A typical value of V p is 1480 ms -1 . Velocity depends primarily on temperature, less on pressure, and very little on salinity. In practice the last 2 terms in the above equation are often ignored. V p is highly variable. As noted above, V p for salt water is approximately 1480 ms -1 .
V p varies between different sediment types and rocks.
Furthermore, the general empirical rule applies that the compressional wave velocity increases in step with density in similar material types. For example, sediment generally becomes more dense with depth of burial, hence their compressional wave velocity also increases with depth. The interaction of acoustic waves with boundaries in the sub-surface is explained in the diagram below. The majority of these surveys are conducted by and for the hydrocarbon exploration industry, in the search for oil and gas reserves. Therefore, the understanding of the above principles are fundamental to the subsequent three lectures.