شناسایی پرتوهای ویژه با روش تئوری پرتو در مخزن سد لتیان به کمک سامانه پرتونگاری مقطعی صوتی

پرتونگاری مقطعی صوتی شاخه ای از دانش سنجش از دور است که با اندازه گیری زمان طی شده امواج صوتی در آب، قادر است متغیرهای محیط آبی را اندازه گیری کند. اولین گام برای استخراج نیمرخ حرارتی مخزن سد با روش پرتونگاری مقطعی صوتی، اولین گام شناسایی پرتوهای ویژه و شبیه‌سازی آن‌ها در مخزن سد است. در این مطالعه به شبیه‌سازی الگوی انتشار امواج صوتی با تئوری پرتو در مخزن سد لتیان پرداخته شد. برای این منظور از داده‌های عمق‌سنجی و سی‌تی‌دی (هدایت-دما-عمق) برای شرایط مرزی، سرعت صوت، نیمرخ دما و شوری در مخزن سد استفاده شد. از دو ایستگاه صوتی پرتونگاری مقطعی صوتی با بسامد 10 کیلوهرتز و 30 کیلوهرتز به فاصله 1.6 کیلومتر برای پرتونگاری مقطعی صوتی استفاده شد. زاویه انتشار نسبت به افق (سطح آب) از منفی 20 تا مثبت 20 درجه با اختلاف 0.1 درجه رهگیری شدند. با توجه به توپوگرافی مخزن، سه پرتو صوتی با زاویه های 0.9، 14 و منفی 20 درجه از بین 36 پرتو رهگیری شده انتخاب شد. نتایج نشان داد که الگوی رهگیری شده در مخزن سد لتیان با اختلاف 4 تا 8 میلی‌ثانیه زمان پیمایش پرتو صوتی را درست تخمین زده است. سه پرتو انتخابی نشان داد که پرتوی که شکست کم‌تری داشته و برخورد کم‌تری به کف مخزن و یا سطح آب دارد سریع‌تر به ایستگاه دوم خواهد رسید و طول کم‌تری خواهد داشت. همچنین تغییر بسامد از 10 کیلوهرتز به 30 کیلوهرتز در زمان پیمایش پرتو صوتی هیچ اثری نداشت. از نتایج این مطالعه و پرتوهای ویژه شناسایی شده، برای محاسبه طول پرتو طی شده جهت استخراج نیمرخ حرارتی در مخزن سد به کمک سامانه پرتونگاری مقطعی صوتی استفاده می‌شود.

identification of special rays using the ray theory method in the latian dam reservoir using coastal acoustic tomography system

extended abstract introduction various measurement tools are used to monitor water environments such as oceans, seas, rivers, and dam reservoirs. acoustic tomography is one of the branches of remote sensing, which is a powerful tool for monitoring the characteristics of water resources, such as water temperature and velocity in different layers of water depth (in various water environments such as the ocean, sea, river, and dam reservoir). this technology is used with the aim of obtaining information from a desired area, without any interference in the characteristics of that area. in acoustic tomography systems with at least two devices, the waves are sent through the transmission part on both sides of the water environment. due to the presence of two important boundaries of the bottom of the bed and the water surface, these waves propagate in the entire water depth. by calculating the travel times of each sound ray sent to the opposite station, the average sound speed is calculated for each path. these calculations are used to understand the temperature and flow rate changes as a function of water depth in a water environment such as a dam reservoir. the basis of the work of layering the water environment in depth to monitor changes in temperature and flow speed is the way sound rays propagate in that water environment. amplitude-independent ray simulation is performed using conductivity-temperature-depth (ctd) data on the transmission line.methodology experiments and measurements were carried out in the reservoir of latian dam, which is located on the jajroud river. in this study, two experiments were conducted with sound frequencies of 10 khz and 30 khz. in each experiment, two acoustic tomography stations were deployed in the dam reservoir. the approximate distance between the two stations was 1617 meters. ctd and bathymetry data were collected. the time of data collection was the 22 and 23 of october 2020. no valve of latian dam was open during data collection. the sound ray propagation pattern in the dam reservoir can be well approximated by the ray tracing method that only considers sound refraction (snell’s refraction law). snell’s law describes the refraction of sound waves in an environment where the speed of sound in separate horizontal layers varies with depth (reflection of sound rays in an environment with variable speed). transmission losses and mirror reflections on the water surface and reservoir bed can also be included in this method. for this purpose, we first interpolate and draw the depth measurement data with a distance of 0.1 meters. then we interpolate the ctd data including depth, temperature and salinity to a distance of 0.1 m. after obtaining the depth, temperature and salinity in every 0.1 meter depth, using the mckenzie relationship, the average depth reference sound speed is obtained. ray simulation identifies transmitted rays (special rays) that are valuable for peak detection and for solving the tuned inverse problem.results and discussion it was done by processing the data obtained from acoustic tomography in the latian dam reservoir and measuring the ray traveling time from one to two and two to one station. the correlation plot (signal-to-noise-travel time) was plotted for the data sent from the one-to-two and two-to-one stations with a frequency of 10 khz. considering the distance of 1617 meters between the two stations and the approximate sound speed of 1470 meters per second in water (considered as an approximation according to ctd data collection), the first peak travel time should occur in the range of 1100 milliseconds. therefore, the plots were plotted between 1080 milliseconds and 100 milliseconds after that. it was observed that the first peak occurred for the data in the range of 1100 ms.to accurately measure the travel time (arrival time) of the first peak that will be used in the selection of the special ray, the first peak (the largest peak) was identified for each data and the travel time of the first peaks was calculated and plotted against the data acquisition time. due to the fact that the travel time of the sound ray and the speed of sound in water are related to the temperature gradient the measurement was done with the frequency of 10 khz and 30 khz on two different days.400 rays launched from the first station were intercepted. 36 rays reached the second station. the results of the simulation of sound wave propagation patterns were investigated and validated by using ctd and by measuring the travel time of the waves. it can be seen that the difference between the travel time simulated by the ray theory method and the travel time obtained from the acoustic tomography and the calculations of the first peak is in the thousandth of a second, which is very insignificant and shows the accuracy of the simulation.conclusion the propagation pattern of sound waves in the latian dam reservoir, which is considered shallow and fresh water, was intercepted, and as a result, three special rays were identified and validated. the results of this ray tracing were evaluated using the acoustic tomography system. the results showed that this model can simulate different sound paths with different propagation angles as well as the travel time of sound waves. changing the frequency has no effect on the radiation pattern in the dam reservoir, but it affects the intensity of the signal.as a result, the radiation interception in the dam reservoir depends on the topography of the reservoir, water level, and bed as boundary conditions and thermal stratification and temperature, salinity, and depth. finally, according to the propagation of rays in the entire depth of the tank, it was observed that the location of the transom should be placed near the water level of the tank, considering the shape of the cross-section of the reservoir.