Exploration Geophysics - Volume 45, Issue 2, 2014

The geology of the north-western Anatolia (Turkey) ranges from hard Mesozoic bedrock in mountainous areas to large sediment-filled, pull-apart basins formed by the North Anatolian Fault zone system. Düzce and Bolu city centres are located in major alluvial basins in the region, and both suffered from severe building damage during the 12 November 1999 Düzce earthquake (Mw = 7.2). In this study, a team consisting of geophysicists and civil engineers collected and interpreted passive array-based microtremor data in the cities of Bolu and Düzce, both of which are localities of urban development located on topographically flat, geologically young alluvial basins of Miocene age. Interpretation of the microtremor data under an assumption of dominant fundamental-mode Rayleigh-wave noise allowed derivation of the shear-wave velocity (Vs) profile. The depth of investigation was ~100 m from spatially-averaged coherency (SPAC) data alone. High-frequency microtremor array data to 25 Hz allows resolution of a surface layer with Vs < 200 m/s and thickness 5 m (Bolu) and 6 m (Düzce). Subsequent inclusion of spectral ratios between horizontal and vertical components of microtremor data (HVSR) in the curve fitting process extends useful frequencies up to a decade lower than those for SPAC alone. This allows resolution of two interfaces of moderate Vs contrasts in soft Miocene and Eocene sediments, first, at a depth in the range 136–209 m, and second, at a depth in the range 2000 to 2200 m.

Imaging of Rayleigh- and Love-wave velocities is very important in detecting geophysical anomalies within the earth. Surface wave velocity imaging studies using ambient noise have provided enhanced and detailed images of velocity anomalies for sedimentary basins, hotspots, and volcanoes in various regions of the Earth (Yang et al., 2008). Cross-correlations of ambient noises observed from the Korea Meteorological Administration (KMA) seismic network provide the short-period Rayleigh-and Love-wave dispersion characteristics of the Korean Peninsula (Cho et al., 2007). Signal whitening and multiple-filter analysis are used to equalise power in signals from different times before noise processing, such as cross-correlation and stacking to extract group velocities from the estimated Green’s functions, which are then used to image the spatially varying dispersion at periods between 1 and 5 s. The analysis method and data used in this paper are the same as those of Cho et al. (2007) except for the addition of the dataset of a new station, HUK. However, this paper notes that Rayleigh- and Love-wave velocity images in short periods show a very different group velocity image for the north-eastern area of the HUK station because additional data was analysed. This velocity anomaly corresponds with the residual anomaly of gravity tomography obtained in prior studies (Yu and Min, 2005; Kim and Oh, 2007). Our results show that a fracture zone concerning the Permo-Triassic collision (Choi et al., 2006; Kwon et al., 2009) exists below the north-eastern sea of the HUK station. In addition, recent studies (de Ridder and Dellinger, 2011; de Ridder and Biondi, 2013; Mordret et al., 2011, 2013a, 2013b, 2013c; Bussat and Kugler, 2011) regarding ambient noise tomography in hydro-carbon fields show that the anomaly might have resulted from the hydro-carbon reservoir. In the near future, the ambient noise tomography (ANT) method can replace seismic survey dominantly using body waves to find oil and gas reservoirs.

A curvilinear-grid perfectly matched layer (PML) absorbing boundary condition for the second-order seismic acoustic wave equation is presented in this paper. The rectangular grids are transformed into curvilinear grids by using a mathematical mapping to fit the curvilinear boundary, and the original wave equation is reformulated under the curvilinear coordinate system. Based on the reformulated wave equation, theoretical expressions and analysis of the curvilinear-grid PML are given. Furthermore, PML model 1 with symmetric form and PML model 2 with asymmetric form are derived from the same acoustic wave equation. By combination with the finite difference (FD) method, these two models are applied to seismic wave modelling with surface topography. The results show that the absorption effect of these two models discretised by the same second-order time difference and second-order space difference are different, and the symmetric-form PML yields better modelling results than the asymmetric-form.

Throughout the early years of the 21st century, multi-component seismic surveying has developed rapidly with larger capacity recording systems and digital three-component sensors. These technologies capture the seismic wavefield more completely than conventional P-wave surveys and allow extra information to be obtained from a converted-wave or ‘PS’ image. However, converted-wave seismic data is much more challenging to acquire and process than conventional P-wave data. Most existing multi-component surveys to date have been deployed in areas with geologically simple structures. This paper presents a case study of a 250 km² 3D three-component seismic survey located in an area with complex structures and faults. The complexity of the converted-wave data obtained in this survey has rarely been seen before, bringing huge technology challenges to the processing and interpretation. After several early failures, an extremely good image of the converted-wave has been obtained by combining a variety of processing algorithms. The structural features seen remain consistent with that of the P-wave image, providing a good foundation for subsequent inversion and reservoir analysis. This article describes the main techniques used, focusing in particular on three of the most challenging steps: converted-wave statics, anisotropy correction, and pre-stack time migration. We hope that this workflow will provide a technical guide, or reference for multi-component seismic data processing for complex structures.

We apply a Hamiltonian particle method, one of the particle methods, to simulate seismic wave propagation in a cracked medium. In the particle method, traction free boundaries can be readily implemented and the spatial resolution can be chosen in an arbitrary manner. Utilisation of the method enables us to simulate seismic wave propagation in a cracked medium and to estimate effective elastic properties derived from the wave phenomena. These features of the particle method bring some advantages of numerical efficiencies (e.g. calculation time, computational memory) and the reduction of time for pre-processing.

We describe first our strategy for the introduction of free surfaces inside a rock mass, i.e. cracks, and to refine the spatial resolution in an efficient way. We then model a 2D cracked medium which contains randomly distributed, randomly oriented, rectilinear, dry and non-intersecting cracks, and simulate the seismic wave propagation of P- and SV-plane waves through the region. We change the crack density in the cracked region and determine the effective velocity in the region. Our results show good agreement with the modified self-consistent theory, one of the effective medium theories. Finally, we investigate the influence of the ratio of crack length to particle spacing on the calculated effective velocities. The effective velocity obtained becomes almost constant when the ratio of crack length to particle spacing is more than ~20. Based on this result, we propose to use more than 20 particles per crack length.

Dike leakage can be the result of a rupture and the formation of loose zones which are not able to stand the water pressure during flooding. Loose zones are significantly more saturated when it rains and floods than the undamaged portion of the dike. Due to the increased water in loose zones, their electrical properties are changed, particularly dielectric permittivity. As a result, these zones have a different ground-penetrating radar (GPR) wave reflection coefficient and are a source of wave diffraction. The interpretation of GPR measurements carried out on a leaking dike during a flood event in Poland is presented in this paper. The GPR attributes, such as an instantaneous phase, envelope, instantaneous frequency averaged over time and traces, have been analysed in the paper the interpretative tools. Also, the averaged spectrum (spectrum calculated from averaged traces) and moving spectrum (averaged spectrum calculated in windows moving along the traces), as well as the phase spectrum, of recorded GPR data were analysed as indicators of the existence of the deterioration of parts of the dike. As shown in the paper, the use of GPR signals attributes and spectra in the interpretation of field measurements can increase the available information about the structure of the dike by highlighting some of the physical properties of its construction.

The Mohar cauldron, located near Mohar village, Shivpuri District, Madhya Pradesh, India, represents an explosive felsic volcanic event. The Mohar Cauldron Complex (MCC) is an important target area for uranium exploration as collapse breccias associated with extensional tectonics are traditionally important for multi-metal deposits, including uranium. Advanced processing and interpretation of the high resolution airborne electromagnetic (AEM), magnetic and radiometric data acquired over the Mohar cauldron and the surrounding environs by Fugro Airborne surveys, successfully mapped the major geological domains in the area based on their distinct geophysical characteristics. Interpretation of the data indicated the presence of three felsic intrusive bodies, only one of which, the MCC, reached the surface and collapsed. Variation in the geophysical characteristics of the three bodies is attributed to variations in hydrothermal alteration. Magnetic signature and radiometric response of the MCC and surrounding area also show signs of intense alteration. AEM data has allowed the boundary of the sediments within the MCC to be mapped accurately, along with the surrounding brecciated zone. Conductivity depth imaging calculated to a depth of 500 m clearly indicated the geometry and disposition of different layers of MCC. 3D voxel modelling of the MCC also allowed for the identification of the different lithologies that constitute the cauldron structure. 3D conductivity isosurfaces provided a thorough understanding of the subsurface distribution of conductivities.