Near-surface Velocity Model Research Articles

Dispersion analysis of passive surface-wave noise generated during hydraulic-fracturing operations

Abstract Surface-wave dispersion analysis is useful for estimating near-surface shear-wave velocity models, designing receiver arrays, and suppressing surface waves. Here, we analyze whether passive seismic noise generated during hydraulic-fracturing operations can be used to extract surface-wave dispersion characteristics. Applying seismic interferometry to noise measurements, we extract surface waves by cross-correlating several minutes of passive records; this approach is distinct from previous studies that used hours or days of passive records for cross-correlation. For comparison, we also perform dispersion analysis for an active-source array that has some receivers in common with the passive array. The active and passive data show good agreement in the dispersive character of the fundamental-mode surface-waves. For the higher mode surface waves, however, active and passive data resolve the dispersive properties at different frequency ranges. To demonstrate an application of dispersion analysis, we invert the observed surface-wave dispersion characteristics to determine the near-surface, one-dimensional shear-wave velocity.

High-resolution near-surface velocity model building using full-waveform inversion—a case study from southwest Sweden

Full-waveform inversion (FWI) is an iterative optimization technique that provides high-resolution models of subsurface properties. Frequency-domain, acoustic FWI was applied to seismic data acquired over a known quick-clay landslide scar in southwest Sweden. We inverted data from three 2-D seismic profiles, 261-572 m long, two of them shot with small charges of dynamite and one with a sledgehammer. To our best knowledge this is the first published application of FWI to sledgehammer data. Both sources provided data suitable for waveform inversion, the sledgehammer data containing even wider frequency spectrum. Inversion was performed for frequency groups between 27.5 and 43.1 Hz for the explosive data and 27.5-51.0 Hz for the sledgehammer. The lowest inverted frequency was limited by the resonance frequency of the standard 28-Hz geophones used in the survey. High-velocity granitic bedrock in the area is undulated and very shallow (15-100 m below the surface), and exhibits a large P-wave velocity contrast to the overlying normally consolidated sediments. In order to mitigate the non-linearity of the inverse problem we designed a multiscale layer-stripping inversion strategy. Obtained P-wave velocity models allowed to delineate the top of the bedrock and revealed distinct layers within the overlying sediments of clays and coarse-grained materials. Models were verified in an extensive set of validating procedures and used for pre-stack depth migration, which confirmed their robustness.

New geophysical models for subsurface velocity structure in the Nechako–Chilcotin plateau from 2.5-D waveform tomography

Seismic inversion is applied to generate physical property models (P-wave velocity and numerical attenuation) for four profiles in the Nechako–Chilcotin plateau region of south-central British Columbia, Canada. A newly developed method that combines three-dimensional (3-D) travel-time inversion and 2.5-dimensional (2.5-D) viscoacoustic full-waveform inversion was applied to generate the geophysical models from vibroseis data acquired along the preexisting crooked roads. These models are useful for the characterization of rock types in terms of their positions and thicknesses, which may be used in conjunction with geological ground truth to infer the extent of lithostratigraphic units in the subsurface. The velocity structures also may be used for future reprocessing of the seismic reflection data to derive improved images based on the better near-surface velocity models. The subsurface geology of the Nechako–Chilcotin plateau region is complex, resulting from multiple stages of tectonic compression and extension, contemporaneous with the deposition of sediments and volcanic material. Several basin structures are identified from the joint interpretation of the waveform tomography velocity models and post-stack time migration images. The combination of these results enables the extrapolation and characterization of geological structures to ∼3 km depth, particularly within the Cenozoic volcanic units that dominate near-surface stratigraphy. Based on the seismic profiles, a fence-diagram geological interpretation that extends to ∼3 km depth illustrates the complex structure of the Jurassic to Neogene stratigraphic sequence.

Blind Test of Methods for Obtaining 2-D Near-Surface Seismic Velocity Models from First-Arrival Traveltimes

Seismic refraction methods are used in environmental and engineering studies to image the shallow subsurface. We present a blind test of inversion and tomographic refraction analysis methods using a synthetic first-arrival-time dataset that was made available to the community in 2010. The data are realistic in terms of the near-surface velocity model, shot-receiver geometry and the data's frequency and added noise. Fourteen estimated models were determined by ten participants using eight different inversion algorithms, with the true model unknown to the participants until it was revealed at a session at the 2011 SAGEEP meeting. The estimated models are generally consistent in terms of their large-scale features, demonstrating the robustness of refraction data inversion in general, and the eight inversion algorithms in particular. When compared to the true model, all of the estimated models contain a smooth expression of its two main features: a large offset in the bedrock and the top of a steeply dipping low-velocity fault zone. The estimated models do not contain a subtle low-velocity zone and other fine-scale features, in accord with conventional wisdom. Together, the results support confidence in the reliability and robustness of modern refraction inversion and tomographic methods.

Wave equation tomographic velocity inversion method based on the Born/Rytov approximation

This paper discusses Born/Rytov approximation tomographic velocity inversion methods constrained by the Fresnel zone. Calculations of the sensitivity kernel function and traveltime residuals are critical in tomographic velocity inversion. Based on the Born/Rytov approximation of the frequency-domain wave equation, we derive the traveltime sensitivity kernels of the wave equation on the band-limited wave field and simultaneously obtain the traveltime residuals based on the Rytov approximation. In contrast to single-ray tomography, the modified velocity inversion method improves the inversion stability. Tests of the near-surface velocity model and field data prove that the proposed method has higher accuracy and Computational efficiency than ray theory tomography and full waveform inversion methods.

Building a three-dimensional near-surface geologic and petrophysical model based on borehole data: A case study from Chémery, Paris Basin, France

The fact that velocity models based on seismic reflection surveys commonly do not consider the near-surface geology necessitates filling the gap between the top of a velocity model and the surface of the Earth. In this study, we present a new workflow to build a shallow geologic model based exclusively on borehole data and corroborated by laboratory measurements. The study area is in Chemery (France), located at the southwestern border of the Paris Basin, where a large amount of borehole data is publicly available. The workflow starts with identifying lithologic interfaces in the boreholes and interpolating them between the boreholes. The three-dimensional (3-D) geometry of the lithologies then allows interpretation of the position, orientation, and offset of fault planes. Given the importance of the fault interpretation in the modeling process, a combination of different approaches is used to obtain the most reasonable structural framework. After creating a 3-D grid, the resulting 3-D structural model is populated with upscaled velocity logs from the boreholes, yielding the final near-surface P-wave velocity model. To better constrain the velocity model, we conducted laboratory measurements of P- and S-wave velocities in dry and water-saturated conditions on all lithologies in the model. The laboratory data were used to populate the 3-D near-surface model with VP/VS ratio values. The presented workflow accounts for one-dimensional borehole data and is much more iterative and time-consuming than workflows based on two-dimensional seismic sections. Nevertheless, the workflow results in a robust 3-D near-surface model allowing for structural interpretations and revealing the 3-D seismic velocity field.

Bedrock mapping in shallow environments using surface-wave analysis

Rayleigh-wave analysis is nowadays a standard tool for retrieving near-surface S-wave velocity models. The method, usually based on the inversion of surface-wave dispersion curves adopting a 1D forward operator, is most often applied to laterally varying sites and often on long and continuous seismic lines. The processing is performed using one of many available wavefield-transform techniques and results in several local dispersion curves estimated along the survey line. The dispersion curves are inverted to provide local S-wave velocity models which are merged to reconstruct 2D/3D structures.

Estimating Elastic Constants From A Raw Seismic Reflection Data By Performing Turning Ray Tomography

A common technique in seismic inversion for estimating near surface velocity model is Turning Ray Tomography (TRT) which would employ the available 2D seismic reflection data in conjunctions with set of mathematical equations to calculate the elastic constants and interpret the lithology of the subsurface of the earth. In addition, knowing the elastic constants is not only an important issue in civil engineering, but also in mining engineering. Thus, it is necessary to describe the mathematics of the medium?s elasticity, taking into consideration homogeneous and isotropic media. In this technique, the velocity model was computed from a 2D seismic reflection data. This resulted in the reconstruction of the velocity model accurately as the seismic waves travel. TRT result showed only a P-wave velocity model. However, this is insufficient to calculate the elastic constants and predict the lithology, which also require an S-wave velocity and a density. The S-wave velocity was calculated by using Castagna?s equation, while the density was computed by applying Gardner?s equation. Furthermore, the remaining elastic constants were calculated by using their relationships with P-wave velocity, S-wave velocity and the density. Consequently, the results showed that the near-surface velocity model derived from TRT was capable of computing the elastic constants.

Near-surface velocity model construction based on a Monte-Carlo scheme

In tomographic statics seismic data processing, it is crucial to determine an optimum base for a near-surface model. In this paper, we consider near-surface model base determination as a global optimum problem. Given information from uphole shooting and the first-arrival times from a surface seismic survey, we present a near-surface velocity model construction method based on a Monte-Carlo sampling scheme using a layered equivalent medium assumption. Compared with traditional least-squares first-arrival tomography, this scheme can delineate a clearer, weathering-layer base, resulting in a better implementation of datuming correction. Examples using synthetic and field data are used to demonstrate the effectiveness of the proposed scheme.

Migration from 3D irregular surfaces: A prestack time migration approach

We have developed a modified 3D prestack time migration (PSTM) scheme that can handle rugged topography as well as high near-surface velocities in land seismic imaging. The proposed topography PSTM can be applied to seismic data recorded on a 3D irregular surface without static corrections. Two effective velocity parameters were found to describe wave propagation through inhomogeneous media above and below a chosen datum. As a result, wave propagation phenomena in the complex near surface, such as near-vertical incidences through a weathering layer and raypaths bending away from vertical in the presence of high near-surface velocities, are correctly considered. The two effective velocity parameters can be estimated by flattening events in imaging gathers. Hence, it is not necessary to have detailed knowledge of the near-surface velocity model and velocity field below the datum when applying topography PSTM. We integrated residual static corrections into topography PSTM. This eliminated the distortions along the events better than conventional residual static corrections, which are usually applied before migration. The computational cost of the topography PSTM was only slightly higher than that of conventional PSTM due to the use of a table-driven algorithm. Three-dimensional synthetic and field data sets were used to test the proposed topography PSTM. High-quality imaging results were obtained.

Tutorial: Incorporating near-surface velocity anomalies in pre-stack depth migration models

Unresolved velocity anomalies in the near surface degrade deeper imaging. As a consequence, great care needs to be taken to ensure that all significant near-surface effects have been dealt with before attempting to build the deeper parts of a velocity–depth model. In order to incorporate velocity anomalies into the model, a range of options can be used, depending on whether the geobody geometry alone is discernible, or whether its velocity distribution is also known. Here I describe current industrial practice for building complex near-surface models, which is based on a range of approximate techniques, as well as the more complete solution offered by the emerging technology of waveform inversion. Although building complex nearsurface models is a painstaking process, a suitable near-surface velocity model can usually be obtained.

Scattering effect on shallow gas-obscured zone imaging in Bohai PL19-3 area

Seismic imaging is challenging in the Bohai Bay PL19-3 area, offshore China. Bohai Bay Field is seismically obscured, making well penetrations the only reliable source of data for subsurface interpretation. With the help of turning-ray tomography, we are able to obtain a reliable near-surface velocity model approximately down to 700 m below sea level, using the first arrivals picked from streamer data. Depth migration using velocities estimated from turning-ray tomography has improved shallow structures and fault definition. However, reservoir level structures from 800 to 1500 m are still poorly imaged. A viscoelastic modeling study with assigned variable Q and shallow velocity profiles, with and without shallow gas-induced scatterers, demonstrates that scattering is the primary controlling phenomenon causing imaging difficulty within the obscured zone. Due to scattering, imaging tests at the target level were unsuccessful even with precise knowledge of velocity.

Waveform tomography of field vibroseis data using an approximate 2D geometry leads to improved velocity models

Waveform tomography, a combination of traveltime tomography (or inversion) and waveform inversion, is applied to vibroseis first-arrival data to generate an interpretable model of P-wave velocity for a site in the Nechako Basin, south-central British Columbia, Canada. We use constrained 3D traveltime inversion followed by 2D full-waveform inversion to process long-offset (14.4 km) first-arrival refraction waveforms, resulting in a velocity model of significantly higher detail than a conventional refraction-statics model generated for a processing workflow. The crooked-line acquisition of the data set makes 2D full-waveform inversion difficult. Thus, a procedure that improves the tractability of waveform tomography processing of vibroseis data recorded on crooked roads is developed to generate a near-surface ([Formula: see text]) velocity model for the study area. The data waveforms are first static corrected using a time shift determined by 3D raytracing, which accounts for the crossline offsets produced by the crooked-line acquisition. The velocity model generated from waveform tomography exhibits substantial improvement when compared with a conventional refraction-statics model. It also shows improved resolution of sharp discontinuities and low-velocity regions when compared to the model from traveltime tomography alone, especially in regions where the geometry errors are moderate. Interpretation of the near-surface velocity model indicates possible subbasins in the Nechako Basin and delineates the Eocene volcanic rocks of the study area. This approach limits the ability of the full-waveform inversion to fit some propagation modes; however, the tractability of the inversion in the near-surface region is improved. This new development is especially useful in studies that do not warrant 3D seismic acquisition and processing.

Application of super-virtual seismic refraction interferometry to enhance first arrivals: A case study from Saudi Arabia

Complex near-surface anomalies are one of the main onshore challenges facing seismic data processors. Refraction tomography is becoming a common technology to estimate an accurate near-surface velocity model. This process involves picking the first arrivals of refracted waves. One of the main challenges with refraction tomography is the low signal-to-noise ratio characterizing the first-break waveform arrivals, especially for the far-offset receivers. This is especially evident in data recorded using reflection acquisition geometry. This low signal-to-noise ratio is caused by signal attenuation due to geometrical spreading of the seismic wavefield, near-surface-generated noise, and amplitude absorption. Super-virtual refraction interferometry improves the quality of the first-break picks by enhancing the amplitude of the refracted waves and attenuating the amplitude of the random noise.

Estimation of the near-surface velocity structure of the Yasur-Yenkahe volcanic complex, Vanuatu

Small-aperture array measurements of seismic noise at seven sites around Yasur volcano (Vanuatu) are performed to estimate the VP and VS velocities of the shallow structure. The spatial autocorrelation (SPAC) and the frequency–wavenumber (f–k) methods are used to determine Rayleigh phase velocity dispersion curves. Phase velocities computed with the SPAC method vary between 580m/s and 960m/s at 1Hz and between 270m/s and 420m/s at 15Hz. F–k dispersion curves show velocities of 300–340m/s and 800–940m/s at 1Hz and 200–230m/s at 15Hz. An inversion technique based on the use of the neighbourhood algorithm has been used to calculate the one-dimensional velocity model at each site. Velocity models reach 200m deep and mainly contain two layers and a half-space. For sites close to the Siwi caldera rims, comparisons with geology and hydrothermal system studies suggest that the two layers highlighted in models may correspond to two large pyroclastic sequences related to caldera collapses based on the flank of an old volcano. Results obtained for the other three sites, located inside the caldera, show the influence of the hydrothermal system on P- and S-wave velocities. For these sites, fluid circulation inside the volcanic deposits causes lower velocities at depth. To obtain a near-surface velocity model of the volcanic structure, each 1D velocity model is spatially extrapolated according to the surface geology. Results highlight four distinct areas, the Siwi caldera edges with high velocities and the resurgent block, the ash plain and the Yasur edifice with lower velocities at depth.

综合地球物理近地表调查技术在复杂山地区地震资料采集中的应用分析<br>Synthetical Geophysical Near-Surface Survey Method Application and Analysis in Complex Mountain Seismic Prospecting

本文针对南方地区复杂近地表条件的地质、地球物理特点，进行综合地球物理近地表结构调查方法组合试验，通过多种地球物理技术手段联合应用分析，总结了一套有针对性的南方复杂山地区表层结构调查方法技术组合。通过多种方法的组合使用和资料的联合解释，建立较准确的近地表速度及岩性结构模型，为地震资料采集的激发、接收因素选择提供依据，也为静校正处理提供可靠的资料，在南盘江和凯理地区的实际应用中较好的指导了井深及井位设计，对提高地震资料信噪比和分辨率取得了较好的效果。 In connection with the southern region of complex near surface condition of geological and geo- physical characteristics, we made comprehensive geophysical near surface structure survey combination ex- perimental research, and summarized a set of targeted of complex surface structure survey method technology in the south area through many geophysical methods. Through combined methods and synthetical explain, to establish a more accurate near-surface velocity and lithology structure model, provide seismic data acquisition excitation and receiving factors, and provide reliable data for static correction processing. In Nanpanjiang and KaiLi regions seismic prospecting, it gives better guidance to the design for well depth and location, and has obtained a better effect to improve signal-to-noise ratio and resolution of Seismic data.

Tomographic velocity model building of the near surface with velocity-inversion interfaces: A test using the Yilmaz model

First-arrival traveltime tomography is a popular approach to building the near-surface velocity models for oil and gas exploration, mining, geoengineering, and environmental studies. However, the presence of velocity-inversion interfaces (VIIs), across which the overlying velocity is higher than the underlying velocity, might corrupt the tomographic solutions. This is because most first-arrival raypaths will not traverse along any VII, such as the top of a low-velocity zone. We have examined the impact of VIIs on first-arrival tomographic velocity model building of the near surface using a synthetic near-surface velocity model. This examination confirms the severe impact of VIIs on first-arrival tomography. When the source-to-receiver offset is greater than the lateral extent of the VIIs, good near-surface velocity models can still be established using a multiscale deformable-layer tomography (DLT), which uses a layer-based model parameterization and a multiscale scheme as regularization. Compared with the results from a commercial grid-based tomography, the DLT delivers much better near-surface statics solutions and less error in the images of deep reflectors.

Surface-wave analysis for building near-surface velocity models — Established approaches and new perspectives

Today, surface-wave analysis is widely adopted for building near-surface S-wave velocity models. The surface-wave method is under continuous and rapid evolution, also thanks to the lively scientific debate among different disciplines, and interest in the technique has increased significantly during the last decade. A comprehensive review of the literature in the main scientific journals provides historical perspective, methodological issues, applications, and most-promising recent approaches. Higher modes in the inversion and retrieval of lateral variations are dealt with in great detail, and the current scientific debate on these topics is reported. A best-practices guideline is also outlined.

Recent applications of turning-ray tomography

Recent applications of 2D and 3D turning-ray tomography show that near-surface velocities are important for structural imaging and reservoir characterization. For structural imaging, we used turning-ray tomography to estimate the near-surface velocities for static corrections followed by prestack time migration and the near-surface velocities for prestack depth migration. Two-dimensional acoustic finite-difference modeling illustrates that wave-equation prestack depth migration is very sensitive to the near-surface velocities. Field data demonstrate that turning-ray tomography followed by prestack time migration helps to produce superior images in complex geologic settings. When the near-surface velocity model is integrated into a background velocity model for prestack depth migration, we find that wave propagation is very sensitive to the velocities immediately below the topography. For shallow-reservoir characterization, we developed and applied azimuthal turning-ray tomography to investigate observed apparent azimuthal-traveltime variations, using a wide-azimuth land seismic survey from a heavy-oil field at Surmont, Canada. We found that the apparent azimuthal velocity variations are not necessarily related to azimuthal anisotropy, or horizontal transverse isotropy (HTI), induced by the stress field or fractures. Near-surface heterogeneity and the acquisition footprint also could result in apparent azimuthal variations.

Use of refraction, reflection, and wave-equation-based tomography for imaging beneath shallow gas: A Trinidad field data example

Shallow localized gas pockets cause challenging problems in seismic imaging because of sags and wipe-out zones they produce on imaged reflectors deep in the section. In addition, the presence of shallow gas generates strong surface-related and interbed multiples, making velocity updating very difficult. When localized gas pockets are very shallow, we have limited information to build a near-surface velocity model using ray-based reflection tomography alone. Diving-wave refraction tomography successfully builds a starting model for the very shallow part. Usual ray-based reflection tomography using single-parameter hyperbolic moveout might need many iterations to update the deeper part of the velocity model. In addition, the method generates a low-velocity anomaly in the deeper part of the model. We present an innovative method for building the final velocity model by combining refraction, reflection, and wave-equation-based tomography. Wave-equation-based tomography effectively generates a detailed update of a shallow velocity field, resolving the gas-sag problem. When applied as the last step, following the refraction and reflection tomography, it resolves the gas-sag problem but fails to remove the low-velocity anomaly generated by the reflection tomography in the deeper part of the model. To improve the methodology, we update the shallow velocity field using refraction tomography followed by wave-equation tomography before updating the deeper part of the model. This step avoids generating the low-velocity anomaly. Refraction and wave-equation-based tomography followed by reflection tomography generates a simpler velocity model, giving better focusing in the deeper part of the image. We illustrate how the methodology successfully improves resolution of gas anomalies and significantly reduces gas sag from an imaged section in the Greater Cassia area, Trinidad.