# Seismic Exploration Methods for Structural Studies and for Active Fault Characterization: A Review
## 2 Review of Reflection Seismology  
Reflection seismology is a geophysical method of investigation of the subsurface at different scales (from metric to crustal) that allows retrieving with high detail the characteristics and geometry of the rock formations underlying the earth's surface [18]. Typically, it is based on the analysis of the response of the subsurface layers to a seismic disturbance generated at the earth's surface by an impulsive or vibratory source. It was originally developed for sedimentary basins, but it is nowadays applied in many other environments, from volcanic areas to intrusive domains [2]. The principles of reflection seismology are very similar to those of sonar and radar: an artificial seismic source releases energy at the surface; the energy propagates inside the earth in the form of seismic waves which are reflected back toward the earth's surface when discontinuities are encountered. To produce a seismic image, the data are intensively processed, the intensity of the processing depending mainly: on the near-surface conditions below the investigated area; on the complexity of the structures to be imaged and finally on the signal-to-noise ratio of the acquired data. In continental environments, seismic data are generally affected by greater noise, which is in part caused by the environment but also linked to near-surface heterogeneities. For example, across fault zones, very pronounced lateral variations of seismic velocity conflict with the assumptions of horizontally stratified earth and constant velocity implied by the method [19]. In the same way, seismic exploration in volcanic environments is strongly hindered by the presence of highly heterogeneous material which complicates wave propagation by absorbing and scattering most of the energy released by the source. Generally, in those environments, the seismic energy is fragmented into a series of seismic phases (such as surface waves and guided waves, head-waves and diffractions) that obliterate the reflected phases of interest and inevitably worsen the quality of the final seismic image; therefore, data will require additional processing steps. Conversely, in marine environments, data processing is simplified by the presence of a homogeneous medium (i.e., the water column) at the surface. On land, the recording device is made by one or more vibratory sources organized in patterns and by a series of geophones placed regularly along a line (or several lines for 3D acquisitions). At sea, the most used source is the airgun (see [PERSON] and [PERSON], [20] and references therein). The utilization of airguns is presently a topic of controversy due to the potential effects on marine life, especially cetaceans, which are still being investigated [21]. As a result, several countries have imposed limitations on the use of this source. As previously mentioned, seismic reflection processing is data dependent, i.e., there is no flow valid for all data, but the processing stream must be adapted to the individual dataset. For example, the static corrections (necessary to remove elevation differences between sources and geophones and the effects of surface velocity anomalies) are fundamental in continental settings but generally not performed in a marine environment. However, three analysis procedures are always applied to seismic data: deconvolution, migration, and velocity analysis [19]. Deconvolution (which is divided into spiking and predictive) is a process which allows to increase the temporal resolution of the seismic trace by removing the source features embedded in the recorded seismogram. In its predictive implementation, it also allows to attenuate multiple reflections, thus improving the interpretability of the data. Velocity analysis plays a crucial role in seismic data processing: in classical applications, as described by [PERSON] [19], it becomes essential to estimate velocities to convert seismic data from the pre-stack to the post-stack domain.