Evaluation of the thickness of bottom silts in the water area of the city pond using GPR (Latvia)

Georadar scanning works in the water area of ​​the city pond were carried out in order to assess the thickness and reserves of bottom sapropel silts. The work was carried out from the ice by laying arbitrarily oriented profiles along the most characteristic directions of possible changes in the bottom topography, taking into account the availability of profiling areas in conditions of melting ice. Profiling was performed using a dipole antenna with a zonding pulse frequency of 150 MHz. Profile geodetical positioning was carried out using a portable GPS receiver mounted directly on the GPR antenna.

    Scheme of location of georadar profiles in the water area of the pond

The location of the profiles was chosen taking into account their minimum deviation from the bottom silt sampling points available at the site with a probe.

An example of a GPR section

In the GPR section below, recalculated to a depth scale, reflective boundaries can be traced in the upper part, corresponding to the bottom of the reservoir, the beds of weak organogenic and plastic mineral silts, as well as the surface of moraine deposits. The position of the boundaries is consistent with the sampling intervals for the corresponding categories of silts. At a depth of about 12 m, a strong reflective boundary can be traced, corresponding to the surface of the Devonian bedrock.

Thickness map of organogenic sapropel silts

Based on the correlation of three reflecting boundaries confidently traced within the entire water area, maps of the surfaces of weak organogenic and plastic mineral silts, as well as the surface of moraine deposits, were constructed. Based on this, maps of the thicknesses of organogenic and mineral silts were calculated.

Map of the total thickness of organogenic and mineral silts

The obtained thickness maps make it possible to estimate by calculation the reserves of both productive sapropel silts for their development, and the total volumes of silts for the dredging project in order to clean up the water area and create an urban recreation area.

Longitudinal section of bottom silts

The line of the longitudinal section through the entire water area is shown in blue on the above maps of bottom silt thicknesses. The section line of the estimated thickness of the silts is assigned taking into account its location at the minimum distance from the bottom sampling points. In most cases, the boundaries between layers of silts with different physical properties constructed from GPR data correlate quite well with bottom soil sampling data.

Estimation of pollution of the bottom layer of silts

In the below section of root-mean-square amplitudes (RMS) of the reflected signal, recalculated from the georadar section, areas of a sharp increase in the amplitudes of the reflection of an electromagnetic pulse are clearly manifested. To a large extent, the increase in the reflectivity of silts is associated with the presence in their composition of an increased content of organic residues and oil products, which are characterized by low values ​​of the dielectric constant and form a sharp jump in the electromagnetic impedance for the radiated pulse. Below on the map of the water area, yellow and red colors indicate areas of alleged pollution by industrial and domestic wastewater from buildings located near the coast. The position of the profile in the EW direction is shown on the map in red.