3D Seismic Imaging of the Irpinia Fault System (Southern Italy) From Multi-Scale Velocity and Attenuation Tomography (original) (raw)
Related papers
2005
In seismology the accurate mapping of faults and fault zones plays an important role as a crucial step for characterizing the earthquake potential of an area. Cross-hole seismic surveys for engineering site investigations can provide high resolution images of subsurface seismic velocity capable of delineating tectonic structures such as vertical or dipping faults juxtaposing different geological formations or step-like structures with good accuracy. The result of velocity reconstruction in travel time seismic tomography is restricted by factors such as ray aperture and distribution. These physical limitations cannot be completely surmounted even in cross borehole acquisitions where the ray coverage is the best one possible. The present paper studies the effects of staggering normal grids as a tool to increase resolution and reduce inversion vagueness and instability. Truncated Singular Value Decomposition as a mathematical tool of least demands on the final solution is utilized, and the inversion scheme is examined with respect to the number of available data. Numerical simulations of a model featuring a fault-like structure are performed and the resulting recovered images are compared against straight grid inversions. Reconstructed images using staggered grids provide a smoothed version of the true model since they basically operate as a moving average filter on the model. On the other hand the final tomograms of the synthetic tests, as a result of shifting the grid, show high accuracy and resolution since both the geometry of the fault and the velocity values throughout the model are better determined. Conventional grid inversion fails to image
Staggered, Adapted and Stacked Grids in Seismic Tomography
61st EAGE Conference and Exhibition, 1999
Adaptive tomography can fit the grid resolution locally to the ray distribution and th e detected velocity anomalies : thus, one reduces the inversion ambiguities and the unnecessary model complexities . Staggered averaged grids obtain similar results , but require simpler algorithms for the ray tracing in the inversion procedure . However , we get slightly beuer images by stacking a number of adapted tomographic grids together .
Model parametrization in seismic tomography: a choice of consequence for the solution quality
Physics of the Earth and Planetary Interiors, 2001
To better assess quality of three-dimensional (3-D) tomographic images and to better define possible improvements to tomographic inversion procedures, one must consider not only data quality and numerical precision of forward and inverse solvers but also appropriateness of model parametrization and display of results. The quality of the forward solution, in particular, strongly depends on parametrization of the velocity field and is of great importance both for calculation of travel times and partial derivatives that characterize the inverse problem. To achieve a quality in model parametrization appropriate to high-precision forward and inverse algorithms and to high-quality data, we propose a three-grid approach encompassing a seismic, a forward, and an inversion grid. The seismic grid is set up in such a way that it may appropriately account for the highest resolution capability (i.e. optimal data) in the data set and that the 3-D velocity structure is adequately represented to the smallest resolvable detail apriori known to exist in real earth structure. Generally, the seismic grid is of uneven grid spacing and it provides the basis for later display and interpretation. The numerical grid allows a numerically stable computation of travel times and partial derivatives. Its specifications are defined by the individual forward solver and it might vary for different numerical techniques. The inversion grid is based on the seismic grid but must be large enough to guarantee uniform and fair resolution in most areas. For optimal data sets the inversion grid may eventually equal the seismic grid but in reality, the spacing of this grid will depend on the illumination qualities of our data set (ray sampling) and on the maximum matrix size we can invert for. The use of the three-grid approach in seismic tomography allows to adequately and evenly account for characteristics of forward and inverse solution algorithms, apriori knowledge of earth's structure, and resolution capability of available data set. This results in possibly more accurate and certainly in more reliable tomographic images since the inversion process may be well-tuned to the particular application and since the three-grid approach allows better assessment of solution quality.
Crosshole seismic waveform tomography – II. Resolution analysis
In an accompanying paper, we used waveform tomography to obtain a velocity model between two boreholes from a real crosshole seismic experiment. As for all inversions of geophysical data, it is important to make an assessment of the final model, to determine which parts of the model are well-resolved and can confidently be used for geological interpretation. In this paper we use checkerboard tests to provide a quantitative estimate of the performance of the inversion and the reliability of the final velocity model. We use the output from the checker-board tests to determine resolvability across the velocity model. Such tests can act as good guides for designing appropriate inversion strategies. Here we discovered that, by including both reference-model and smoothing constraints in initial inversions, and then relaxing the smoothing constraint for later inversions, an optimum velocity image was obtained. Additionally , we noticed that the performance of the inversion was dependent on a relationship between velocity perturbation and checkerboard grid-size: larger velocity perturbations were better-resolved when the grid-size was also increased. Our results suggest that model assessment is an essential step prior to interpreting features in waveform tomographic images. Waveform tomography is a powerful tool that can yield quantitative images of physical properties of the earth media. Compared to traveltime tomography, a velocity image generated by waveform tomography will have significantly better resolution. However, one significant question remains, 'how reliable is the velocity image, and can we use it to make a direct geological interpretation?' Can we use the observed velocity contrasts to distinguish individual geological layers? In this paper, we conduct a series of checkerboard tests on a waveform tomographic velocity model that was obtained from a real crosshole seismic data set. The aims of these test are to reveal the resolving power of the inversion when dealing with real data, and to provide an indication of reliability of the inversion result. Generally speaking, a geophysical tomographic solution is not unique. It depends on the quality of the data, data selection, the inversion method employed, and the model parametrization. Tomo-graphic resolution can be very poor in regions where the distribution of sources and receivers is irregular. Additionally, it is common to apply model constraints to the inverse problem to produce a practical solution. The effect of these factors upon the inversion solution is difficult to quantify, especially when dealing with real seismic data. It is thus questionable whether we can use the final velocity model to infer the earth's properties correctly. In this paper we use checkerboard tests to verify the final velocity model obtained from a waveform tomographic inversion. Checker-board testing has been used commonly in traveltime tomography (Inoue et al. 1990; Zelt 1998; Zelt & Barton 1998; Morgan et al. 2002), but has not yet been used in an application of waveform tomography to real seismic data. We set up a checkerboard consisting of rows and columns of alternating positive and negative velocity anomalies, superimposed on the final velocity model. The velocity perturbations are a percentage of the actual velocity value, and thus are spatially varying. Based on the checkerboard model, we generate a synthetic data set using the same frequency-domain finite difference scheme as in the inversion itself, and then invert these data using exactly the same method, procedure, constraints, and parametrization as used for the tomographic inversion of the real data. Resolvability at any point of the model space is defined in terms of the ratio of recovered velocity anomaly to the real velocity perturbation. In the resolution analysis tests, we test the effect of: the reference-model constraint, the model smoothness constraint, and the effect of the combination that we applied in the inversion of the real data. We also mimic the real data acquisition with an irregular source/receiver geometry. Then we test the effect of the irregular ray coverage in the real experiment by setting up an ideal crosshole configuration, consisting of regular sources for each of the cells in one borehole and regular receivers spanning over all cells at the other borehole. Finally, we test the effect of varying the magnitude of the velocity perturbation and the cell-size of the checkerboard.
Analysis of the resolution function in seismic prestack depth imaging
Geophysical Prospecting, 2002
We consider the problem of estimating subsurface quantities such as velocity or reflectivity from seismic measurements. Because of a limited aperture and bandlimited signals, the output from a seismic prestack reconstruction method is a distorted or blurred image. This distortion can be computed using the concept of resolution function, which is a quantity readily accessible in the Fourier space of the model. The key parameter is the scattering wavenumber, which at a particular image point is defined by the incident and scattered ray directions in a given background model. Any location in any background model can be considered. In general, the resolution function will depend on the following four quantities: the background velocity model, the frequency bandwidth, the wavefield type and the acquisition geometry.
2010
A new dataset of first P-wave arrival times is used to derive the 3D tomographic model of the Campania-Lucania region in the southern Apennines (Italy). We address the issue related to the non-uniqueness of the tomographic inversion solution through massive numerical experimentation based on the global exploration of the model parameter space starting from a large variety of physically plausible initial models. The average of all the realizations is adopted as the best-fit solution and the uncertainty of the model parameters is studied using a statistical approach based on a Monte Carlo-type analysis. How the uncertainty in the initial model, earthquake locations, and data influences the inversion result is studied by considering separately the individual effects. Checkerboard tests are performed to estimate the resolving power of the dataset. Re-located seismicity in a reliable new 3D tomographic model allows us to correlate the earthquake distribution with the main seismogenic structures present in the area.
Staggered versus adapted grids for the joint 3D inversion of surface and well data
SEG Technical Program Expanded Abstracts 1999, 1999
Staggered grids can increase the resolution of seismic tomography while reducing the null space ambiguities: thus, they allow us to approximate the physical resolution limits of the seismic experiments in the reconstructed Earth image. However, when regular grids are adopted for this approach, we may not fully exploit their potentialities, because they cannot fit the natural space-variant resolution of seismic waves, unlike adaptive irregular grids.
2013
Introduction. Delay time tomography based on local earthquakes can provide a detailed three-dimensional image of seismic velocity structure in areas which are expected to be affected by strong earthquakes. Refined earthquake locations in a reliable velocity model, allow to detect and characterize crustal structures, and embedded active faults with a high seismogenic potential (Eberhart-Philipps, 1993). On the other hand, the availability of a velocity model and the seismicity distribution allow to study the relationship between the geometry and mechanical behavior of a fault or faults system and the physical properties of the host environment (Michael and Eberhart-Phillips, 1991). The accuracy of earthquake locations is strongly controlled by several factors, among which the geometry of the network, the number of available phases especially the capability of identifying both P and S phases, the accuracy of arrival time readings and the knowledge of the crustal structure (Pavlis, 1986). In addition, the use of 1D layered velocity models can introduce systematic errors in the estimation of P-and S-travel times due to the presence of large-scale three-dimensional heterogeneities in the propagation medium (Matrullo et al., 2013). In order to reduce the effect of using a 1D velocity model, relative locations and double differences techniques can be used. With these apporaches, the effect of a poor knowledge of the structure can be cancelled out for two events, very close in space, recorded at the same station travel as they travel following nearly the same path except nearby the source zone (Waldhauser and Ellsworth, 2000). However, Michelini and Lomax (2004) emphasized that systematic errors on earthquake location using double-differences may also be caused if the velocity model is not accurate. In this respect, a joint inversion of hypocenter and velocity parameters could allow to overcome the simplistic assumptions of the location methods mentioned above. The methods used today for the joint tomographic inversion have made substantial progress with respect to the basic theory originally developed by Aki and Lee (1990). Recent methods include efficient techniques for 3D ray tracing calculation using the eikonal equation (Vidale, 1990) for firstarrival traveltimes estimation on a given finite-difference grid in which the precision of traveltime calculation can be significantly improved by a successive integration of the slowness along the ray (e.g, Latorre et al., 2004). High-resolution imaging of the sub-surface with local earthquake data requires the use of large and consistent data sets of first arrival times. The quality and resolution of the medium image depends not only on the source/receiver coverage of the target region but also on the accuracy of the travel time measurements. The common procedure of reading the arrival time of a phase (picking) involves the manual measurement of P-and S-arrival times on recordings of a single event at a time. Systematic errors can be introduced due to inadequate working procedures such as the interaction between the process of picking and the result of the location. The inconsistency of the data can remain unnoticed when the events are analyzed independently from each other, but it may clearly appear when performing a joint determination of the hypocentral and velocity model parameters and reducing the inconsistency would require a complete picking revision. The knoweledge of the S-wave add important constraints to the earthquake location problem. The S-phase is important to derive physical parameters of the subsurfaces. The correct reading of the arrival times of these waves can be complicated by various factors, such as the superposition of the tail of the P-wave, the presence of converted waves generated at different interfaces, and the splitting of S-waves caused by seismic velocity anisotropy
Physical criteria for adaptive seismic tomography
SEG Technical Program Expanded Abstracts 1997, 1997
Traveltime inversion is a cost-effective tool for estimating the velocity model for complex geological structures. However, such estimates may be non-unique, especially if the velocity field is inverted by regular grids. The quality of the tomographic images may be degraded by the poor local ray coverage and by the shape mismatch between the velocity anomalies and the grid (Vesnaver 1994). We can master these mathematical and geometrical drawbacks by adapting the grid shape and its parameter number to the available rays (Böhm et al. 1996).