and IIB). The lower two units were also significantly deformed, whereas the two overlying units remained relatively undeformed. This indicated that, during the Pliocene and Lower Pleistocene (units III and IIB), sedimentation was mainly controlled by tectonic activities, whereas sedimentation after the Lower Pleistocene (units IIA and I) was mainly influenced by periodically repeated sea-level changes rather than tectonic controls. PubDate: 2022-03-18
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Abstract: Abstract Acoustic data collected by multibeam echosounders (MBES) are increasingly used for high resolution seabed mapping. The relationships between substrate properties and the acoustic response of the seafloor depends on the acoustic angle of incidence and the operating frequency of the sonar, and these dependencies can be analysed for discrimination of benthic substrates or habitats. An outstanding challenge for angular MBES mapping at a high spatial resolution is discontinuity; acoustic data are seldom represented at a full range of incidence angles across an entire survey area, hindering continuous spatial mapping. Given quantifiable relationships between MBES data at various incidence angles and frequencies, we propose to use multiple imputation to achieve complete estimates of angular MBES data over full survey extents at a high spatial resolution for seabed mapping. The primary goals of this study are (i) to evaluate the effectiveness of multiple imputation for producing accurate estimates of angular backscatter intensity and substrate penetration information, and (ii) to evaluate the usefulness of imputed angular data for benthic habitat and substrate mapping at a high spatial resolution. Using a multi-frequency case study, acoustic soundings were first aggregated to homogenous seabed units at a high spatial resolution via image segmentation. The effectiveness and limitations of imputation were explored in this context by simulating various amounts of missing angular data, and results suggested that a substantial proportion of missing measurements (> 40%) could be imputed with little error using Multiple Imputation by Chained Equations (MICE). The usefulness of imputed angular data for seabed mapping was then evaluated empirically by using MICE to generate multiple stochastic versions of a dataset with missing angular measurements. The complete, imputed datasets were used to model the distribution of substrate properties observed from ground-truth samples using Random Forest and neural networks. Model results were pooled for continuous spatial prediction and estimates of confidence were derived to reflect uncertainty resulting from multiple imputations. In addition to enabling continuous spatial prediction, the high-resolution imputed angular models performed favourably compared to broader segmentations or non-angular data. PubDate: 2022-03-09 DOI: 10.1007/s11001-022-09471-3
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Abstract: Abstract The study investigates sea-level measurements observed from 12 tide gauge sites over the Indian coastal area during the last two decades. Initially, the rise of sea-level (slope) and acceleration is estimated by fitting the linear and parabolic equations in the recorded data from tide gauge measurements. The estimated results showed the abrupt change of relative sea-level measurements at the distinct site depends upon the tide gauge site locations. To accurately analyse the regional coastal sea level pattern, the tide gauge time series data are decomposed in sine and cosine functions at different frequencies followed by the spectral analysis. Analysis of the results confirmed that the tide gauges peaks do not occur at the fixed period at each tide gauge site, instead they repeat with varying time periods. The spectrum peaks width fluctuate at distinct sites and the general pattern of frequency spectrum does not follow a unique model. Such type of characteristic variation with the time is possibly because of the effective variables, which affects the steadiness of sea-level changes. The study concludes that the experimental results from the Indian coastal region must be included during the comparison of global data sets and other contemporary oceanic models. PubDate: 2022-02-23 DOI: 10.1007/s11001-022-09468-y
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Abstract: Abstract In ocean-bottom node (OBN) seismic exploration, a ghost is a common interference wave that affects the accuracy of seismic data interpretation. Receiver de-ghosting can be achieved using dual-sensor summation technology, which employs a hydrophone and geophone to collect seismic signals. The differences between the two receivers cause the polarities of the ghost wave signals to be opposite; therefore, the ghost waves can be eliminated by adding these receivers. However, there are differences between the actual data obtained from the hydrophone and geophone with regard to frequency, phase, and amplitude, thereby preventing them from being directly summated. Therefore, the frequency, phase and amplitude of both data records must be matched for consistency before dual-sensor summation can be conducted. In addition, some noise and ghosts will remain during data processing, resulting in a reduction in the signal-to-noise ratio of the data, making it necessary to adopt noise and residual ghost suppression methods. In this study, a wavelet analysis was newly introduced to the dual-sensor summation process. Specifically, the wavelet spectrum whitening method was proposed for the frequency matching of dual-sensor data, and the nonlinear wavelet transform threshold method of the wavelet denoising method was applied to suppress the noise and residual ghost. On this basis, a new dual-sensor process flow in OBN seismic exploration was developed. The feasibility and effectiveness of the method were verified using actual data. The method proposed in this study will help to improve the accuracy of future data processing. PubDate: 2022-02-17 DOI: 10.1007/s11001-022-09467-z
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Abstract: Abstract Seismic feature characterization in the time domain remains a challenge for geoscientists, thus motivating transformation to the frequency domain. This study exposes an unreported buildup through a frequency domain transformation of seismic sections using constrained least squares spectral analysis for spectral decomposition. The imaged buildup appears at an anomalous low frequency of 5 Hz and vanishes at frequencies higher than 10 Hz and is traced in time domain. The geological context and feature morphology suggest that the buildup seems to be a reef. Further, quantitative scoring using regional, geophysical and geological criteria suggest a possible isolated carbonate buildup. This suggests a future exploration prospect and opens the scope for further geophysical investigation, developing conceptual models and practical evaluation. PubDate: 2022-01-31 DOI: 10.1007/s11001-022-09466-0
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Abstract: Abstract Analysis of anisotropy from velocity data is essential for improving the hydrocarbon reservoir characterization. The anisotropy of a medium is affected by the mechanical strength, presence of fracture, mineral distribution of the rock, and its degree affects the seismic velocity. We attempted to characterize the anisotropy of the gas hydrate bearing sediments in the offshore Mahanadi basin using three wells. Initially, the presence of anisotropy was investigated by estimating the stiffness coefficients and Thomsen’s parameters (epsilon, gamma and delta) assuming a horizontal transversely medium using dipole S-wave (upper and lower) velocities. The natural fractures were identified from the formation image data. The strong anisotropy is associated with the presence of natural fractures and lower values of the elastic modulus. Most of the strong and weak anisotropy zones are oriented in the NW to W direction of the study area. Our study suggests that the anisotropy in gas hydrate bearing sediment is stress-induced due to the presence of pore filling fractures, and the change of mechanical behavior. The higher positive values of epsilon and delta with gamma represent either dry solid gas hydrate or free gas filled in the fracture of the sediments as observed in the crossplot analysis. Finally, we modeled P-wave and S-wave velocities by incorporating the Thomsen’s parameters. S-wave velocity is less effective than P-wave velocity at 90° angle of fracture relative to the symmetry axis and the modeled P-wave velocity increases upto 2.8% in the gas hydrate bearing sediments. PubDate: 2022-01-23 DOI: 10.1007/s11001-021-09465-7
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Abstract: Abstract Coastal dynamics are the result of several processes controlling the balance between sediment input and output over time. The beach system is not always able to maintain a neutral coastal balance due to natural and anthropogenic causes. We present an integrated marine geology, geomorphological and sea-level rise analysis in the coastal sector between Torre delle Ciavole and Capo Calavà (North-Eastern Sicily, Italy).This sector is characterized by high uplift rates and frequent seismicity (mainly generated by the very active Vulcano-Tindari Fault System), promoting the development of mass-wasting processes in the coastal and offshore sectors. A main erosive feature observed in the area is the head of the Gioiosa Marea submarine canyon, located at some meters of depth, few hundred meters far the coastline. The main morphological features of the canyon were reconstructed through the analysis of high-resolution multibeam data, indicating that the canyon is active, as also testified by the comparison of time-lapse aerial photos. Due to this active setting, the study area is exposed to multiple geohazards, among which we deal with: (1) retrogressive instability at the head of the Gioiosa Marea submarine canyon, (2) coastal erosion favored by the downlope funnelling of littoral drift at the canyon head, (3) flooding scenario at 2100 using the IPCC (Intergovernmental Panel on Climate Change) and Rahmstorf sea-level projections. The consequences associated with these geohazards are amplified by the strong anthropization pressures occurring along in this sector. Our results provide key insights regarding the future scenarios of this coastal sector, revealing the effects of the retrogressive activity associated with the canyon head on the coastal strip. We also present the first management tool for the application of forecasting studies by local administrations. PubDate: 2022-01-08 DOI: 10.1007/s11001-021-09463-9
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Abstract: Abstract Surface Related Multiple Elimination (SRME) usually suffers the issue of either over-attenuation that damages the primaries or under-attenuation that leaves strong residual multiples. This dilemma happens commonly when SRME is combined with least-squares subtraction. Here we introduce a more sophisticated subtraction approach that facilitates better separation of multiples from primaries. Curvelet-domain subtraction transforms both the data and the multiple model into the curvelet domain, where different frequency bands (scales) and event directions (orientations) are represented by a finite number of curvelet coefficients. When combined with adaptive subtraction in the time–space domain, this method can handle model prediction errors to achieve effective subtraction. We demonstrate this method on two 2D surveys from the TAiwan Integrated GEodynamics Research (TAIGER) project. With a careful parameter determination flow, our result shows curvelet-domain subtraction outperforms least-squares subtraction in all geological settings. We also present one failed case where specific geological condition hinders proper multiple subtraction. We further demonstrate that even for data acquired with short cables, curvelet-domain subtraction can still provide better results than least-squares subtraction. We recommend this method as the standard processing flow for multi-channel seismic data. PubDate: 2022-01-06 DOI: 10.1007/s11001-021-09464-8
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Abstract: Abstract This study investigates the real-time sea-level measurement from six tide gauge stations in the coastal area of Japan. Initially, the six co-located global navigation satellite station sites vertical velocity has been observed followed by estimation of the relative tide gauge velocities. The results showed that the estimated relative sea-level changes abruptly at different sites depending on the location of tide gauge stations. To accurately analyse the regional tide gauge patterns at each site, a continuous wavelet transforms based on Morse wavelet as a default transformation has been applied. Analysis showed that the peaks of tide gauges have not occurred at a static time at each site, rather they were repeating with the variation of time. The width of the spectrum peaks varied with different sites and the overall spectrum frequency was not unique. These characteristics variation with time may be due to the variables that affect the stability of the sea level measurements. Finally, the kernel extreme learning machine (KELM) approach based on variational mode decomposition is used to reconstruct the tide gauge measurements. The correlation coefficients between observed and reconstructed data sets are very high varying from 0.91 to 0.96. The estimated root mean square error between two data sets is varying from 0.04 to 0.06 m at the entire region. This means that the KELM reconstructed tide gauge showed good agreement with the observed tidal waveforms. Thus, it can be concluded that the proposed techniques can be implemented for effective real-time sea-level monitoring changes with the available data. PubDate: 2021-12-02 DOI: 10.1007/s11001-021-09460-y
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