Sonic logging

Sonic logging is a well logging tool that provides a formation’s interval transit time, designated as Δ t {displaystyle {Delta }t} , which is a measure of a formation’s capacity to transmit seismic waves. Geologically, this capacity varies with lithology and rock textures, most notably decreasing with an increasing effective porosity. This means that a sonic log can be used to calculate the porosity of a formation if the seismic velocity of the rock matrix, V m a t {displaystyle V_{mat}} , and pore fluid, V l {displaystyle V_{l}} , are known, which is very useful for hydrocarbon exploration. Sonic logging is a well logging tool that provides a formation’s interval transit time, designated as Δ t {displaystyle {Delta }t} , which is a measure of a formation’s capacity to transmit seismic waves. Geologically, this capacity varies with lithology and rock textures, most notably decreasing with an increasing effective porosity. This means that a sonic log can be used to calculate the porosity of a formation if the seismic velocity of the rock matrix, V m a t {displaystyle V_{mat}} , and pore fluid, V l {displaystyle V_{l}} , are known, which is very useful for hydrocarbon exploration. The velocity is calculated by measuring the travel time from the piezoelectric transmitter to the receiver, normally with the units microsecond per foot (a measure of slowness). To compensate for the variations in the drilling mud thickness, there are actually two receivers, one near and one far. This is because the travel time within the drilling mud will be common for both, so the travel time within the formation is given by: where t f a r {displaystyle {t_{far}}} = travel time to far receiver; t n e a r {displaystyle {t_{near}}} = travel time to near receiver. If it is necessary to compensate for tool tilt and variations in the borehole width then both up-down and down-up arrays can be used and an average can be calculated. Overall this gives a sonic log that can be made up of 1 or 2 pulse generators and 2 or 4 detectors, all located in single unit called a “sonde”, which is lowered down the well.