Sonic Logs first appeared in 1957. Sonic logs rely on the properties inherent in Snell’s Law to propagate sound from a logging tool through the rock to receivers located on the same logging tool.
Sonic logs require a fluid filled borehole to operate properly. Modern logs can make most measurements in both open and cased holes. Open-hole logging refers to logging operations that are performed on a well before the wellbore has been cased and cemented. This means that the logging is done through the bare rock sides of the formation. Cased-hole logging involves retrieving logging measurements through the well casing, or a metal piping that is inserted into the well.
Basic Principle of How Logs Work
Sonic logging tools emit a sound pulse every second. The arrival of this pulse is detected at an array of receivers a few feet from the transmitter. The difference in time elapsed between the arrival of sound at the receivers is the desired travel time, the sound wave travels through the formation while undergoing dispersion (spreading of the wave energy in time and space) and attenuation (loss of energy through absorption of energy by the formations). Essentially the basic principle of these sonic logging tools is to measure the travel time of sound through rock. Newer generation logging tools can use a cross correlation of waveforms to determine this travel time. As well as the compressional wave that is detected in the first described cross correlation methods also detect the shear, Stoneley and mud waves.
Formations are commonly split into two categories, fast and slow. Fast formations are a rock in which the shear velocity is faster than the compressional velocity in the fluid or mud, when the shear velocity is slower than the compressional velocity it is classified as a slow formation. In addition, subsurface rock formations typically exhibit elastic anisotropy. This anisotropy is typically in the form of vertical and/or horizontal transverse isotropy (VTI/HTI).
Energy Sources for Acoustic Logs
Tools today typically operate in the 5 – 10kHz frequency range. In our Sonic Logging Examples we use two different types of excitation method, monopole (Figure 1) and a Dipole (Figure 2) source.
The difference between the two is that Monopole sources emit sound uniformly in all directions, commonly referred to as axisymmetric or radially symmetric sources. At the critical angle the energy is refracted so that it travels parallel to the borehole inside the rock, the energy is refracted back into the borehole and is recorded at the receivers. Sound velocity is the inverse of slowness, which in the formation is the difference in time between the arrival time of the refracted sound energy at the receivers.
The dipole source exerts a differential pressure on one side of the transmitter element which creates a flexural wave in the borehole. Flexural waves are dispersive but at low frequencies they travel down the borehole at the formation shear velocity. Receivers must be calibrated to detect this flexural wave. If the wavelength of the flexural wave is at least 3x the diameter of the borehole, the wave travels near the formation bulk-shear velocity. However, because this is a dispersive mode, if the wavelength is shorter, this flexural mode will travel slower than the shear velocity and dispersion corrections are needed.
Acoustic Transmission Modes
Monopole sources can generate both body and surface waves. Dipole sources can only generate surface waves. Body waves refer to the waves that travel in the formation. Surface waves are waves that travel on the wall of the borehole or bounce between transmitter, wall and receiver.
- Fast compressional waves, longitudinal or P-waves are the fastest acoustic waves and are the first waves to return to the receiver. It has been used successfully for years as a porosity indicator.
The velocity of this wave doesn’t vary much with frequency. The spectrum of the wave depends on the source excitation frequency. Sonic logging tools usually operate in the 3-15kHz range.
- Surface compressional waves, also referred to as the leaky compressional wave follow the fast-compressional waves. The amplitude of the surface wave varies with the formation Poisson’s ratio. This wave is dispersive, meaning low frequencies travel faster than higher frequencies
- Shear body waves are generated by conversion of the compressional fluid wave when it refracts into the rock from thee wellbore. This refracted wave returning is referred to the shear head wave.Monopole sonic logs can not detect these waves in a slow formation because refraction cannot occur. Just like the compressional waves generated, the shear waves can be used to predict porosity
- Stoneley waves are guided waves generated by a monopole source. Their arrival time is typically after the shear wave. The annulus between the tool and the wall of the borehole acts as a waveguide for Stoneley waves.
The amplitude of the Stoneley wave is dependent on the permeability of the formation. Higher permeability absorbs more energy.
Note: Not all modes of transmission have been covered in this article