The sonic or acoustic log was developed in the 1950's to provide a detailed record of acoustic velocities along a well trajectory. If interval travel times and depth intervals corresponding to those travel times were recorded, then a velocity depth profile could be constructed. This profile could then be used to convert seismic events, recorded in two-way travel times, to images that were plotted as a function of depth.
It soon became apparent that the sonic travel times could also be used for other purposes, such as porosity estimation and gas detection -as well as lithology assessment when used together
with density and neutron tools. Most important of all, the sonic tool can be used to evaluate mechanical properties of the rock when used in conjunction with the density tool.
As a porosity tool, the sonic wireline device was often run in combination with the SP-Gamma Ray-Resistivity tool string. Because this combination of tools was slick, and did not require pad contact, it was generally the first set of tools to be run in the hole during a log run. As such, the sonic log often provided the first indication of porosity during the log run. With advances in seismic prospecting and improvements in acoustic logging, the sonic tool is now enjoying a resurgence in seismic applications.
This discussion focuses primarily on how the sonic tool is used to evaluate porosity, and briefly touches upon mechanical properties and seismic applications. For information on other sonic applications, refer to the following IPIMS discussions on borehole velocity measurements and borehole visualization:
Discipline Series Topic Subtopic
Petroleum Geophysics 3-D Seismic and Other Geophysical Methods Other Geophysical Techniques Borehole Velocity Measurements Formation Evaluation Wireline Well Logging Borehole Imaging Borehole Imaging Technology
PRINCIPLES
Two types of body waves travel within the formation:
Compressional waves, or P-waves, are waves of compression and expansion in which small particle vibrations occur in the same direction the wave is traveling. The compressional wave can propagate through both solids and fluids. P-wave data is acquired by conventional sonic tools for evaluating formation porosity.
Shear waves, or S-waves, are waves of shearing action in which rock particle motion is perpendicular to the direction of wave propagation. Only in a solid medium that has rigidity can the motion of the particles perpendicular to the wave propagation be accommodated. Hence the shear wave can only exist in solids and not in fluids. This is because solids have shear strength while liquids do not. Shear data is used in such applications as rock mechanics, formation anisotropy, permeability, and formation fluid evaluation.
The speed of P-and S-waves is controlled by rock mechanical properties, such as rock density and elastic dynamic constants. In fluid-saturated rock, these properties depend on the amount and type of fluid present, the composition of rock grains, and the degree of inter-grain cementation.
Because soft, loosely consolidated rock exhibits smaller elastic stiffness, sound waves will travel slower in soft rock than in hard rock.
In acoustic logging, an acoustic pulse -produced by alternate expansions and contractions of a transducer -is emitted by a transmitter. A typical pulse of this sort is shown in Figure 1 (Typical transmitter pulse; courtesy of Schlumberger Well Services).
Figure 1
Part of the acoustic energy traverses the mud, impinges on the borehole wall at the critical angle of incidence, passes along the formation close to the borehole wall, reenters the mud, and arrives at a receiver, where it is converted into an electrical signal (Figure 2 Signal generated at the receiver by various wave arrivals; courtesy of Schlumberger Well Services).
Figure 2
The sonic tool has at least one pair of transmitters and receivers, as depicted in Figure 3:
Acoustic Pulse Recording in a Borehole. A magnetostrictive alloy or piezoelectric crystal with a resonance frequency between 5 to 20 kHz is used as material for these transducers.
Figure 3
The transmitter sends out pulses with an oscillatory waveform that generates either compressional or shear waves.
The compressional (P) wave generated by the transmitter in the borehole fluid will travel in all directions until it hits the borehole wall. At the borehole wall the P-wave will continue in the rock as a fast P-wave, but some of the P-wave energy at the wall will be converted to a shear (S) wave in the rock. (Although both waves will expand in all directions from the point of impact, only the path along the wall is sketched in the above graphic of the Acoustic Pulse Recording. The wave traveling along the borehole wall will continuously produce compressional waves back into the borehole as indicated by the small arrows.
However, the velocity of the wave-front in the formation will out-run the P-waves created in the borehole because the P-wave velocity of the formation is higher than velocity of the borehole fluid.
The P-wave that travels the shortest distance through the mud will be the first one to arrive at the receiver (known as the first arrival).
The shear (S) wave front travelling along the borehole wall will also create secondary P-waves in the fluid, since a fluid can only sustain compression waves and has no shear strength. As a result, there is a continuous conversion of S-waves back into P-waves along the borehole wall. The shortest P-and S-wave paths will not be identical, due to refraction of the waves on the borehole wall. According to Snell’s law, the slow S-wave will refract less to the normal than the fast P-wave.
As depicted the above graphic, the P-wave which represents the converted S-wave will arrive later than the leading P-wave ("first arrival" or "first break") which represents the P-wave velocity of the formation.
This first P-wave arrival is what triggers the sonic tool to record. The tool transmits about ten pulses per second, and the time is measured between the transmission and the first arrival. The actual parameter measured is the reciprocal velocity, called travel time (t), expressed for convenience in microseconds per foot.
With the velocity V, expressed in feet per second and t in sec, the following relation is valid:
V
= 10
t
6
The velocity of the compressional wave depends on the elastic properties of the rock matrix and the fluids in the pore space. The measured travel time is therefore a function of the rock matrix, the fluid type and the porosity.