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SUMMARY The fidelity of depth seismic imaging depends on the accuracy of the velocity models used for wavefield reconstruction. Models can be decomposed in two components corresponding to large scale and small scale variations. In practice, the large scale velocity model component can be estimated with high accuracy using repeated migration/ tomography cycles, but the small scale component cannot. Therefore, wavefield reconstruction does not completely describe the recorded data and migrated images are perturbed by artifacts. There are two possible ways to address this problem: improve wavefield reconstruction by estimating more accurate velocity models and image using conventional techniques (e.g. wavefield cross correlation), or reconstruct wavefields with conventional methods using the known smooth velocity model, and improve the imaging condition to alleviate the artifacts caused by the imprecise reconstruction, as suggested in this paper. In this paper, the unknown component of the velocity model is described as a random function with local spatial correlations. Imaging data perturbed by such random variations is characterized by statistical instability, i.e. various wavefield components image at wrong locations that depend on the actual realization of the random model. Statistical stability can be achieved by local wavefield averaging either in spatial windows defined in the vicinity of the data acquisition locations, or in local windows defined in the vicinity of image points. We use the latter approach and show that the technique is effective in attenuating imaging artifacts without being hampered by some of the limitations of data-space alternatives. INTRODUCTION Seismic imaging in complex media requires accurate knowledge of the medium velocity. Assuming single scattering, imaging requires propagation of the recorded wavefields from the acquisition surface, followed by the application of an imaging condition highlighting locations where scattering occurs, i.e. where reflectors are present. The main requirement for good-quality imaging is accurate knowledge of the velocity model. Errors in the model used for imaging lead to inaccurate reconstruction of the seismic wavefields and to distortions of the migrated images. In a realistic seismic experiment the velocity model is not known exactly. Migration velocity analysis produces large scale approximations of the model, but fine scale variations remain elusive. Therefore, even if the broad kinematics of the seismic wavefields are reconstructed correctly, the extrapolated wavefields also contain distortions that lead to image artifacts obstructing the image of the geologic structure under consideration. While it is certainly true that even the recovery of a long-wave background may prove to be a challenge in some circumstances, we do not attempt to address that issue in this paper. Instead, we concentrate solely on the problem of dealing with the effect of small scale random variations. There are two ways in which we can approach this problem: The first option is to improve the velocity analysis methods to estimate the small-scale variations in the model. Such techniques take advantage of all information contained in seismic wavefields and are not limited to kinematic information of selected events picked from the data. Examples of techniques in this category are waveform inversion.
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (1.00)
Summary We discuss the phenomenon of'turning noise into signal' (one of the main properties of seismic interferometry) in the light of changing worldviews, starting with the ordered view of the nineteenth century, via the chaotic world of the twentieth century, to the present view, in which the chaos is tamed. Introduction It is by now well-known in ultrasonics (Weaver and Lobkis, 2001), geophysics (Campillo and Paul, 2003; Draganov et al., 2007) and underwater acoustics (Roux et al. 2004) that the cross-correlation of diffuse wave fields recorded by two different receivers yields the response at one of the receiver positions as if there were a source at the other. This phenomenon is often named'Green's function retrieval by cross-correlation', whereas in the seismic literature the term'seismic interferometry' is commonly used (after Schuster, 2004). Although seismic interferometry can be applied to noise as well as controlled source data (see also the 2006 July/August issue of Geophysics), in this paper we only consider its aspect of turning noise into signal. In the present age of chaos theory, one of the most striking properties of seismic interferometry is its robustness.
Non-uniform Scaling Behavior In Ultra Low Frequency Geomagnetic Data In Relationship With Seismicity
Telesca, Luciano (Institute of Methodologies for Environmental Analysis, National Research Council,Italy) | Lapenna, Vincenzo (Institute of Methodologies for Environmental Analysis, National Research Council,Italy) | Macchiato, Maria (Dipartimento di Scienze Fisiche, INFM, Universita¿ "Federico II", Naples, Italy) | Hattori, Katsumi (Department of Earth Sciences, Faculty of Science, Chiba University, Japan)
Summary In this paper the uniformity and stability of the scaling behavior of the temporal fluctuations in Ultra Low Frequency (ULF) geomagnetic data observed during 2000 at Izu Peninsula in Japan are analyzed in relationship with the seismic activity, occurred in the monitored area during the observation period. The detrended fluctuation analysis (DFA) is used to identify and quantify deviations from uniform power-law scaling. Our results reveal that the scaling behavior of ULF geomagnetic data appears unstable and not-uniform in relationship with the occurrence of large earthquakes and intense seismic clusters. Introduction Short-term earthquake prediction is still a debated question. Many studies have shown the existence of seismic precursory signatures in several geophysical signals. Recently, it has been shown that earthquake-related electromagnetic phenomena can be considered as very promising for short-term earthquake prediction. Many clear precursory signatures in a wide frequency range (DC-VHF) have been reported (Hayakawa and Fujinawa, 1994; Hayakawa, 1999; Hayakawa and Molchanov, 2002). The Ultra Low Frequency (ULF) geomagnetic signals seem to be very suited phenomena for seismic short-term prediction, because, in comparison with other higher frequency ranges, they have an advantage in propagation in the crust due to skin depth (Fraser-Smith et al., 1990; Bernardi et al., 1991; Molchanov et al., 1992; Kopytenko et al., 1993; Hayakawa et al., 1996; Hayakawa et al., 1999; Kawate et al., 1998; Hayakawa et al., 2000; Uyeda et al., 2002; Hattori et al., 2002; Hattori et al., 2004a; Hattori et al., 2004b; Hattori, 2004; Hattori et al., 2006). The observed ULF data are superposition of some signals. The first one is a natural global signal such as a daily variation of the geomagnetic field and magnetic pulsations originated from the solar-terrestrial interaction and most intense signals. The second one is artificial signal generated by a direct current driven train and a factory and it is considered to be regional one. The third one is a local signal around the magnetometer such as vibration of the ground, movement of magnetic devices, inner circuit noises. In general, the signal associated with crustal activity is very weak. Several methods, like polarization analysis (Hayakawa et al., 1996; Hattori et al., 2002), fractal analysis (Hayakawa et al., 2000), principal component analysis (PCA) and singular spectral analysis (Hattori et al., 2006), have been used efficiently to recognize earthquakerelated patterns in ULF data. Data On June 26, 2000, an official alarm was issued for imminent volcanic activity of volcano Oyama, Miyakejima Island (34.09°N, 139.51°E) by the Japan Meteorological Agency based on increased occurrences of small earthquakes under the island. In the next morning, at several km west of the island, there was an indication of undersea eruption and the seismic swarm activity started almost simultaneously. After a while, a large scale depression at the summit the volcano occurred on July 8. The depression kept growing. Although the last major volcanic event was on August 29, 2000, toxic gas emission is still continuing. Earthquake epicenters also migrated from the island first westward and then northwestward.
- Geophysics > Seismic Surveying (1.00)
- Geophysics > Magnetic Surveying (1.00)