Effects of geophysical parameters on the seismic expression of the Maghlaq Fault, Malta: insights from outcrop-based 2D seismic modeling

Abstract

Faults are heterogenous zones comprising highly complex geometries, which can be studied in detail from field outcrops. The internal complexity of a fault zone is however difficult to interpret from seismic images, due to resolution and illumination limitations of the data. To improve the seismic interpretation of subsurface structures, synthetic seismic can be simulated from outcrop-derived geological models. Such seismic modeling reveals the potential seismic expression of structural and stratigraphic features observed in the field. In this study, a 2D Point-Spread Function based modeling approach is applied to investigate the seismic expression of the Maghlaq Fault, Malta, which is a carbonate-hosted normal fault zone comprising complex hanging wall geometries. Geological models of the fault zone are created based on both geological interpretations of virtual outcrop models of the hanging wall of the fault as well as conceptual extrapolations. The geological models are divided into lithostratigraphic units, which are further assigned realistic elastic properties (i.e. seismic velocities and density) in order to create reflectivity models. Pre-stack Depth Migration images are simulated from the seismic modeling, predicting the seismic characteristics of the Maghlaq Fault. The study further analyzes the effects of various geophysical survey parameters on the seismic images, by systematically varying each individual parameter, such as the dominant frequency, level of noise, angle of maximum illumination, incident angle and wavelet type. The resulting 2D seismic sections generated in this study show that the seismic expression of the Maghlaq Fault differs to some extent for the four different geological models, due to the significant variation in hanging wall geometry of the four geological input models. Nevertheless, some consistent seismic characteristics are found in all seismic images, regardless of the variation in structural and stratigraphic input. Furthermore, the obtained results from the sensitivity analyses highlight the geophysical parameter dependency of both the detectability and the seismic expression of the fault zone. The dominant frequency, noise level and angle of maximum illumination are the parameters which have the greatest impact on the seismic images of the Maghlaq Fault zone. This thesis is a contribution to improve the understanding of seismic imaging of normal fault zones within carbonates, which hopefully can aid seismic interpretation of similar structures in the subsurface.