Acoustic Logs

The acoustic log exploration technique includes those techniques that use a transducer to transmit an acoustic wave through the fluid in the well and surrounding elastic materials. Several different types of acoustic logs are used, based on the frequencies used, the way the signal is recorded, and the purpose of the log. All these logs require fluid in the well to couple the signal to the surrounding rocks. There are four main types: acoustic velocity, acoustic waveform, cement bond, and acoustic televiewer.

Acoustic logs are used to determine the lithology and porosity of the rocks surrounding the well. This information can be helpful for determining future well locations and potential areas for well bore stimulation. The log when combined with other logs run provides the gbasis for a detailed analysis pr lithologies, alteration, stratigraphy, etc.

• Cement Bond Log
• Cross-Dipole Acoustic Log
• Single-Well And Cross-Well Seismic

The geophysical/well logging service company conducts the down hole logging operation and produces both digital and hard copy logs. The Drilling contractor trips the drill pipe and bit and conditions the well bore for logging.

Most acoustic-velocity probes employ magnetorestrictive or piezoelectric transducers that convert electrical energy to acoustic energy. Most of the transducers are pulsed from 2 to 10 or more times per second, and the acoustic energy emitted has a frequency in the range of 20 to 35 kHz. Acoustic probes are centralized with bow springs or rubber fingers so the travel path to and from the rock will be of consistent length. Some of the energy moving through the rock is refracted back to the receivers. The receivers reconvert the acoustic energy to an electrical signal, which is transmitted up the cable. At the surface, the entire signal may be recorded digitally for acoustic waveform logging, or the transit time between two receivers may be recorded for velocity logging. Amplitude of portions of the acoustic wave also may be recorded; that technique is described later under waveform logging.

Probes are constructed of low-velocity materials, producing the shortest travel path for the acoustic pulse through the borehole fluid and the adjacent rocks, which have a velocity faster than that of the fluid.

An unstable well bore (sluffing, wash outs, etc) can be of concern in any well logging operation. In extreme condition, the loss of the logging tool down hole could possibly result in the loss of the hole and would require premature abandonment or the necessity to side track to complete the well drilling operation.

Well logging is a standard operation associated with the drilling permit approval and is included in the downhole analysis of the drilling program.

EPA-Environmental Geophysics-Geophysical Methods-Acoustic Logging