Near-surface Heterogeneities Research Articles

The impact of receiver arrays on suppressing seismic speckle scattering noise caused by the meter-scale near-surface heterogeneity

Managing seismic scattering noise in geophysical surveying, especially from near-surface heterogeneity, presents significant challenges. We revisit the role of receiver arrays in mitigating such noise in regions with near-surface meter-scale heterogeneity. Moving beyond traditional approaches that focus on suppressing coherent noise from near-surface arrivals and random ambient noise, our research highlights the substantial impact of speckle scattering noise. This noise, resulting from near-ballistic scattering on small-scale heterogeneities, introduces considerable trace-to-trace variability in phase and amplitude, complicating data processing. By integrating a novel model of random multiplicative noise with sophisticated statistical techniques, we determine the efficacy of array stacking in significantly reducing this type of noise and decreasing the dispersion of phase and amplitudes. Our analytical and numerical analyses reveal a notable trend: the reduction in phase and amplitude spread amplifies with the array size. A key finding of our study is the identification of a [Formula: see text] law for phase spread reduction, analogous yet distinct from the well-known [Formula: see text] law observed in amplitude attenuation of additive ambient noise. In our context, this law implies that the standard deviation of residual phase disturbances diminishes in proportion to [Formula: see text], where N denotes the array size. This insight holds particular significance for smaller arrays, such as those with nine receivers commonly used in desert environments.

Effect of near-surface heterogeneities on the pore-water pressure distribution and slope stability

Effect of near-surface heterogeneities on the pore-water pressure distribution and slope stability

Noise masking of near-surface scattering (heterogeneities) on subsurface seismic reflectivity

Abstract Near-surface velocity variations are the main cause of seismic scattering in exploration seismology. Many studies create the near-surface heterogeneity as velocity models that have random velocity distribution, random objects, or irregular subsurface topography to study and mitigate the resultant scattering effects of the near-surface layer. Von Kármán (self-similar) method is a known method in the literatures for modeling heterogeneous earth in a statistical way. This research modifies the self-similar method, and throughout the work, it has proven that the self-similar provides a robust method for generating realistic near-surface velocity models with different spatial velocity distributions. This study creates four-velocity models with simple subsurface layering and structure, three of which include a near-surface layer in three different degrees of velocity heterogeneity. Synthetic acoustic seismic reflections are produced for the four-velocity models to investigate the resultant scattering effects of the near-surface velocity heterogeneity on the quality of seismic waveform coherency. Spectacular negative observations are witnessed of the near-surface layer involvement to the quality of seismic reflection coherency that increases as velocity dramatically varies. Subtracting the scattering noise, which is modeled using an exact heterogeneous model, enhances seismic reflection coherency for the subsurface layers, but waveforms that are affected by scattering must be reconstructed for true amplitude and seismic waveform analysis.

Modelling of subsurface geological structure related to anomalous magnetotelluric phase data exceeding 90° in Central India

SUMMARY We note anomalous phase (>90°) in magnetotelluric (MT) measurements at several stations along the Narmada river course normal to the Tan Shear Zone (TSZ) in central India. We made efforts to derive a possible cause of the anomalous phase by comparing the results of numerical modelling based on the tectonics of the TSZ with the measured impedance tensor data. A zone of multiple reactivations of the TSZ leads to the formation of a damage zone and conjugate Riedel shear faults parallel and normal to the TSZ, respectively. The conjugate Riedel shear faults act as a pathway for the Narmada River course. The multiple tectonic activities associated with the TSZ induce surficial heterogeneity that results in distortion in the MT data. The Mohr circle and phase tensor analysis establish the effect of distortion in the data. To understand the cause of distortion as well as anomalous phase in the present data, we carried out synthetic 3-D modelling. We modelled the damage zone and conjugate Riedel shear faults by a cross shape. From the results, we noted that the near-surface 3-D heterogeneity across the TSZ critically distorted the YX component of the MT impedance tensor due to the strong current channelling. When the current flows across the conjugate Riedel shear zone (Narmada river course) charges are accumulated along the boundaries of near-surface heterogeneity. The charges associated with the current channelling brought out the reversion of the electric field direction that is reflected in the form of an anomalous phase in the YX component of the impedance tensor in measured data.

Numerical investigation of influence of near-surface lateral heterogeneities on propagation of Rayleigh waves

Recent research on earthquake disasters has provided evidence that Rayleigh waves play a crucial role in causing damage to both underground and surface structures during earthquakes. In this study, we utilize the finite difference method to investigate the impact of the interaction between Rayleigh waves and near-surface heterogeneities on seismic ground response. Specifically, we analyze the effects of different types of cavities (vacuum cavity, water-filled cavity, and soft-soil-filled cavity) as well as granite boulders on the propagation of Rayleigh waves. Furthermore, we examine how the buried depth and size of the heterogeneous structure, as well as the distance from the center of the structure, influence the amplitude and frequency spectrum of Rayleigh waves. Our numerical results reveal that the presence of cavities leads to larger amplitudes of Rayleigh waves compared to the free field, with the amplification effects becoming more pronounced as the cavities become shallower. Although the amplitude of Rayleigh waves generally decreases with increasing offset, the complex interaction between Rayleigh waves and the heterogeneous structure causes significantly smaller amplitudes on the far offset side of the cavity, while the near offset side exhibits significant amplification phenomena. Moreover, the characteristics of the Rayleigh-waves field vary depending on the filling material inside the cavities. For instance, the wall of a water-filled cavity clearly shows the presence of Scholte waves, whereas a soft-soil-filled cavity exhibits an energy traps phenomenon. Additionally, the conversion phenomena of Rayleigh waves generated by these two types of cavities are characterized by strong energy and smooth continuity. On the other hand, granite boulders have a relatively minor influence on Rayleigh wave propagation at the far-source side, except for partial scattering of waves. However, at the near-source side, significant amplitude amplification occurs in the vicinity of granite boulders, particularly for the high-frequency radial component. Therefore, considering the intricate interaction between Rayleigh waves and near-surface lateral heterogeneities, the findings of this study can serve as valuable guidance for seismic micro-zonation and the design of observation systems using the high-frequency surface-wave method.

Integrated structural, geophysical and remote sensing data for characterizing extensional linked fault systems and related land deformation hazards at Cairo-Suez District, Egypt

A tiered framework has been developed to map and characterize extensional linked fault systems at Cairo-Suez District, Egypt to better understand seismic and land deformation hazards. As a test site, the present tiered approach is applied on the Cairo-Suez District (CSD), Egypt, which is considered as an active seismogenic zone that encompasses complex deformation settings. Tier 1, which involves outcrop observations and structural mapping, reveals four geometries belonging to soft- and hard-linkage transfer zones (TZs). Considering the mapped TZs, the near-surface heterogeneity is characterized using a 2D-electrical resistivity tomography (2D-ERT) technique at “tier 2” applying an advanced inversion scheme (AIS). Inversion results of the 2D-ERT profiles indicate a subsurface continuity of the mapped surface geological and structural features including shale smears of fault zones and shale layer distributions. At “tier 2”, we particularly identify a highly deformed shale bed of 15-m thick, that accommodate fault slips as indicated by internal gouge zones. We further represent all local hazard factors including slope, drainage density, land use/land cover and displacement maps using the “Fuzzy-Shannon Entropy Model”. Overall, our findings show that most of the delineated deformation hazards in the test site are related to hard-linked TZs. At “tier 3”, we ultimately provide our recommendation for regional and site-specific engineering mitigation and to avoid active faults by defining setback-zone buffer (i.e., no build zone) correlated with the local fault activity and near-surface geometry. For Egypt, an economically developing country, this approach represents a major advancement from conventional geologic and geotechnical investigations through introducing a tiered framework for risk-relevant characterization of the extensional linked fault systems to apply adequate procedures/precautions by engineers for urban planning.

АНАЛИТИЧЕСКИЕ РЕШЕНИЯ ЗАДАЧ КОМПЕНСАЦИИ НЕОДНОРОДНОСТИ ВЕРХНЕЙ ЧАСТИ РАЗРЕЗА В СЕЙСМОРАЗВЕДКЕ МОВ

The solution of the problem of compensation of the near-surface heterogeneity (NSH) is an obligatory component of the digital processing technology of CDP data. The main result in the theory of solving this problem was determined by the equation of the normal traveltime curve of the common mid point with an additive component describing static corrections at the points of shotpoint and geophone. The substantiation of methods for compensation of heterogeneity based on the structure of high-speed model of the NSH is carried out in the present paper. Three variants of the NSH model corresponding to the zone of low velocities, heterogeneity of underlying bedrock deposits and variable relief of daytime surface are considered. The research subject is analytical methods for solving the problem of field continuation aimed at eliminating the influence of NSH on the kinematic parameters of wave field: method of static corrections and parametric method. Applications of methods are determined by the ratio of near-surface section velocities and underlying deposits. The requirements to the methodology of field observations and digital data processing ensuring the uniqueness of problem solution are considered for the compensation problem of heterogeneity of low-velocity zone.

3D Phase tensor inversion and Joint inversion using impedance and phase tensor components aiming better subsurface mapping and interpretation in Dhanjori basin, Singhbhum craton, eastern India

Dhanjori formation, a significant member of Singhbhum Craton, Eastern India, is well-known for its complex mineralogical character and huge content of Au-mineralization. This 2.1 Ga old Paleoproterozoic formation is composed of siliciclastic metasedimentary rocks interlayered with mafic/ultramafic volcanic rocks. Traces of gold has been spotted by many researchers in the sulfide-hosted mafic-ultramafic volcanic rocks and quartz pebble conglomerate units by different studies. Regional Audio Magnetotelluric (AMT) survey was conducted in 10 kHz–10 Hz frequency range over various lithological units of Dhanjori and adjacent Iron Ore Group. The key challenge faced during the analysis of magnetotelluric (MT) data is due to the presence of near-surface heterogeneities. The amplitude of the MT response gets distorted by galvanic effects which further create difficulties in modelling and inversion. If the effect is not removed in advance, it might lead to erroneous subsurface modelling and interpretations. On the contrary, the phase tensor data does not show any change in values even in the presence of galvanic distortion. Though phase tensors are the computed values from impedance tensor, still the response is free from any shallow level ambiguities unlike impedance data. We have performed 3-D inversion of phase tensor data to generate a conductivity model of Dhanjori basin. The objective was to verify whether the inclusion of phase tensor components in the inversion routine was able to provide some extra information that might add value to the analysis. Model from phase tensor inversion indicates only relative variation in the subsurface conductivity structure due to lack of amplitude information in the data. Hence, we also performed 3D joint inversion including both impedance and phase tensor components for obtaining the final conductivity model of Dhanjori to recover the absolute subsurface conductivity structure unaffected by galvanic distortion. Correlation of the conductivity model with available geoscientific information in Dhanjori and adjacent geological units helped us to relate the conductors to the sulfide mineralization prospect.

The effect of initial and prior models on phase tensor inversion of distorted magnetotelluric data

Magnetotelluric (MT) data are often distorted by near-surface small-scale lateral heterogeneities. Inverting distorted MT data may produce artifacts or false anomalies, leading to unreliable interpretation. This problem can be avoided by inverting the phase tensor (PT), which is known to be free of galvanic distortion. However, PT inversion is known to strongly depend on the initial or prior model, because the PT itself does not contain absolute resistivity information. To obtain a reliable inversion result from a set of PT data, a proper initial or prior model is crucial. In this study, a one-dimensional mean resistivity profile estimated from the average sum-of-squared-elements impedance was chosen as an initial model, because it was proven to be less sensitive to galvanic distortion. Examples with synthetic data showed that PT inversion using such an initial and prior model is a viable approach for inverting galvanically distorted MT data. In addition, the present paper considers a situation, where the distortion is not purely galvanic. A simple synthetic study indicated that the PT is affected by inductive distortion, and thus, such inversion results should be interpreted with caution.Graphic

Surface sum-frequency generation from chiral medium by elliptically polarized light beyond plane-wave approximation and coplanar geometry of incidence

Aiming to study the nonlinear response of the surface of isotropic chiral medium, we obtained analytical expression relating the transverse amplitudes of the spatial Fourier-spectra of two incident arbitrary polarized fundamental beams and one signal reflected beam at the sum-frequency within the first-order approximation by their divergence angles. The calculations, carried out in paraxial approximation, simultaneously take into account the spatial dispersion of the bulk of the medium, its near-surface heterogeneity and the transverse finiteness of the three interacting light beams with arbitrary amplitude profiles and orientation in space. A special compact form for the final formulas was found, which makes use of effective nonlinear transformation tensors, the components of which are solely determined by the geometry of incidence of the beams and the material constants of the medium. A possibility of ‘switching off’ the certain mechanisms of nonlinear response by choosing the specific polarization states of the incident beams is discussed.

Investigation of topographic site effects using 3D waveform modelling: amplification, polarization and torsional motions in the case study of Arquata del Tronto (Italy)

The combined effect of topography and near-surface heterogeneities on the seismic response is hardly predictable and may lead to an aggravation of the ground motion. We apply physics-based numerical simulations of 3D seismic wave propagation to highlight these effects in the case study of Arquata del Tronto, a municipality in the Apennines that includes a historical village on a hill and a hamlet on the flat terrain of an alluvial basin. The two hamlets suffered different damage during the 2016 seismic sequence in Central Italy. We analyze the linear visco-elastic seismic response for vertically incident plane waves in terms of spectral amplification, polarization and induced torsional motion within the frequency band 1–8 Hz over a 1 km2 square area, with spatial resolution 25 m. To discern the effects of topography from those of the sub-surface structure we iterate the numerical simulations for three different versions of the sub-surface model: one homogeneous, one with a surficial weathering layer and a soil basin and one with a complex internal setting. The numerical results confirm the correlation between topographic curvature and amplification and support a correlation between the induced torsional motion and the topographic slope. On the other hand we find that polarization does not necessarily imply ground motion amplification. In the frequency band above 4 Hz the topography-related effects are mainly aggravated by the presence of the weathering layer, even though they do not exceed the soil-related effects in the flat-topography basin. The geological setting below the weathering layer plays a recognizable role in the topography-related site response only for frequencies below 4 Hz.

Synchroqueezed wavelet transform based groundroll suppression

In seismic exploration, groundroll often exists polluting land seismic data. This is a form of source-generated noise, and near-surface heterogeneities often scatter this kind of noise. Groundroll suppression is always a focused problem especially in engineering geophysics. In this paper, we utilize synchrosqueezing wavelet transform which can provide a high resolution time-frequency map and has strong reconstruction ability to remove the groundroll. We build three models to illustrate how our method works in detail. Two field data tests prove the proposed method can do a good job in groundroll suppression. Compared with the conventional groundroll suppression method, our proposed method can get a much cleaner profile and the removed noise contains much less reflections.

Detection of near-surface heterogeneities at archaeological sites using seismic diffractions

Abstract The detection of shallow buried ancient structures or objects of cultural heritage is a primary challenge for seismic surveys at archaeological sites. The knowledge of the distribution of shallow objects can assist archaeologists’ study of the past without making excavations. Excavations lead to surface exposure of the buried objects and potential damages and preservation issues. The seismic response arising from localized archaeological targets is encoded in diffractions, which can be used to locate the objects. However, the energy of a diffracted wave is usually weak and masked behind the strong presence of other coherent signals or coherent noise in the data (e.g., surface waves, specular reflections). This makes it difficult to detect and interpret reliably.

High-frequency acoustic land full-waveform inversion: a case study from the Sultanate of Oman

Applications of full-waveform inversion (FWI) to land seismic data exhibit specific challenges, largely associated with elastic effects and near-surface heterogeneities. This explains why only a few land FWI case studies have been published to date (Mothi et al., 2012; Mei et al., 2014). In recent years, the acquisition design of land surveys has improved dramatically and now offers ideal conditions for FWI: dense sampling, long offsets and full azimuths (FAZ), and very low frequencies down to 1.5 Hz (Mahrooqi et al., 2012). The first published application of 3D land FWI to this new, ultra-low frequency, data set was encouraging (Stopin et al., 2014). The authors demonstrated the capability to recover the long spatial wavelength components of the velocity model, robustly and efficiently, using diving wave FWI and basic data pre-processing. It was proposed that the resulting velocity model be used as a starting model for reflection-based tomography. Limitations in obtaining a high-resolution velocity model (> 6 Hz) from FWI were identified, and attributed to the acoustic assumption and a weak signal-to-noise ratio.

Reservoir delineation beneath a heterogeneous shallow gas overburden using ‘True-3D’ seismic imaging approaches

Imaging through a heterogeneous shallow gas-charged overburden, such as a gas cloud, presents several imaging challenges and is a demanding problem to solve. Our preferred technical solution for imaging beneath gas clouds is to utilize converted wave imaging (Radzi et al., 2015), but this is not always available or cost effective and velocity model building is still difficult. Many previous case studies have been produced from Malaysia which demonstrate subsurface imaging techniques and improvements for fields affected by gas clouds, e.g., Akalin et al. (2010); El Kady et al. (2012); Abd Rahim et al. (2013); Ghazali et al. (2016) and Gudipati et al. (2018). In this paper, we describe a new comprehensive high-density experimental project to readdress these ever-challenging seismic issues by imaging the reservoir from both above and within existing boreholes. The integration of multiple technologies has significantly improved the subsurface images of the field including better-quality velocity models below gas clouds. The new data reveal a larger scale of near-surface heterogeneities than previously expected and future studies will selectively reprocess subsets of the acquired data in order to optimize the images; and, by extension to other similar fields, address a cost-effective imaging strategy.

Parametric method of compensation for near-surface heterogeneity in processing CDP data

Compensation for near-surface heterogeneity is an important part of the seismic reflection method. The problem is solved by continuing the observed wavefield to a horizontal datum. Traditionally, two reflection traveltime curves determined from survey and datum lines are related by static shift corrections. In this paper, we solve a continuation problem in which the relation between the traveltimes of the observed and continued wavefields is given by the root-mean-square velocity equation for a two-layer earth model. A comparative analysis of the two methods of compensation for surface heterogeneity due to the variable altitude of the ground surface is made.

Imaging near-surface heterogeneities by natural migration of backscattered surface waves: Field data test

We have developed a methodology for detecting the presence of near-surface heterogeneities by naturally migrating backscattered surface waves in controlled-source data. The near-surface heterogeneities must be located within a depth of approximately one-third the dominant wavelength [Formula: see text] of the strong surface-wave arrivals. This natural migration method does not require knowledge of the near-surface phase-velocity distribution because it uses the recorded data to approximate the Green’s functions for migration. Prior to migration, the backscattered data are separated from the original records, and the band-passed filtered data are migrated to give an estimate of the migration image at a depth of approximately one-third [Formula: see text]. Each band-passed data set gives a migration image at a different depth. Results with synthetic data and field data recorded over known faults validate the effectiveness of this method. Migrating the surface waves in recorded 2D and 3D data sets accurately reveals the locations of known faults. The limitation of this method is that it requires a dense array of receivers with a geophone interval less than approximately one-half [Formula: see text].

Dealing with spatial sampling sparseness and irregularity in 3D OBC seismic data offshore Abu Dhabi

Due mainly to commercial and operational constraints, seismic data are often sparsely and irregularly sampled, leading to several challenges in processing of 3D OBC seismic data offshore Abu Dhabi. Conventional linear noise attenuation techniques based on multi-channel filters are not effective with Scholte waves as they are usually aliased with typical sampling intervals in 3D OBC seismic data (e.g., 25 m source and receiver point intervals), and sometimes scattered because of near-surface heterogeneity. To address them, we apply model-based surface wave attenuation, Surface Wave Analysis Modelling and Inversion (SWAMI), which enables an estimate of local near-surface properties by analysing dispersion curves. Thus, both direct and scattered Scholte waves are effectively modelled and attenuated without suffering from under-sampling. A data interpolation and regularization technique called Matching Pursuit Fourier Interpolation (MPFI) is then applied to enhance spatial sampling. MPFI employs an anti-aliasing capability so optimum data reconstruction can be performed for any frequency range. In addition to the regularization aspect, MPFI with a 5D implementation (4 spatial coordinates and time) is targeted to densify receiver line interval and extend source lines, which consequently enhances fold, offset and azimuth distributions of the data.The implementation of the two techniques successfully addresses processing challenges in sparsely and irregularly sampled OBC seismic data.

Imaging near-surface heterogeneities by natural migration of backscattered surface waves

S U M M A R Y We present a migration method that does not require a velocity model to migrate backscattered surface waves to their projected locations on the surface. This migration method, denoted as natural migration, uses recorded Green’s functions along the surface instead of simulated Green’s functions. The key assumptions are that the scattering bodies are within the depth interrogated by the surface waves, and the Green’s functions are recorded with dense receiver sampling along the free surface. This natural migration takes into account all orders of multiples, mode conversions and non-linear effects of surface waves in the data. The natural imaging formulae are derived for both active source and ambient-noise data, and computer simulations show that natural migration can effectively image near-surface heterogeneities with typical ambient-noise sources and geophone distributions.

Innovative Processing Approaches to Overcome Sampling Sparseness and Irregularity in 3D OBC Seismic Data Offshore Abu Dhabi

Due mainly to commercial and operational constraints, seismic data are often sparsely and irregularly sampled, leading to several challenges in processing of 3D OBC seismic data offshore Abu Dhabi. Conventional linear noise attenuation techniques are not effective with Scholte waves as they are usually aliased with typical sampling interval in 3D OBC seismic data, and sometimes scattered because of near-surface heterogeneity. To address this, we apply model-based surface wave attenuation, Surface Wave Analysis Modeling and Inversion (SWAMI), which enables an estimate of local near-surface properties by analysing dispersion curves. Thus, both direct and scattered Scholte waves are effectively modelled and attenuated without suffering a lack of spatial sampling. We also highlight the shortcomings of the application of interferometry to scattered noise attenuation for sparse acquisition geometry. Matching Pursuit Fourier Interpolation (MPFI) is then implemented to deal with insufficient sampling in crossline direction caused by acquisition geometry. MPFI employs anti-aliasing capability so optimum data reconstruction can be performed for any frequency ranges. In addition to regularization aspect, MPFI with 5D implementation (4 spatial coordinates and time) is targeted to densify receiver line interval and extend source lines, which consequently enhances fold, offset and azimuth distributions of the data.