Non-Seismic Methods on Shallow Water Environments

Abstract

Characterization of inland water is a topic of great interest due to the broad spectrum of potential applications. Applied geophysic boasts different techniques adapted to retrieve useful information about these kinds of environments. Certainly, the most common geophysical techniques used in shallow waters are the seismic methods. However, there are some situations in which seismic methods could fail. Although nowadays it does not exist a method able to solve completely this task, electromagnetic techniques are a cost efficient tool to provide useful information. Thanks to their versatility, we concentrated our attention on the possibility of the Ground Penetrating Radar (GPR) and of the low induction number electromagnetic multi-frequency soundings measurements, carried from the water surface. We started from acquisitions performed in controlled settings. We described how we reproduced the field condition of a riverine GPR survey in laboratory experimentation. We selected a 1500 MHz GPR antenna, and we studied five types of riverine bottom sediments. We developed two different approaches to interpret the GPR responses of the sediments: the velocity and the amplitude analysis. The amplitude analysis developed is particularly innovative and fit very well the field requirements. We tried to estimate the sediments porosities by some mixing rules by the electromagnetic properties founded with both the analysis performed. The comparison among the porosities provided by the GPR measurements and the porosities measured by direct methods confirm the accuracy of the velocity analysis and it highlights the poor reliability of the amplitude analysis. Successively, we tested our methodology in survey condition. We conducted an integrated geophysical campaign on a stretch of the river Po in order to check the GPR ability to discriminate the variability of riverbed sediments through an analysis of the bottom reflection amplitudes. We conducted continuous profiles with a 200MHz GPR system and a handheld broadband electromagnetic sensor. A conductivity meter and a TDR provided punctual measurements of the water conductivity, permittivity and temperature. The processing and the interpretation of both the GEM-2 and GPR data were enhanced by the reciprocal results and by integration with the punctual measurements of the electromagnetic properties of the water. The GPR measurements provided maps of the bathymetry and of the bottom reflection amplitude. The high reflectivity of the riverbed, derived from the GPR interpretation, agreed with the results of the direct sampling campaign that followed the geophysical survey. The variability of the bottom reflection amplitudes map, which was not confirmed by the direct sampling, could also have been caused by scattering phenomena due to the riverbed clasts which are dimensionally comparable to the wavelength of the radar pulse. About the multi-frequency electromagnetic sensor, we analyzed the induction number, the depth of investigation (DOI) and the sensitivity of our experimental setup by forward modeling varying the water depth, the frequency and the bottom sediment resistivity. The simulations led to an optimization of the choice of the frequencies that could be reliably used for the interpretation. The 3406 Hz signal had a DOI in the PO water (27 Ωm) of 2.5m and provided sediment resistivities higher than 100 Ωm. We applied a bathymetric correction to the conductivity data using the water depths obtained from the GPR data. We plotted a map of the river bottom resistivity and compared this map to the results of a direct sediment sampling campaign. The resistivity values (from 120 to 240Ωm) were compatible with the saturated gravel with pebbles in a sandy matrix that resulted from the direct sampling, and with the known geology