Finite-difference approximation of wave equation: a study case of the SIMA velocity model

Synthetic seismograms enable to model the theoretical seismic response of the Earth interior due to different structural features and changes in the physical properties of crust and mantle. This approximation provides a best understanding of the real seismic data recorded in field experiments. In this paper, we are showing the development and application of a new scheme based on a multi-order explicit finite-difference algorithm for acoustic waves in a 2D heterogeneous media. The results of the modeling are compared with the seismic data acquired within the SIMA project providing new insight about the internal structure of the subsurface allowing improving the velocity model obtained in previous works.Peer Reviewe

Forward modeling of SIMA seismic line

© 2015 Kamal Gueraoui et al. In this paper, a reviewed velocity model is proposed along the North-South high-resolution profile SIMA (Seismic Imaging of the Moroccan Atlas), acquired in 2010 and crossing the four major geological zones in Morocco, the High Atlas, the Middle Atlas, the western edge of the Rif Mountains and the Sahara craton. The tremendous changes in topography of the studied area and the complex near surface layer led us to better focus on the shallow subsurface zone, and establish a revised 550 km long P-wave velocity model, based on forward modeling.Peer Reviewe

New numerical and theoretical model to characterize the upper crustal structure of the moroccan atlas from wide-angle seismic reflection data

© 2015 Shagun Mudgil, Harika Nambula and B. Bharathi. The upper crust beneath the Moroccan Atlas has been modeled using the travel time of wide-angle arrivals from the SIMA (Seismic Imaging of the Moroccan Atlas) dataset. The detailed knowledge of the internal structure of this orogen allows understanding its uncommon characteristics, featuring high topography, moderate tectonic shortening and moderate crustal thickening. The -700 km long SIMA wide-angle reflection seismic profile has provided a high resolution geophysical data set to obtain a shallow P-wave velocity model along the transect. The seismic data processing has enabled us to accurately pick the first arrivals of the seismic records. Subsequently, the development of a numerical code to mathematically model the hodochrones defined by the first arrivals, has provided us the P-wave velocity structure of the crust down to 10 km. The resulting model shows a detailed image of the Atlas upper crust and reveals several relevant features that help to understand the structure of the orogen and its composition.SIMA was funded by a grant from the Spanish Science Foundation (FECYT), involves several institutions like ICTJA-CSIC, Salamanca University and Autonomous University of Barcelona, the Scientific Institute of Rabat, University Cadi Ayyad in Marrakech, FST of Errachidia, University Sidi Mohammed ben Abdellah in Fes, and was supported as part of PICASSO by grant EAR 0808939 from the NSF Continental Dynamics ProgramPeer Reviewe

A 700 km long crustal transect across northern Morocco

Two controlled-source wide angle seismic reflection experiments have been acquired recently (2010 and 2011) in northern Africa across Morocco. A lithospheric scale transect can be constructed by joining both data sets. Hence, an approximately 700 km-long seismic velocity cross section can be derived. From south-to-north the transect goes from the Sahara Platform, south of Merzouga, to Tanger in the north. The first experiment, SIMA, aimed to constrain the crustal structure across the Atlas Mountains. The Rif, the orogenic belt located just south of the coast of Alboran Sea, was the target of the second experiment, RIFSIS. In both cases 900 recording instruments (TEXANS) from the IRIS-PASSCAL instrument center were used to record the acoustic energy generated by explosion shots. In both experiments the shots consisted of 1 TM of explosives fired in ~30 m deep boreholes. Although the data quality varies from shot to shot, key seismic phases as Pg, PmP, Pn, and a few intra-crustal arrivals have been identified to constrain the velocity-depth structure along the whole transect. Forward modelling of the seismic reflection/refraction phases reveals a crust consisting of 3 layers in average. The Moho topography shows from south to north a relatively moderate crustal root beneath the High Atlas, which can reach 40-42 km depth. The crust is thicker beneath the Rif where the Moho is imaged as an asymmetric feature that locally defines a crustal root reaching depths of 50 km and suggesting a crustal imbrication. P wave velocities are rather low in the crust and upper mantle. First arrivals/reflections tomography supports the forward modelling results. Low fold wide-angle stacks obtained by using hyperbolic move-out reveals the geometry of the Moho along the entire transect. Beneath the Atlas, the moderate crustal root inferred is not isostatically consistent with the high surface elevations, hence supporting the idea of a 'mantle plume' as main contributor to the Atlas Mountains topography.Peer Reviewe

Crustal structure and velocity model of the Moroccan Atlas from refraction/wide angle data. Implications for its tectonic evolution

The Atlas Mountain Range is an intra-continental Cenozoic orogenic belt located at the southern edge of the diffuse plate boundary zone separating Africa and Europe. Its western part, the Moroccan Atlas, has long been under the scope of scientist regarding the origin of its high topographies, locally exceeding 4000 m. Geological studies indicate that this mountain belt has experienced low to moderate shortening. Furthermore, the later decreases as topography increases towards the west. These observations rise the question about the origin of the Atlas Mountains topography. Potential field studies indicate that an astenospheric upwelling supports the Atlas high elevations. However, these models depend strongly on the Moho topography and depth. Refraction/wide angle experiments carried out in the 80¿s suggested that the crust is thin and the Moho relatively flat. However, the proposed crustal structure and velocity inversions are not in agreement with the present models of this mountain belt. With the goal of improving the knowledge of the Moho boundary geometry and the velocity structure of the crust, a refraction/wide angle experiment was carried out in spring 2010 by an international team: the SIMA (Seismic Imaging of the Moroccan Atlas) experiment. A ~700 km long profile, going from Tanger to the Sahara Desert, south of Merzouga, recorded, every 400-1000 m, the energy of 6, 1 tn shots. Even with a low signal/noise ratio, the data allows the identification of crustal phases (Ps, Pg and PiP) and Moho reflected/refracted phases (PmP and Pn). Very weak subcrustal energy appers in some shot gathers. Forward modeling pictures a 3 layers crust and shows the Moho as an asymmetric feature that locally defines a crustal root, suggesting that the crust is imbricated. The crust-mantle boundary is modeled at relatively shallow depths that are in accordance with the results of other geophysical data, thus supporting the idea of a `mantle plume¿ as main contributor to the Atlas mountains topography. P wave velocities are low in the crust and upper mantle. First arrivals/ reflections tomography supports the forward modelling results. Further investigations are needed to unravel the source of the low velocities and to constrain the existence of mantle reflectors.Peer Reviewe