Key acquisition parameters are described. Recommended values are by no means definitive and a certain degree of variation (e.g., 20 %) can be always expected.
Receiver Array: A linear receiver array deployed parallel to the road is used. Although it does not have to be close to the road, it is recommended that the offline distance (between the array and the road center) be maintained fairly constant (for example, within ±30%) throughout the entire survey. Dimension (D) of the array should be at least equal to or greater than the maximum depth of investigation (Zmax): D = mZmax (1 ≤ m ≤ 3). Then, receiver spacing (dx) is determined by number of channels (N) available approximately as dx=D/N. It is usually a rounded number (e.g., 1-m, 4-ft, etc.). Once dx is determined, then it determines the shallowest depth investigated (Zmin) roughly as Zmin=pdx (1/3 ≤ p ≤ 1.0). In any case, more channels are an advantage that can increase the resolution of dispersion processing. Although forty-eight (48) channels are most desirable for a survey aiming at Zmax = 100 m, because of the 1-D nature of the array a 24-channel acquisition often may be used with dx = 5 m or so.
Recording Parameters: A sampling interval of 4 ms (dt = 4 ms) and total recording time of 10 sec (T= 10 sec) are most recommended for an urban survey near major highway(s). Total recording time (T) is determined in such a way that there is at least one occurrence of passive surface wave generation during recording. Therefore, it can be reduced or increased depending on local situations related to the surface wave generation. In case of surveys near roads, for example, there should be a vehicle passing near the survey area at least once during recording. The longer T is not always better. This is because the chance of recording surface waves generated at different locations (azimuths) on the road increases as well and it will generally degrade the data processing resolution unless those locations are well apart in azimuth (for example, 90° or more). If the main source point is fixed in location, however, the longer T will be better as multiple generations from the same point will constructively contribute to the dispersion imaging process. A fixed source point of major surface waves is observed when you hear a jolting sound coming from nearly the same spot (azimuth) on the road as vehicles pass over. Vertical stacking during recording is strongly discouraged (unless T is significantly limited) because it will increase the chance of recording multi- azimuth surface waves, as previously explained. Instead, saving individual records separately is recommended. Then, the dispersion imaging process can be applied to individual records and then these image records can be vertically stacked together to enhance the definition of the image. However, if T is limited to a shorter time (e.g., < 5 sec) by the recording device, then a few times of vertical stacking (e.g., stacking 3-sec recording three times) may be necessary either during recording or post-acquisition stage. If dynamic range is a controllable option with a recording device, choosing the highest value, along with the highest gain available, will always be a benefit.