SONIC VELOCITY/VARIABLE DENSITY
Sound waves traveling through rock will have a certain velocity and attenuation that is a function of the rock’s properties. Such properties as density, rigidity, porosity, saturation and fracturing will cause sound to be altered. The sonic log can be very useful in understanding rock formations by measuring these resulting waves. Information concerning how tight the rock is, whether it is saturated, fractured, or if the formation is above the regional water table, can be understood with this method. Combining this with resistivity and gamma-ray logs can aid in understanding rock properties as a greater whole than merely the individual logs.
SNC/VDL Sonic Velocity/Variable Density
• DT Delta Time (∆t): A measurement of the travel time of the first arrival (known formally as P-wave arrival)
• VDL Variable Density Log: A graphic presentation of the full wave recorded at a receiver concerning the depth
• ITT Integrated Transit Time: The integration of the ∆t wave over a distance traveled between two corresponding depths, usually represented by marks scaled as a millisecond between depth intervals.
• SPOR Sonic Porosity: If the matrix velocity of the formation is known, using Wyllie’s equation, log porosity can be derived.
The sonic tool needs to be centralized and conducted in fluid-filled boreholes of water or drilling mud. In shallow wells, a pressure gradient can affect the logs along with borehole diameters. Lithological correlation is possible with drill cuttings, core, or other geophysical measurements.