The best source for seismic reflection is not always the most practical. Parameters such as cost, size, access, surface materials, and depth of penetration must all be taken into account. As always, a sledgehammer always supplies the most bang for the buck when practical. If the ground surface is too soft for a sledgehammer, you might consider a downhole seisgun. Small explosives are ideal in terms of portability and power, but for obvious reasons are often not feasible. Less portable sources like weight drops and vibrators should be considered when access allows and required depth of penetration exceeds that of a sledgehammer or seisgun. Although depth of penetration varies widely depending on the geology and cultural noise, you can expect to see somewhere in the range of 0.25 - 0.5 seconds with the latter two sources.