The sonic or acoustic log measures the travel time of an elastic wave through the formation. This travel time information can also be used to derive the velocity of elastic waves through the formation.
Use: Its main use is to provide information to support and calibrate seismic data and to derive the porosity of a formation.
Basic terms in acoustic problems
Period
The period of the wave is defined as the duration of one cycle, and is generally measured in microseconds.
Frequency
This corresponds to the number of complete cycles per second and is measured in Hertz (Hz). 1 Hertz = 1 cycle/second. Frequency is the inverse of the period, hence f = 1/T.
Wavelength
This is the distance travelled in one cycle by a wave front. It is equal to the ratio of the propagation speed ( v ) and the frequency ( f ) i.e., \(\lambda = v/f\)