The Journal of the Acoustical Society of Korea (한국음향학회지)

The Acoustical Society of Korea (한국음향학회)
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5-MHz Volume Backscattering Strength Measurements from Suspended Sediment Concentrations

5 MHz 신호를 이용한 부유물의 농도에 따른 후방산란강도 측정

  • Lee, Changil (Department of Marine Sciences and Convergent Technology, Hanyang University) ;
  • Choi, Jee Woong (Department of Marine Sciences and Convergent Technology, Hanyang University)
  • 이찬길 (한양대학교 해양융합과학과 해양음향연구실) ;
  • 최지웅 (한양대학교 해양융합과학과 해양음향연구실)
  • Received : 2012.08.06
  • Accepted : 2012.11.10
  • Published : 2013.01.31
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Abstract

The erosion, suspension, and transport of sediment frequently occur in the coastal waters and estuarine. These processes often generate the so-called fluid mud layer, which is defined as a high-concentration aqueous suspension of fine grained sediment (> 10 g/l), consisting mainly of silt and clay-size particles. Therefore the high-resolution ultrasound is mostly used to detect or monitor the fluid mud layer. Because the sound attenuation tends to increase rapidly with the suspended sediment concentration, it is necessary to consider the accurate attenuation correction to estimate the backscattering strengths from the suspended sediment layers. In this paper, the volume backscattering strengths with various suspended sediment concentrations were measured using 5-MHz ultrasound signal in a small-scale water tank. The sound attenuation due to the viscosity and scattering from suspended sediment particles was predicted by the Richard's model and applied to the sonar equation to estimate the volume backscattering strengths from the suspended sediment concentrations. For the case that the additional attenuation was not considered, the volume backscattering strengths increased to the concentration of 20 g/l, and over this point, the backscattering strengths were roughly constant. However, for the case that the attenuation due to the suspended sediment concentration was considered, the backscattering strengths increased with the concentration.

연안 및 하구지역은 퇴적물의 침식, 부유, 이동이 활발하게 이루어지며, 이로 인해 유성머드와 같은 고농도의 부유퇴적물 환경이 조성된다. 유성머드는 주로 실트, 점토와 같이 매주 작은 입자들로 이루어져 있는 10 g/l 이상의 고농도 환경으로써, 유성머드의 분포 및 특성 파악을 위해 초음파와 같이 분해능이 우수한 높은 주파수 특성을 이용한 연구가 필요하다. 음파가 부유퇴적물에 의해 산란될 때, 부유퇴적물의 농도가 높아질수록 후방산란강도의 세기가 증가하지만, 점성 및 산란에 의한 감쇠도 함께 증가하기 때문에 농도에 따른 올바른 감쇠계수 보상이 요구된다. 본 논문에서는 5 MHz 주파수 특성을 바탕으로 간이 수조에서 농도에 따른 가상의 유성머드 환경을 조성한 뒤, 후방산란강도를 측정하였다. 또한 Richards의 감쇠계수 모델을 이용하여 부유물의 특성 및 농도에 따른 감쇠계수를 계산하였고, 이를 후방산란강도를 계산하는 소나방정식에 적용하여 부유물 농도에 따른 후방산란강도를 예측하였다. 그 결과 부유물에 의한 감쇠를 보상하지 않았을 경우 부유물 농도 증가에 따라 후방산란강도 값이 함께 증가하는 경향을 보이다가 일정 농도(20 g/l) 이후부터는 부유물의 농도가 증가하여도 후방산란강도가 더 이상 증가하지 않았다. 반면, 농도에 따른 감쇠계수를 보상해 주었을 경우는 부유물 농도 증가에 따라 후방산란강도가 연속적으로 증가하였다. 이러한 결과는 고농도의 부유퇴적물 환경에서 음파에 의한 산란강도를 정확히 예측하기 위해서는 부유물 농도에 따른 감쇠효과 고려가 필요함을 보여준다.

Keywords

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