Arrival Traveltime Data Research Articles

Binary structure constrained gravity inversion based on seismic first arrival travel time data

Gravity exploration method is one of the important methods for deep mineral resource exploration, but gravity data inversion has limited resolution ability in the vertical direction. In order to improve the vertical resolution of gravity data inversion, we propose a binary structure constrained gravity inversion method based on seismic first arrival travel time data. This method effectively reconstructs a density model with high vertical resolution by transferring the structural information of a high-resolution velocity model reconstructed by seismic data inversion to gravity data inversion through the binary structure constrained technique. This strategy eliminates the need to integrate both gravity and seismic methods into a single inversion framework, avoiding both the difference in convergence speeds between the two methods, as well as getting rid of the complexity associated with calculating structural coupling terms. Theoretical simulations show that the fuzzy c-means cluster analysis technique can accurately extract the target region of the velocity model reconstructed by seismic data inversion. Under the constraint of seismic structural information, the resolution of reconstructed density model is much higher than that of separate gravity data inversion, which proves that high resolution seismic information can improve the vertical resolution of gravity data inversion. Compared with the traditional cross-gradient joint inversion, the binary structure constrained gravity inversion method can further improve the resolution of the density model, especially in the reconstruction of the anomaly interface, which verifies that the method has certain effectiveness.

Seismic Travel Time Tomography in Modeling Low Velocity Anomalies between the Boreholes

Travel time cross-hole seismic tomography is applied to describing the structure of the subsurface. The sources are placed at one borehole and some receivers are placed in the others. First arrival travel time data that received by each receiver is used as the input data in seismic tomography method. This research is devided into three steps. The first step is reconstructing the synthetic model based on field parameters. Field parameters are divided into 24 receivers and 45 receivers. The second step is applying inversion process for the field data that consists of five pairs bore holes. The last step is testing quality of tomogram with resolution test. Data processing using FAST software produces an explicit shape and resemble the initial model reconstruction of synthetic model with 45 receivers. The tomography processing in field data indicates cavities in several place between the bore holes. Cavities are identified on BH2A-BH1, BH4A-BH2A and BH4A-BH5 with elongated and rounded structure. In resolution tests using a checker-board, anomalies still can be identified up to 2 meter x 2 meter size. Travel time cross-hole seismic tomography analysis proves this mothod is very good to describing subsurface structure and boundary layer. Size and anomalies position can be recognized and interpreted easily.

Modelling and inversion of single-ended refraction data from the shot gathers of multifold deep seismic reflection profiling-an approach for deriving the shallow velocity structure

SUMMARY A multifold crustal-scale deep seismic near-vertical reflection profile generates a large number of single-ended shot gathers, which provide redundant data sets because of overlapping coverage of the shallow refractors. We present an approach for deriving the shallow velocity structure by modelling and inversion of single-ended seismic refraction first arrival traveltime data. We apply this method to a data set acquired with a 12-km long spread with 100 m spacing of shots and receivers, of the Neoproterozoic Marwar basin in the NW Indian shield. The approach is shown to be quite successful for delineating the shallow refractor depths, steep dips and velocities, even in the absence of regular reverse refraction profiles. The study reveals two-layered sedimentary formations, Malani volcanics and a complicated basement configuration of the Marwar basin, and provides a measure of resolution and uncertainty of the estimated model parameters. A seismic section of the near-trace gather is found to be qualitatively consistent with the derived structural features of the basin. The relative highs and lows, observed in the Bouguer gravity profile, further corroborate the derived velocity model. The present approach can be especially useful in offshore areas and elsewhere, where the single-ended multifold seismic profiles are the only available data sets.

2.5D seismic velocity modelling in the south-eastern Romanian Carpathians Orogen and its foreland

The DACIA-PLAN (Danube and Carpathian Integrated Action on Processes in the Lithosphere and Neotectonics) deep seismic reflection survey was performed in August–September 2001, with the objective of obtaining new information on the deep structure of the external Carpathians nappe system and the architecture of the Tertiary/Quaternary basins developed within and adjacent to the Vrancea zone, including the rapidly subsiding Focsani Basin. The DACIA-PLAN profile is about 140 km long, having a roughly WNW–ESE direction, from near the southeast Transylvanian Basin, across the mountainous south-eastern Carpathians and their foreland to near the Danube River. A high resolution 2.5D velocity model of the upper crust along the seismic profile has been determined from a tomographic inversion of the DACIA-PLAN first arrival data. The results show that the data fairly accurately resolve the transition from sediment to crystalline basement beneath the Focsani Basin, where industry seismic data are available for correlation, at depths up to about 10 km. Beneath the external Carpathians nappes, apparent basement (material with velocities above 5.8 km/s) lies at depths as shallow as 3–4 km, which is less than previously surmised on the basis of geological observations. The first arrival travel-time data suggest that there is significant lateral structural heterogeneity on the apparent basement surface in this area, suggesting that the high velocity material may be involved in Carpathian thrusting.

跡津川断層周辺における地殻構造と地震活動

The Atotsugawa fault system, which is located in the northern region of Central Honshu, Japan, is composed of the Amidagawara, Atotsugawa, Mannami, Mozumi-Sukenobe, and Ushikubi faults. The Atotsugawa fault in the central part is creeping where seismicity is low. In contrast, the edges of the creeping zone are locked showing relatively high seismicity. In order to study the relationship between the distribution of earthquakes and the fault structure, we analyzed seismic survey data along and across the fault system, and relocated hypocenters using a 1-D velocity structure determined in this study. P-wave velocity structures around the Atotsugawa fault system are determined by comparing first arrival travel time data from explosion surveys with the travel times from forward calculations by a ray tracing method. On the basis of the P-wave velocity structures, we have found that the crust around the Atotsugawa fault system consists of three layers, an upper crust which includes a surface layer, a middle crust and a lower crust. Furthermore, two distinct reflectors are located at depths of about 11km and 20km below the Atotsugawa fault system. The depth of the shallower reflector is close to that of low resistivity layer. Comparing the relocated hypocenters with the depths of the reflectors, the shallower reflector is roughly coincident with the base of the seismogenic layer and the second reflector is several kilometers deeper. In addition, seismicity is concentrated in the upper crust (with velocities of 5.9-6.2km/s) and only a few earthquakes occur at the bottom of the middle crust. The difference in seismic structures between the creeping zone and the other regions is not clear. This is probably because the creeping zone does not have a significantly different velocity structure that is detectable from these data.

BGPR_Reconstruct: A MATLAB ® ray-tracing program for nonlinear inversion of first arrival travel time data from zero-offset borehole radar

A MATLAB program was developed to invert first arrival travel time picks from zero offset profiling borehole ground penetrating radar traces to obtain the electromagnetic wave propagation velocities in soil. Zero-offset profiling refers to a mode of operation wherein the centers of the bistatic antennae being lowered to the same depth below ground for each measurement. The inversion uses a simulated annealing optimization routine, whereby the model attempts to reduce the root mean square error between the measured and modeled travel time by perturbing the velocity in a ray tracing routine. Measurement uncertainty is incorporated through the presentation of the ensemble mean and standard deviation from the results of a Monte Carlo simulation. The program features a pre-processor to modify or delete travel time information from the profile before inversion and post-processing through presentation of the ensemble statistics of the water contents inferred from the velocity profile. The program includes a novel application of a graphical user interface to animate the velocity fitting routine.

Physical modelling of multi-layered transversely isotropic media

Physical modelling facilities and anisotropic inversiontechniques are applied in this study to recover the average and interval anisotropy parameters, and average P-wave velocity field information, in layered transversely isotropic media. The elastic parameters recovered are the vertical P-wave velocity (α0), the vertical S-wave velocity (β0), the P-wave anisotropy (ε) and the near-vertical P-wave anisotropy (δ). The experimental models consist of isotropic and anisotropic simulating materials with vertical axes of symmetry. Inversion codes developed by Li et al. (1998) are used to invert traveltime data to recover the average anisotropy parameters ε and δ. The physical modelling transmission experiments simulate walkaway VSP field surveys. The first arrival traveltime data, recorded at different ray angles, are required to determine the average vertical P and S wave velocities α0 and α0 and are also used as input to the inversion program that recovers the average elastic parameters. The interval parameters for each particular layer are recovered by inversion codes that utilise the average elastic parameters of the multilayered models.Physical modelling transmission data obtained from experiments indicate that the effect of the transducer size must be considered. Variations in the interval anisotropy parameters ε and δ recovered from the two-layer and three-layer models indicate that corrections must be applied to compensate for the large size of the piezoelectric transducers. The accuracy of the inversion results was observed to decrease as the ratio of the transducer width to the model thickness increased. When the ratio of the size of the transducers to the thickness of the model is too large, the effects of transducer size must be taken into account.Point-sized transducers were successfully used to overcome the transducer size effects, and hence obtain results that are more accurate. The accurate determination of these parameters in multilayered media enables the computation of the correct velocity field. Successful recovery of elastic parameters in the physical modelling laboratory may lead to direct application of this analysis to walkaway VSP surveys in the field. The use of correct velocity functions will result in improved seismic data processing and interpretation.

A comparative study of 2D and 3D crosswell tomography

Crosswell tomography can generate high-resolution velocity images that can be used directly in interpretation or as inputs to migration processing. Two-dimensional tomography is computationally efficient and has been widely used to invert crosswell data. Real geological structures, however, are usually 3D, and it is important to assess 3D effects on first arrival traveltimes and account for these effects in traveltime tomography. In this paper, we investigate how 3D structures affect first arrival traveltime data in crosswell surveys, and present a 3D tomography algorithm that minimizes a model objective function consisting of the second order directional derivatives of the model parameters. Our 3D forward modelling results show that 3D effects can become significant in a 2D crosswell survey when the 3D structure is close to the 2D survey plane. Furthermore, the 3D effects in the data can be propagated to the final inversion results. Three dimensional tomography counts for off-plane ray-paths and hence can produce better velocity images over 2D inversion in 3D environments. Because of the restriction from the geometry, 3D tomography of crosswell data from 2D surveys cannot provide any lateral resolution.

A study of the effects of transducer size on physical modeling experiments for recovering anisotropic elastic parameters

The effect of finite size transducers on elastic parameters recovered from laboratory measurements has been studied in this paper. Ultrasonic transmission experiments in the physical modeling laboratory were used to acquire the first arrival traveltime data at different ray angles, to simulate walkaway VSP surveys in the field. The elastic parameters of the materials were then recovered by our developed inversion techniques, using velocities at different directions inverted from the first arrival traveltimes. In our laboratory experiments, “point” transducers and “large” transducers were used to carry out the transmission experiments. The finite size of the transducers introduced systematic errors in the first break picks along the survey line, and hence the determined velocity field was inaccurate. These systematic errors cannot be corrected by the inversion technique. Numerical simulations were also conducted to study the differences between experiments using large and point transducers. From our experimental measurements, accurate elastic parameter values are obtained when the ratio of the sample‐thickness to transducer‐diameter is greater than 36. Decreasing the size of the transducers is highly recommended when conducting laboratory experiments to recover elastic parameters. A method of offset correction is also implemented in an attempt to decrease the measurement errors.

An improved iterative reconstruction algorithm for traveltime tomography

Traveltime tomography is a technique to reconstruct acoustic, seismic, or electromagnetic wave-speed distributions from first arrival traveltime data. The ray paths that should be used for tomographic techniques strongly depend on the wave-speed distribution. In this paper, a new method is proposed for finding out the ray paths from Fermat's principle, that means the traveltime of the ray path should be a minimum value. The problem of finding out the ray path is actually an optimum problem. Our new method uses the idea to find out the shortest path in a weighted directed graph to solve the problem. The ray paths found out by this method are used in the iterative reconstruction algorithm. Computer simulation result produced by this reconstruction algorithm is better than that by the conventional ones. It also shows that the new algorithm is effective with good convergency and stability.

Inverse scattering, seismic traveltime tomography, and neural networks

AbstractInverse scattering methods for reconstructing sound‐wave‐speed structure in the dimensions have been shown to be equivalent to inverting line integrals when the scattered field is of sufficiently high frequency and the scattering is sufficiently weak. Seismic traveltime tomography uses first arrival traveltime data to invert for wave‐speed structure. Of course, the traveltime is itself a line integral along a refracting ray path through the medium being probed. The similarity between these two inversion problems is discussed. One type of neural network‐the Hopfield net‐may be used to improve the traveltime inversion. We find that, by taking advantage of the general relationship between least‐squares solution and generalized inverses, the neural networks approach eliminates the need for inverting singular or poorly conditioned matrices and therefore also eliminates the need for the damping term often used to regularize such inversions. This procedure produces reconstructions with fewer artifacts and faster convergence than those attained previously using damped least‐squares methods.

Stable iterative reconstruction algorithm for nonlinear traveltime tomography

Reconstruction of acoustic, seismic, or electromagnetic wave-speed distributions from first arrival traveltime data is the goal of traveltime tomography. The reconstruction problem is nonlinear, because the ray paths that should be used for tomographic backprojection techniques can depend strongly on the unknown wave speeds. In the author's analysis, Fermat's principle is used to show that trial wave-speed models which produce any ray paths with traveltime smaller than the measured traveltime are not feasible models. Furthermore, for a given set of trial ray paths, non-feasible models can be classified by their total number of 'feasibility violations', i.e. the number of ray paths with traveltime less than that measured. Fermat's principle is subsequently used to convexify the fully nonlinear traveltime tomography problem.