Geophysical approach to the environmental study of a coastal plain

Abstract

in the study of coastal plains affected by soil and water salination, a knowledge of several geological aspects, such as structural features, depth to basement, stratigraphy of sedimentary cover, relationships between the phreatic aquifer and underlying aquifers, and the latter's structure, is basic to gaining an adequate understanding of both the causes and possible evolution of salination. In this framework, geophysical techniques can play a very important role. \ud \ud To improve the available geophysical information about the Muravera coastal plain, Sardinia, Italy, which is affected by severe soil and water salination, previously acquired electrical resistivity, reflection seismic and gravity data have been reprocessed, and a new seismic reflection survey has been conducted. Moreover, in order to give better support to the geological and hydrogeological interpretation of geophysical data, three boreholes were drilled. Reprocessed electrical data indicate the presence of a wide, electrically homogeneous low-resistivity zone associated with salination phenomena. Reprocessed reflection data provide useful information on the near-surface stratigraphy. The combined interpretation of resistivity and seismic results, supported by one calibration borehole, elucidates the relationships between the phreatic aquifer and the underlying confined aquifer. A new seismic reflection survey gives information on the depth to, and structure of, the Paleozoic basement, as well as on stratigraphic conditions of Pleistocene-Holocene sediments. Finally, the combined interpretation of seismic, gravity, and well data results in a geological section containing most of the information considered essential, such as the interface between Holocene alluvium and Pleistocene alluvium, the thickness of the latter, and the structure and composition of the Paleozoic basement. \ud \ud The work as a whole shows how the combined application of geophysical techniques can in this specific situation provide wide-ranging and high-quality information that is essential for the realistic mathematical modeling of aquifer contamination, and can enable the rational planning of exploratory drillings

Combined time domain electromagnetic soundings and gravimetry to determine marine intrusion in a detrital coastal aquifer (Southern Spain)

“…The detection of seawater at depth normally relies on different electric methods as it is usually easily identified given the low resistivity of saltwater-saturated rocks (Gondwe, 1991;Kafri et al, 1997;Chen, 1999;Hopkins and Richardson, 1999;Choudhury et al, 2001;Guérin et al, 2001;Balia et al, 2003;Goldman et al, 2003;Wilson et al, 2005). Porosity is a parameter that must be taken into account due to the controlling role it can play in seawater saturation of a porous material.…”

Medium to Shallow Depth Stratigraphic Assessment Based on the Application of Geophysical Techniques

“…The coastal plain covered in this section is a fluvial valley that also includes a river delta (Balia et al, 2003). The surface geology of the plain and its surroundings is characterized, from bottom to top, by a Paleozoic metamorphic complex outcropping on the edges of the plain, and Pleistocene and Holocene sediments and alluvium, up to a few hundred meters thick, overlying the Paleozoic bedrock.…”

“…The lack of a coherent signal in the lower part of the section, from CMP trace #270 to the northwest, may be attributed to relatively homogeneous granite. Balia et al, 2003) Given the aim of the work, a detailed knowledge of the Pleistocene-Holocene cover was of primary interest. Thus, the data pertaining to the southernmost part of the seismic profile were processed separately, especially refining velocity analysis for shallower events.…”

Identification of the Saline Zone in a Coastal Aquifer Using Electrical Tomography Data and Simulation

“…Consequently, the good agreement between aquifer hydraulic conductivities calculated from the interpreted resistivity soundings and those deduced from pumping test analysis proves the essential contribution of geophysical methods in the determination of the aquifer' parameters. Balia et al (2003) applied a combination of geophysical techniques in order to show the fundamental role of geophysical survey techniques in the environmental study of coastal areas. This approach was applied at the Muravera plain in southern Sardinia, Italy, which is affected by severe water salination.…”