Geophysical investigations in the Flumendosa River Delta, Sardinia (Italy) — Seismic reflection imaging

Abstract

A geophysical investigation that included seismic-reflection surveying and time-domain electromagnetics (EM) was carried out in the Flumendosa River Delta plain in southeastern Sardinia, Italy. The objective was to improve knowledge of geologic and hydrogeologic controls on a highly productive aquifer hosted in thick Quaternary deposits and known to be affected by an extensive saltwater intrusion. The seismic reflection survey, whose results are reported here, aimed to image the Paleozoic bedrock topography and to obtain detailed structural and stratigraphic information on the sequence of largely fluvial sediments extending from the surface down to bedrock. The survey consisted of two inline profiles, nearly parallel to the coastline and 1 km inland. The sources (0.25 kg of explosives buried at 2 m depth) and receivers (50-Hz vertical geophones) produced a twelvefold data set with common midpoints every 2.5 m. Detailed integrated velocity analysis (constant velocity gathers, constant velocity stacks, and semblance plots) and frequency-wavenumber-domain dip moveout (DMO) proved to be the most important processing steps in producing the two stacked sections. Both sections were characterized by distinct seismic units bounded by quasi-continuous reflectors. The lowermost reflection, imaged on section 1 at two-way traveltimes between 310 ms (~300 m) and 580 ms (~530 m) and on section 2 between 200 ms (~190 m) and 65 ms (~52 m), was interpreted to be Paleozoic bedrock. A maximum depth twice as deep as expected was the primary and somewhat surprising result. Imaging of oblique progradational facies — another major finding — proved the existence of undocumented marine Pleistocene sediments that could help in revising the area's geology.