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4ТЕХНОГЕННАЯ ТРАНСФОРМАЦИЯ ПРИРОДНОЙ СРЕДЫ
УДК 504 Giambattista Piazzese
Graduated - Master of Science in Environmental Engineering - Polytechnic of Turin (Italy)
e-mail: gpiazzese90@gmail.com
A SUB-BOTTOM PROFILER SURVEY ON THE SIRIO LAKE
In the present article, through an integrated geophysical and geological study, we intend to determine the thickness of sediments deposited on the bottom of Lake Sirio (Ivrea - TURIN), through geophysical surveys carried out with a Subbottom Profiler (Seismic Sediment Profiler). This survey was also carried out in order to highlight any geological discontinuity, which may possibly indicate an interaction between the waters of the lake in question and sub-river sources that feed the lake itself, since superficially it does not have tributaries. In addition to the geophysical surveys, measurements of electrical conductivity and water temperature of the lake were performed in order to find a variation of these parameters, which could indicate the presence of a recharge area of the reservoir by underground sources.
In the first part of the elaboration, the geology of the area in question is introduced, based on authoritative studies conducted in the past, relative to the geology of the complex called the Morainian Amphitheater of Ivrea (following, MAI).
Subsequently, the discussion continues with the analysis of geophysical data for the estimation of the sediment thickness of the bottom of the basin, trying to correlate the results obtained by the seismic profiler, with some results of studies previously carried out on the Sirio or on geologically lakes. analogues, belonging to the same morainic amphitheater.
Keywords: Sub-bottom profiler, Morainian Amphiteater of Ivrea, thickness of sediments
The lake Sirio, the surface water body object of the integrated geophysical and geological study of the present work of Thesis, belongs to the geological complex called "Morainic Amphitheater of Ivrea" (hereafter referred to as MAI). [6] It is a complex of morainic imposing circles that owes its origin to the mechanical action of the Dora Baltea glacier, exerted by the Valle d'Aosta during the
tangible proof of the extension that the glacialism developed in Valle d'Aosta and surrounding areas has reached in the Quaternary: as regards its area extension, the terminal moraines of the MAI are up to about 120 kilometers from the current glaciers of Mont Blanc and extend up to about half the width of the Po Valley between the Alps and the Monferrato (Fig. 1).
Pleistocene glaciations. This complex represents the most
(C)
Figure 1. Map of the main geomorphological sectors of the Amphitheater of Ivrea, Piedmont, Italy (from Gianotti,
2008 - modified)
© Giambattista Piazzese, 2018
As described in the scientific literature [6], in relation to some studies concerning MAI (F. Gianotti, 2007), from a geomorphological point of view, this morainic amphitheater is characterized by four sectors: the complex of morainic circles, the internal depression, the rocky reliefs of exaration and the external fluvioglacial plane. It is right to point out that glacial genesis deposits should be considered and classified according to the process that generated them (melting processes, till-tilling, till-tilling, deformation, etc.), to the sedimentation environment (depending on whether this is aquatic or subaerial) and their position relative to the glacier
(supraglacial, subglacial, endoglacial, marginal or proglacial). The studies found in the literature attest that each of the four geomorphological sectors mentioned manifests forms and deposits intrinsically linked to a certain position with respect to the glacier.
A complex of morainic circles like that of the MAI [6], is the most suitable place to favor the genesis of forms of marginal position: its constituent elements (mainly the moraines and the so-called kame terraces) have in fact formed on the surface at the edge of the glacier.
Figure 2. Geographical and geological framing of the MAI (from Gianotti, 2008)
Figure 3. Geological section of the MAI (from Gianotti, 2008)
As shown in the geological section in Fig. 3, Lake Sirio has been formed on granulites, and is separated from the Dora Baltea always by granulitic material. The granulitic facies is formed under particular conditions of pressure and temperature (at depths of 15 ^45 km, temperatures of 700 ^ 900 ° C and pressures of 0.4 ^ 1.2 GPa). Therefore the area of interest in which it is located the lake must reasonably have formed in conditions of high pressures and high temperatures and medium to high depths.
Lake Water Electrical conductivity
The specific electrical conductivity of Lake Sirio, detected on the day of 28/04/16, assumes values ranging from a minimum of 213 ^S / cm to a maximum of 219 ^S / cm. For the first 5 meters from the free surface the conductivity is maintained constant with a value of 213
and then rises of about 0.5 ^S / cm per meter up to 15 m of depth. Just the stretch from 5 to 15 m is that in which the conductivity change more abruptly while it stands between 218 and 219 ^S / cm up to 22 m, depth to which the relief ends.
Lake Water Temperature
The water temperature of Lake Sirio detected on 28 April 2016, starting from the free surface up to a depth of 22 m, assumes along the water column values ranging from a maximum of 14.9 ° C on the surface, up to at a minimum of 5.7 ° C at a depth of 22m. The lake shows the phenomenon of the "thermocline" between 5 and 8 m of depth; in this stretch the temperature passes abruptly from 13.5 ° C to 7.4 ° C, while over 8 m it starts to have a much weaker gradient tending to reach 6 ° C.
We tried to cover the surface of the lake in a more homogeneous way, to highlight possible peculiarities of the bottom relative to the thickness of the sediments and the identification of possible discontinuities. For geo-
Specific Electrical C'oiiductrvitylfjS/cm]
212 213 214 215 216 217 218 219 220
referencing a GARMIN GPS receiver of the 421 series was used (Fig. 7c). Fig. 8 shows a photograph of the instrumentation installed on the boat used.
Figure 4 - Survey of specific electrical Conductivity
Figure 5 - Survey of Temperature (28 April 2016)
Fig. 6 Profiles of the survey on the lake on the day of 28 April 2016.
Fig. 7 a) Laptop for real-time display of the survey b) Graphic output of the survey in real time c) Garmin GPS receiver installed on the sub-bottom S A-Rl AT1R2T21
-
A-Tl \ / A-T1R2 \ / Sabbia
/ Granulite ; . ... . ... •••.•
Fig. 8 Sub-bottom profiler (immersed in water) and GPS installed on the boat used
Fig. 9 Schematic example of the transmi
nd reflection of
the pulse emitted by the Sub-bottom to the different interfaces
The subbottom profiler is an instrument that transmits an acoustic impulse which, in correspondence with a discontinuity in the elastic properties of the material, is partly reflected and partly transmitted. The reflected signal is received by the transducer which in turn sends it via the hardware unit to the display program in the connected laptop. The operating principle is therefore based on the reflections that originate at the interfaces between materials with different speed of sound propagation and density. The system, to create a seismic section, uses the energy reflected by the interfaces where there is a variation of acoustic impedance Z = d • V [kgm-3 m-s-1 = kg-m-2s-1] being the density of the material and V the propagation velocity of the compression waves in the material. To clarify the following considerations, in addition to the definition of acoustic impedance Z, it is advisable to use the diagram in Fig. 9, where A is assumed equal to 1, while T and R are respectively coefficient of Transmission and coefficient of Reflection
The method used for the reliefs allows to reconstruct the bathymetry with a good detail working at high frequencies, while at low frequencies it is possible to penetrate deeply to define the lithostratigraphic sequences present. The depth of the survey depends on the frequency used, on the presence or absence of vegetation on the bottom and on the type and granulometry of the sediments; in fact, the presence of coarse sediments induces the phenomenon of scattering of the acoustic wave, consequently causing an attenuation of the signal. The relief with subbottom profiler allows to obtain the measurement of the sediment thicknesses on large areas of seabed, and to know the original topography of the bottom of the reservoir. Moreover, it is easily applicable in a context of certain georeferencing and allows the postprocessing of information acquired with specific software
(in this case the Reflexw 5.0 software was used). The instrument used during the surveys is a Sub Bottom profiler model "Bathy 2010PCTM CHIRP". CHIRP is a system based on the emission of a frequency modulation pulse, that is the emission of a "controlled waveform". The signal of the instrument used consists of several cycles of sinusoidal waves. For these measurements, the instrument has been used in CW (Continuous Wave) mode. In the case in question two transducers were used, both acting both from sources and from receivers for the reflected signal. Possible problems may be caused by the presence of air bubbles near the transducers. The frequencies found in the received signals related to the measurements made, vary from 2 to 6.7 kHz and the wavelengths I in play range from 0, 3 to 0.5 m. This datum is important, because if the signal emitted encounters objects (geological formations) having dimensions of an order of magnitude comparable to the wavelength, there are problems of legibility of the thickness of the layer. The purpose of the following elaboration will be to highlight the separation surface, more or less regular, between layers with different acoustic impedance. Moreover, with respect to the raw data (Raw Data), multiple reflections (or reflected echoes - Fig. 10) that occur and have the following characteristics have been eliminated as far as possible:
- The reflection is constantly localized at a depth double compared to the first reflector (bottom of the lake);
- The false reflector follows the morphology of the fund, but amplifies its slopes;
- Continuous and evident signs are observed even at high depths;
- If the acoustic impedance contrast is very high, multiple reflection can be repeated several times at regular intervals.
4
Fig. 10 - Schematization of the problem of multiple reflections (reflected echoes). a) seismogram with multiple reflections; b) seismogram without multiple reflections - modified by "Bathymetric Monitoring - Stratigraphic: methodology of direct survey of the thickness of the sediments in the reservoirs by means of Subbottom Profiler"
Bazzoffi P., Bassignana A [1].
In Fig. 11, the possible situation due to the nonoptimal opening of the subbottom profiler emission cone is reconstructed; that is, since the aperture of the emission cone is a function of the frequency of the transducer employed, it typically manifests irregularities in the shape
of a "side lobe" (sidelobe - Fig. 11) which may disturb the echo. Depending on the opening of the cone of emission of the instrument and of the irregularities of the bathymetry, it is therefore possible to investigate in a more or less precise way the area under examination. In
some cases, it is not possible to have sufficient resolutive between two different geological formations from the power and therefore an adequate ability to distinguish point of view of acoustic properties.
Fig. 11 - Example of the variation of the sidelobe as a function of the frequency from "Monitoring Bathymetric -Stratigraphic: methodology of direct survey of the thickness of the sediments in the reservoirs by means of Subbottom
Profiler", Bazzoffi P., Bassignana A [1].
Besides the dependence on the frequency at which the measurements are made, we can find ourselves in unclear situations in which there are two distinct reflections as if there were two different layers, whereas in reality this can occur when the subbottom emission cone investigates a
measurements in correspondence of strong bathymetric irregularities
Now, we pass to make under exam the result of our survey after many operations to clean the raw data, specifically:
1. Time Cut - Over 80 ms, there was substantially noise. Investigation depth is about 60 m
2. Energy Decay - Recovers the decay of energy due to various attenuations
3. Bandpass Filter - Between frequencies of 2 kHz e 7 kHz to exclude from frequency band analyzed the noise
4. Muting - Only attenuates the sediments ^ resets the signals above the background reflection.
For example, we can observe Profile 12 as representative sample.
In profile 12 (Fig. 13), it is noted that for the first ten meters of the relief, up to about 90 m, there is no material with low acoustic impedance. Probably in this stretch emerges a very reflective material that does not allow the optimal penetration of the signal emitted from the sub -bottom in depth. In the central depression, there is the presence of a layer of sediments of about 2 meters thick, which gradually becomes thinner, the more the bottom rises in altitude. Around 300 m, there is no longer the
clear separation between the two materials and we tried to define it with a manual picking operation (red line that divides the different material with different acoustic impedance). Also in this case there is the probable residual noise despite the muting operation.
Fig.13 - Profile 12 in reference at Fig. 6 (Vertical exaggeration 8.57)
Conclusions
Following the elaborations carried out, it emerges that from the methodological point of view, the instrument is suitable for the determination of the thickness of lake sediments for the study in question.
From the analysis of the profiles, Lake Sirio appears to have substantially a bedrock of granulitic material, on which it places a "mantle" of fine sediments that varies from a minimum of a few centimeters to a maximum of 2 meters. However, the frequencies used for the relief allow, as mentioned, to have a vertical resolution of about 15 cm while, using instruments that investigate at higher frequencies, the resolution increases and can become of the order of centimeter.
Normally, sediment dating occurs through the analysis of Radon isotopes as a function of their decay time. According to some studies [4] (Facchinelli et Alii, 2005), which dealt with the sedimentation speed of Lake Sirio, it results to have a speed of about 0.5 mm / year. Therefore, in the last 2.000 years, about one meter of sediment would have been deposited. In this case, from the measurements made with the sediment profiler, there are maximum thicknesses of 2 meters and therefore, it results in a considerably lower sedimentation rate, since
very irregular bathymetric zone, with locally elevated slopes (Fig. 12).
Fig.12 - Possible problem of the emission cone due to
the origin of the lake is attributable to about 12000 years ago, coinciding with the withdrawal of the Wurm glaciers. This may reasonably be due to the total lack of solid transport due to the absence of Sirio tributaries. Other studies on geologically similar lakes at Sirio [9] (Lami et Alii, 2000, which dealt with Lake Candia) report speeds of about 0.3 mm / year, more in accordance with the measures carried out in this work.
With regard to future developments, it could be interesting to investigate the inside of the sediment layer by means of a survey that reaches the granulite and acoustic instruments and higher resolution to study the variations of acoustic characteristics of the material, internal to the same layer.
References
1. Bazzoffi P., Bassignana A. (2011) Bathymetric Monitoring - Stratigraphic: methodology of direct survey of the thickness of the sediments in the reservoirs by means of Subbottom Profiler, Regione Basilicata, pp.18-22
2. D.L. Bell, W.J. Porter, A.S. Westneat. (1983) Progress in the use of acoustics to classify marine sediments - Submarine Signal Division, Raytheon Company, Porthsmouth Rhode Island, pp. 357
3. E. Hamilton, 1972, Compressionalwaveattenuation in marine sediments, Geophysics, Vol. 37, NO. 4 pp. 620-646
4. Facchinelli A., Magnoni M., Perroni U., Sacchi E., (2005). Post-depositional processes in lake sediments traced by heavy metals and radionuclides: a case study from Lake Sirio. (Ivrea, Northern Italy) - RMZ - Materials and Geoenvironment, Vol. 52., N.1 pp. 31-33
5. Federico Sacco (1938). The Piedmontese Glacialism, - L'Universo, 16, Florence, pp. 337-352.
6. Franco Gianotti (2007). The Morainic Amphitheater of Ivrea, pp. 80-142
7. Gianotti F., Grosso F., Forno M. G., Pini R. (2015). Stratigraphy of the cataglacial sequence in the "Collid'Ivrea" area and preliminary pollen of the Chiaverano lacustrine deposits, Italian Journal of Quaternary Science, pp. 213-228, ISSN 0394-3356.
8. John E. Ehrenberg, Thomas C. Torkelson (2000) .FM slide (chirp) signals: a technique for significantly improving the signal-to-noise performance in the hydroacous assessment system - Fishery Research 47, Elsevier Science.
9. Lami A., Marchetto A., White R., Appleby P.G., Guilizzoni P., (2000). The last ca. 2000 years palaeolimnology of Lake Candia (N. Italy): inorganic geochemistry, fossil pigments and temperature time-series analyses - J. Limnol 59 (1), pp. 31-46.
10. Pietro Azario, (1970). De Bello Canapiciano: The war in the Canavese, Lions Club, Ivrea, pp. 217-231.
11. Piedmont Region, Water Resources Planning Department (2004). Studies and studies aimed at preparing the WATER PROTECTION PLAN (Legislative Decree 152/99).
12. Roberto Romeo (2009) .Doctor Thesis - High resolution Integrated Geophysical study of marine deposits and substrate structure of the Miramare Riviera (Gulf of Trieste), PhD in Environmental Sciences, Trieste, pp. 197.
13. Sambuelli L., Bohm G., Capizzi P., Cardarelli E., Cosentino P. (2011). Comparison between GPR measurements and ultrasonic tomography with different inversion algorithms: Nanjing Geophysics Research Institute, UK, ISSN 1742-2132.
14. Geological Service of Italy. Illustrative notes of the Geological Map of Italy scale 1: 100,000, sheets 56 and 57 Turin - Vercelli.
15. General variant 2003 Town Plan for the Municipality of Chiaverano (TO), www.comune.chiaverano.to.it/.
УДК 504.54
О.В. Абросимова, А.А. Макарова O.V. Abrosimova, A.A. Makarov
Саратовский государственный технический Yuri Gagarin State Technical University of Saratov
университет имени Гагарина Ю.А. 77 Politechnicheskaya str., Saratov, 410054
410054, г. Саратов, ул. Политехническая, 77
e-mail: ecology.saratov@gmail.com
ЭКОЛОГИЧЕСКИЙ АНАЛИЗ ПОЧВЕННЫХ РАЗНОСТЕЙ УРБОСИСТЕМ Г. САРАТОВА
Проведена комплексная оценка различных ландшафтных структур города Саратова. Проанализированы кислотно-щелочные свойства, содержание гумуса в почве и суммарная концентрация тяжелых металлов. Установлена взаимосвязь полученных показателей от мест расположения точек пробоотбора, их функциональной и ландшафтно-структурной характеристики.
Ключевые термины: ландшафтная структура, антропогенное воздействие, техногенное загрязнение, почвенные структуры, урболандшафтное районирование, промышленная зона, суммарная концентрация
ECOLOGICAL ANALYSIS OF THE SOIL DIFFERENCES OF THE URBAN SYSTEMS OF
SARATOV
We evaluated the different landscape structures of the city Saratov. We analyzed the acid-alkaline properties, the content of humus in soil and the total concentration of heavy metals. We established the relationship between the
© Абросимова О.В., Макарова А.А., 2018
