Combination of Geophysical Technques to Characterize Sediments (Ebro Delta, Spain)

The Ebro Delta (Spain) faces anthropogenic and climate changes that can cause a significant decrease on its subaerial surface. One of the causes of this reduction is subsidence that take place due to the compacting of the sediments. In order to identify zones vulnerable to subsidence, we have defined several targets such as: characterization of Holocene sediments (sands versus clays/silts and location of organics) and delineation of the bottom of deltaic sediments. Regarding these objectives, we have acquired geophysical data sets at different environments of the Ebro Delta (from delta plain to marshes among others). The methodology includes combination of seismic methods (surface waves, refraction and reflection methods) and electrical resistivity tomography for sediment characterization as well as the use of the H/V spectral ratio of microtremor measurements to delineate the base of the Holocene sediments. We show the results for one line located on the delta plain and close to alluvial fans. Geophysical models are characterized by a high-resistivity and high-velocity layer at around 25 m depth interpreted as the base of the deltaic sediments. H/V results provide evidence of maximum thickness of deltaic sediments located at the right side of the river mouth.