First-arrival Tomography Research Articles

Application of first-arrival tomography to characterize a quick clay landslide site in Southwest Sweden

First-arrival traveltime tomography was applied to high-resolution seismic data acquired over a known quick-clay landslide scar near the Gota River in southwest Sweden in order to reveal the geometry and physical properties of clay-related normally consolidated sediments. Investigated area proved to be a challenging environment for tomographic imaging because of large P-wave velocity variations, ranging from 500 to 6000 m/s, and relatively steeply-dipping bedrock. Despite these challenges, P-wave velocity models were obtained down to ca. 150 m for two key 2D seismic profiles (each about 500-m long) intersecting over the landslide scar. The models portrait the sandwich-like structure of marine clays and coarse-grained consolidated sediments, but the estimated resolution (20 m) is too small to distinguish thin layers within this structure. Modelled velocity structures match well the results of reflection seismic processing and resistivity tomography available along the same profiles.

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.

Depth-Recursive Tomography of the Bohemian Massif at the CEL09 Transect—Part A: Resolution Estimates and Deblurring Aspects

The refraction CEL09 profile from the CELEBRATION 2000 project intersects the main terranes of the Bohemian Massif in the NW–SE direction: the Saxothuringian, the Tepla-Barrandian, the Moldanubian and the Moravo-Silesian. In its easternmost part, it crosses the Western Outer Carpathians overthrust westward onto the Bohemian Massif. Only the first 450 km were surveyed with the densest deployment of shot points providing data suitable for a reliable geological interpretation. The first-arrival depth-recursive tomography was applied here to derive a P-wave velocity image of the upper and middle crust (Part A). The proper interpretation of the obtained velocity features is the subject of the accompanying paper (Part B). The attained resolution in the velocity image is shown to be superior as compared with the previous CEL09 models based also on the more uncertain later arrivals of reflection waves. The applied DRTG (depth-recursive tomography on grid) method is based on a regular network of refraction grid rays generated for iteratively updated starting models. Only the distinct first arrivals with minimum uncertainty are used for the DRTG inversions to yield the maximum resolution. Thanks to the full control of the data fit by the grid rays used, the statistical lateral resolution could be determined at single grid depths for the chosen confidence levels. Thus, the lateral sizes of the anomalies that can be yet resolved are determined in dependence on their depths and their velocity excesses. The defocusing of the imaged features is studied on the basis of the spatial responses to spike excitation. The calculated spatial responses also allowed the edge smearing of the velocity anomalies to be assessed. Special attention is paid to the imaging of low-velocity zones that are usually suppressed by the smoothing measures used in standard tomographic methods. An improvement was achieved if the smoothing was suggested with regard to the occurrence of the low-velocity zones repeatedly appearing in higher iterations. The gained deblurring effect concerns both the negative and positive anomalies as documented on the velocity features interpreted in the accompanying paper.

Crustal velocity structure of the southern Nechako basin, British Columbia, from wide-angle seismic traveltime inversion1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

In the BATHOLITHSonland seismic project, a refraction – wide-angle reflection survey was shot in 2009 across the Coast Mountains and Interior Plateau of central British Columbia. Part of the seismic profile crossed the Nechako Basin, a Jurassic–Cretaceous basin with potential for hydrocarbons within sedimentary strata that underlies widespread volcanic rocks. Along this 205 km-long line segment, eight large explosive shots were fired into 980 seismometers. Forward and inverse modelling of the traveltime data were conducted with two independent methods: ray-tracing based modelling of first and secondary arrivals, and a higher resolution wavefront-based first-arrival seismic tomography. Material with velocities less than 5.0 km/s is interpreted as sedimentary rocks of the Nechako Basin, while velocities from 5.0–6.0 km/s may correspond to interlayered sedimentary and volcanic rocks. The greatest thickness of sedimentary rocks in the basin is found in the central 110 km of the profile. Two sub-basins were identified in this region, with widths of 20–50 km and maximum sedimentary depths of 2.5 and 3.3 km. Such features are well-defined in the velocity model, since resolution tests indicate that features with widths greater than ∼13 km are reliable. Beneath the sedimentary rocks, seismic velocities increase more slowly with depth — from 6.0 km/s just below the basin to 6.3 km/s at ∼17 km in depth, and then to 6.8–7.0 km/s at the base of the crust. The Moho is found at a depth of 33.5–35 km beneath the profile, and mantle velocities are high at 8.05–8.10 km/s.

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.

Crosswell monitoring using virtual sources and horizontal wells

Time-lapse crosswell seismic provides an efficient way to monitor the migration of a [Formula: see text] plume or its leakage after [Formula: see text] injection into a geologic formation. Recently, crosswell seismic has become a powerful tool for monitoring underground variations, using the concept of a virtual source, with virtual sources positioned at the receivers installed in the well and thus the positions of sources and receivers can be invariant during monitoring. However, time-lapse crosswell seismic using vertical wells and virtual sources has difficulty in describing the front of a [Formula: see text] plume, which usually is parallel to the vertical wells, and in obtaining sufficient ray coverage for the first-arrival tomography. These problems arise because of the theoretical downward-illumination-directivity limitation of the virtual source. We have developed an effective monitoring method that uses virtual sources and two horizontal wells: one above and one below the [Formula: see text]sequestration reservoir. In our method, we redatum the traces that are recorded at geophones in horizontal wells from sources on the surface. The redatumed traces then become virtual traces recorded at geophones in the lower well and sent from virtual sources at the positions of the geophones in the upper well. The geometry of our method has advantages for locating the front of the [Formula: see text] plume, which is normal to the horizontal wells, compared with either real or virtual sources. The method also is advantageous in acquiring full ray coverage between the wells, and that coverage is superior to coverage acquired using vertical crosswell seismic with virtual sources. In addition, we can avoid problems related to any potential change in the medium above the reservoir and in the source and receiver positions. The results of applying our method to synthetic data that simulate [Formula: see text]-sequestration monitoring show that the front of a [Formula: see text] plume in the reservoir is depicted accurately in a velocity tomogram. The new method also can be used to monitor a reservoir during production of heavy oil.

Constrained deformable layer tomostatics

Although first-arrival tomography provides an effective way to estimate near-surface velocities and static corrections, the undulation of velocity interfaces such as the base of the weathered zone may not be easily determined by this method. The main reason is that first arrivals are insensitive to small geometric changes in velocity interfaces because their raypaths tend to traverse along those interfaces. To improve the solution of interface geometry, we developed a deformable layer tomostatics method that approximates the near-surface velocity field as several layers of constant velocity and variable thickness that can be inverted for the geometry of the velocity interfaces. We use a multiscale model parameterization in the inversion for interface geometry. Synthetic and field data tests showed that the method can determine the interface geometry. Constraining the depth range of the basal boundary of the weathered zone increases the convergence rate of the iterative inversion process. Tests on field data showed greater reflection coherency in a stacked section based on constrained static corrections than in one from unconstrained static corrections. The method yielded a better match with statics computed from sand-dune curves than does a match obtained by using two commercial grid tomography packages.

Eocene and Neogene volcanic rocks in the southeastern Nechako Basin, British Columbia: interpretation of the Canadian Hunter seismic reflection surveys using first-arrival tomography

Geological investigation of the near-surface in the southeastern Nechako Basin is difficult. Shallow seismic reflection imaging is poor due in part to an extensive cover of Eocene and Neogene volcanic rocks. Outcrops of these volcanic rocks, and the primarily Cretaceous bedrock, are commonly obscured by Quaternary deposits and vegetation. Estimates of near-surface P-wave velocity are derived from the tomographic inversion of seismic first-arrivals, an effective tool when seismic imaging is poor. Tomographic model velocities are in agreement with sonic logs and laboratory samples, except for those from the Neogene Chilcotin Group. Cretaceous sedimentary rocks have velocities of ∼2800–4200 ms–1. The Eocene Endako and Ootsa Lake groups, which have velocities of ∼3000–4200 ms–1, are not distinguishable based on velocity. The velocity, the character (density, focus, and penetration depth) of rays, and ties with well and surface geology constrain the subsurface extent of the Endako Group adjacent to well b-82-C. The Chilcotin Group typically exhibits velocities (∼2400–3000 ms–1) lower than corresponding velocities from sonic logs (4500–5200 ms–1) and laboratory measurements (5000–5200 ms–1). These low model velocities may be due to the presence of high porosity, brecciated rocks near to the surface, in comparison with the other measurements that have focussed on lower porosity massive lavas. The lowest mean velocities, located to the southeast, are related to anomalously thick, high porosity, breccia-rich deposits of Chilcotin Group. This conclusion is consistent with the interpretation that the Chilcotin Group is thicker in paleo river valleys.

Seismic first-arrival tomography with functional description of traveltimes

A two-dimensional smoothness-constrained least-squares inversion scheme was applied to seismic first-arrival time data. The inversion scheme was based on the functional description of traveltimes; thus, it did not require a step of ray tracing, and traveltimes were obtained by a finite-difference eikonal solver. The Laplacian difference of cell slownesses was used for the smoothness constraint. Model velocities were obtained by matrix inversion including the QR decomposition and iterative LSQR method. The Jacobian matrix of the partial derivatives was constructed by a finite-difference approximation based on the perturbation of the cell slowness. Since the construction of the Jacobian matrix was the most time-consuming step of the inversion scheme, Broyden's update was used for this matrix, and it was replaced by its numerical approximation obtained from Broyden's method after the third iteration and for all subsequent iterations to expedite the inversion process. This significantly improved the computational performance of the scheme by reducing the computer time for the calculation of the Jacobian matrix. The algorithm was tested by using a number of synthetic and field data sets. The test studies included both surface seismic refraction and crosshole seismic data acquisition configurations. Also image appraisal analyses were performed for the solutions obtained from both surface and crosshole field data sets by calculating model covariance and model resolution matrices. The presented algorithm yielded satisfactory results during the test studies. The stability and fast convergence rate were the main characteristics of the algorithm.

Testing robust inversion strategies for three-dimensional Moho topography based on CELEBRATION 2000 data

SUMMARY In this paper, we present results of the 3-D tomographic modelling of the crustal structure and Moho topography applied to data recorded in SE Poland during the CELEBRATION 2000 seismic experiment. The target area covers ca. 500 km × 500 km and represents a complex geological setting from old Precambrian platform (East European Craton, EEC), through the crustal blocks (terranes) that form the Trans-European Suture Zone, to the young Alpine orogen—the Carpathians. We test two different inversion strategies using two different algorithms: (i) coupled inversion of Pg and PmP arrivals to constrain both the crustal velocities and the Moho depths; (ii) decoupled inversion of PmP arrivals only using previously obtained smooth 3-D crustal velocity model. The coupled inversion of 11 700 Pg and 3100 PmP arrivals results in a much smoother crustal velocity field than the one previously obtained by inversion of first arrivals only. Also, the obtained Moho structure is much smoother than the Moho map compiled from the existing 2-D models. Decoupled inversion of the PmP reflections provides Moho structure comparable in resolution to the compiled map. Synthetic tests indicate that with our data set we are able to resolve larger than 100-km-size Moho structures. The modelled Moho is shallowest in the area of the Upper Silesian Block (ca. 32 km), then it is deepening by ca. 10 km over 100-km-wide zone along the margin of the EEC and finally it reaches up to 48 km depth in the area of the EEC. Our favoured strategy for modelling large refraction/wide-angle reflection data set consists of derivation of a smooth crustal velocity model by a first-arrival tomography and then complementing this model by a decoupled inversion of PmP reflections in order to constrain minimum-structure Moho topography.

Three-dimensional seismic modelling of the crustal structure between East European Craton and the Carpathians in SE Poland based on CELEBRATION 2000 data

SUMMARY In this paper, we present results of 3-D first-arrival tomography applied to data recorded in SE Poland during the CELEBRATION 2000 seismic experiment. The target area covers ca. 500 × 500 km 2 and represents a complex geological setting from old Precambrian platform (East European Craton, EEC), through the crustal blocks (terranes) that form the TransEuropean Suture Zone (TESZ) to the young Alpine orogen—the Carpathians. Contrasting velocity distributions in various geological units makes the tomographic inversion challenging. For this reason, much attention was paid to the inversion methodology. We tested ‘multioffset’ and ‘multiscale’ approaches. By increasing the offset range in the ‘multioffset’ inversion we have independently constrained different depth ranges of our model. We have found that the ‘multiscale’ method, that is, the gradual stepping from bigger to smaller model cells produced the preferred solution. Resolution of the resultant crustal model was determined by performing checkerboard and restoration resolution tests. Lateral resolution of the order of 25 km is inferred down to 10 km depth and 50 km down to 25‐30 km depth. We have also applied a quasi-Monte Carlo analysis to determine the absolute errors of model parameters, which to our knowledge, is the first such attempt in case of a 3-D wide-angle data set. The mean uncertainties of our tomographic velocities are 0.1 km s −1 , but some anomalies are recovered with