Chapter 4: Sonic Velocity in Carbonate Sediments and Rocks
Compressional wave and shear wave velocities (VP and Vs) of 295 carbonate minicores from different areas and different ages were measured under variable confining and pore fluid pressures. The samples range from unconsolidated carbonate mud to completely lithified limestones. The results of the measurements show that, unlike siliciclastic and shaly sediments, pure carbonate rocks show little direct correlation between acoustic properties (VP and Vs) and depth of burial or age. Velocity inversions with increasing depth are thus common. Sonic velocity in carbonates is more controlled by the combined effect of depositional lithology and diagenetic processes, such as cementation and dissolution.
At 8 MPa effective pressure, VP ranges from 1700 to 6500 m/s and Vs from 700 to 3400 m/s. These ranges are caused mainly by variations in the amount of porosity and porosity type and not by variations in mineralogy. Measured velocities generally show a positive correlation with density and an inverse correlation with porosity, but departures from general trends can be as high as 2500 m/s. These deviations can be explained by the occurrence of different pore types that form during specific diagenetic phases. Commonly used correlations such as Gardner's law (VP versus density) and the time-average equation (VP versus porosity) should be modified because they often result in velocities that are too low for carbonates.
Velocity measurements in unconsolidated carbonate mud at different stages of experimental compaction show that velocity increases due to compaction are lower than observed velocity increases at decreasing porosities in natural rocks. This difference shows that diagenetic changes such as cementation and dissolution, which may predate or accompany compaction, influence velocity more than simple compaction at increasing burial depth.