## 3 Methods
### Seismic Refraction by Streamer  
The seismic refraction method, due to its versatility, is one of the most commonly used geophysical methods in engineering, mining, groundwater exploration and environmental investigations. Based on favourable density contrasts that generally exist between geological materials, the refraction method is used to provide detailed information on the distribution and thicknesses of subsurface layers with characteristic seismic velocities. Overburden and basement rocks may be classified to some degree to discriminate for example, surficial sediments from consolidated soils or highly fractured rock from competent rock. This technique is widely used for the assessment of pipeline cable burial and for the assessment of river dredging. Operations involve laying out a seismic streamer with several pressure-sensitive hydrophone receivers (usually 24 or 48), at the takeout points on the streamer. Hydrophone spacing is strongly dependent on the depth of search and the desired resolution for a given survey (Figure 2). A pattern of shot points is then executed within and off the ends of the cable and the seismic wave arrivals for each hydrophone are recorded in the seismograph. The key piece of recorded information is the time of the first arrival. This arrival is the direct wave, or more commonly, the refracted wave which occurswhen seismic energy propagates along a geological interface having a sufficiently great velocity contrast. This contrast must consist of a higher velocity zone underlying a lower velocity zone, fortunately the most common geological condition.  
Interpretation of the seismic data involves resolving the number of velocity layers present, the velocity of each layer, and the travel time taken to travel from a given refractor up to the ground surface. This time is then multiplied by the velocity of each overburden layer to obtain the thickness of each layer at that point.  
According to Equation (1) [29], the method consists of laying a receiver array (generally a streamer made of 2 \(\times\) 24 hydrophones spaced 5 m) on the riverbed and performs shots with an air gun seismic source at specific locations: central shot, in the centre of the streamer, near shots, at both extremities of the streamer, in close position to hydrophones 1 and 48.  
\[\frac{sin\theta_{1}}{V_{1}}=\frac{sin\theta_{2}}{V_{2}}=p \tag{1}\]  
\(\theta_{1}\) is the incident angle, \(\theta_{2}\) the angle of refraction, \(V_{1}\) and \(V_{2}\) are, respectively, the velocity of the first and second layer, \(p\) is called the ray-path parameter.  
The acquisition boat carries positioning and navigation systems, the recording system and a radio link for the source triggering. The acquisition boat is moored close to the end of the array to ease connections with the seismic recorder. The operation boat carries air-gun, compressed-air bottle and streamer. The streamer is deployed and recovered by hand and its position is recorded at three to five locations marked with surface buoys. The contact of the streamer with the riverbed is ensured through small dead weights. Air gun is deployed close to riverbed. Shots are performed successively at each of the specific locations. Both ends of streamer (i.e., position of hydrophones 1 and 48) and each shot are positioned using DGPS positioning at each mooring line (Figure 2).  
Figure 2: Cont.