2014 SPA Contest - Siddique Akhtar Ehsan - From the surface Topography to the Upper mantle beneath Central-Iberian-Zone. The ALCUDIA Seismic Experiments

Seismic (from the Greek word Seismos for “shake, vibration of Earth”) wave is the energy that moves through and around the Earth. The seismic waves are messengers that convey information about the interior of the Earth. They are caused by the sudden break of a rock mass within the Earth or they can also be generated by an explosion. The Earth’s outermost shell contains solid and rocky layer called lithosphere. The lithosphere can be divided into the crust and uppermost mantle. The deep seismic reflection data set provide high quality images of the crust and upper mantle. The seismic experiments are also used in hydrocarbon (Oil/Gas) industry to place key constraints on the subsurface geological structures and composition.

In summer 2007 and 2012, multi-seismic experiments namely ALCUDIA, which include normal incidence and wide-agnle data sets were acquired across the central and southern part of the Iberian Peninsula. The seismic experiments provided a new insight into the structure and nature of the lithosphere beneath these areas. A high quality image, ~230 km long, down to 45 km depth (~15 s TWTT) is provided by the normal incidence data set. Based on the reflectivity characteristics, the image can be divided into an upper and mid-lower crust, ~13 km and ~18 km thick, respectively. The wide-angle seismic transect extended the crustal section towards the north across the Madrid Basin. This, latter data set also sample the CIZ until the CU. This is ~280 km long profile which provides very strong constraints on the distribution of physical properties (P wave and S wave velocities, Poisson's ratio) of the upper lithosphere. The PiP and PmP seismic phases constrain two discontinuities: the brittle to ductile discontinuity at ~13-19 km and the Moho boundary at ~31-35.5 km. 

The normal incidence deep seismic reflection image is shown below. Along the x-axis, top and bottom, are the Common depth point (CDP) locations and distance of the image, respectively. Along the y-axis, is the depth. The crust (~31 km thick) is more reflective than the upper mantle.

(a) Uninterpreted poststack time migrated (15 s) and depth converted section. The black arrows highlight the migration artifacts. (b) The geological cross section along the ALCUDIA normal incidence (NI) deep seismic reflection profile. (c) The suggested interpretation of the migrated and depth converted ALCUDIA-NI profile. The black thick lines have been interpreted as faults and/or major shear zones; the red lines are indicative of the trends of the most prominent reflectivity fabric. A blue line highlights the crust mantle transition. Abbreviations: UC, Upper crust; MC,Middle crust; LC, Lower crust; DL, Decollement level; HW, Hanging wall; FW, Footwall; DLCR, Dense lower crustal reflectivity; DFWR, Dense footwall reflectivity; x, y, z, imbricate thrust systems.