Investigation of Flow Condition in the Haftbarm Lake Using Acoustic Tomography Technology

Flow velocity measurement is an important hydrological factor in dam reservoirs, lakes, river and coastal planning/management, control of water resources and environmental conservation. A wide variety of techniques such as Current Meter and tracking float methods have been used to measure flow velocity. However, it is very difficult to measure cross-sectional average velocity in unsteady flows or during extreme hydrological events, such as flooding. Therefore, establishing the method and technology for flow velocity measurement is a crucial issue. Acoustic tomography systems offer powerful technique for measuring the flow velocity in oceans, seas, rivers and lakes. Ocean Acoustic Tomography (OAT) employs high-powered signals with frequencies less than 1 kHz to measure meso scale ocean currents. Coastal Acoustic Tomography System (CATS) applies OAT to coastal waters. Because CATS transmits signals at frequencies up to 10 kHz, it can be used in smaller water areas. To use this technique in shallow aquatic environment and in the shorter ranges as much as hundred meters, these systems must transmit sound at much higher frequencies. As a result, Fluvial Acoustic Tomography System (FATS) uses a second-generation - CATS that transmits sound at a frequency of 30 kHz to measure flow velocity and water temperature. Although, FATS is widely used to measure flow features in rivers and estuaries, it has never been used in the lakes. This study shows the result of first acoustical tomography experiment in one of the freshwater Haftbarm Lakes, located western part of Shiraz. This study shows the first acoustical tomography experiment in a lake to measure the flow velocity. Reciprocal sound transmissions were performed between the two acoustic stations located diagonally on both sides of the lake during the period of July 7, 2017. The air temperature ranged from 32 °C to 33 °C and there was not meaningful wind. Sound pulses of the FATS were simultaneously transmitted from transducers every 40 second at a timing synchronized with a GPS clock. The length of sound transmission line was 262 m and the central frequency was set to 30 kHz. The velocity data was successfully collected. FATS uses travel-time tomography approach. Based on the arrival time of acoustic signals at the upstream and downstream stations, the sound speed and flow velocity along the sound ray path are computable. To accurately identify the arrival time of a traveling acoustic signal mixed with noise, the transmission signal was phase- modulated by applying a pseudo-random sequence called an M-sequence. The FATS transmission signal was modulated with a 9th-order M-sequence (511 digits). A three cycles per digit (Q-value) was also selected as a suitable value to transmit the phase-modulated sound from the broadband transducers. The arrival times of the acoustic data were 176.96 and 177 msec at station 1 and station 2, respectively. Therefore, the differential of -0.004 msec was observed. The minus value shows the existence of an insignificant flow to the station 1. The results of calculations showed the speed of sound in water was about 1481 m/s during the experiment period. The flow velocity was estimated as much as 1.5 cm/s. Since, the flow velocity resolution of FATS is 0.1 cm/s, the measured value was meaningful. However, other instruments such as mechanical current meter cannot measure the flow velocity under the 3 cm/s. Therefore, the results of acoustic tomography experiment did not compare with other methods. There are two possibilities for observing the flow in the lake: 1) the effect of wind on the Lake Surface or 2) alluvium aquifer recharge where is located under the lake. Continuous measurements of the flow velocity were conducted in a shallow lake using Fluvial Acoustic Tomography System (FATS), a state-of-the-art acoustic system. The FATS was equipped with a couple of 30-kHz broadband transducers with horizontally omnidirectional and vertically hemispherical beam patterns which can be used to estimate the cross-sectional average velocity from multiple ray paths that cover the cross-section of a water body. This study shows the recharge of alluvium aquifer located under the lake. This outcome is confirmed by the previous studies that investigated about the hydrogeological situation of the aquifer located under the lake. In conclusion, this study shows the possibility of determination of ground flow inflow/outflow (discharge/recharge) into or out of the lakes.