WELLOG ACOUSTIC LOG INTERPRETATION

Part III, Page 1

INTRODUCTION:

Acoustic logging began with experiments reported as early as 1948. Methods involving velocity logging occurred in the early 1950’s with determination of porosity from velocity in 1956 by Wyllie et al. Other, later developments in acoustic logging applied to amplitude measurement and the effect of amplitude in casing cement bond in 1960. By 1964 it was observed that in wells containing bond from casing to cement to formation provided formation data through the casing. In 1967 – 1968 acoustic pictures of the borehole wall were developed. Borehole compensation of acoustic logging systems was developed in the early 1970’s. Further advancements in waveform presentation and computer use in the late 1970’s and early 1980’s gave rise to modern acoustic logging. Improved circuitry using microcomputer and solid-state technology has given multi-frequency acoustic logging and other improvements in borehole acoustic televiewer logging.

PRINCIPLES OF ACOUSTICS:

The velocity and attenuation of sound waves are greatly affected by the formations they pass through. As a consequence, the use of sound waves has proven valuable in determining many properties of formations and the fluids contained within them. In fact, waveforms are complex, and many sources consider them to contain more information than can be interpreted!

PARTICLE MOTION:

All objects that vibrate have an equilibrium position and are subject to a force called a restoring force that tries to return the object to its equilibrium position. Elasticity causes a substance to resist deformation and to return to its original shape. These substances are referred to as being “elastic”. Temporary displacement in elastic materials cause an oscillating deformation after the deformation is released. This oscillating motion called a “wave” continues until the equilibrium position is reached. The motion has “frequency” which refers to the number of oscillations in a unit of time and “wavelength” which refers to the distance between similar points from one wave to another.

Time and frequency are inversely related.

F = 1/T

Wavelength is based on velocity of the waveform within a given medium.

Velocity (V) = distance/time

Velocity (V) = F * l

l = V/F

Where: l is the symbol for wavelength

F is frequency in Hertz abbreviated Hz

T is Time

V is velocity

Example: Compressional wave velocity in a water saturated unconsolidated sandstone has a velocity of 4200 ft./s.

What is the wavelength of a 100 Hz waveform in this material?

Solution: Wavelength = 4200 / 100 = 42 feet.

As particles are displaced, energy is transferred to other particles. Acoustic energy is propagated through an elastic medium because of continued transfer of acoustic energy. The continued propagation is called an “elastic wave” or “acoustic wave”.

ACOUSTIC WAVES:

A wave front of acoustic energy moves outward from the source of the disturbance at a certain velocity. Waveform velocity is a function of the physical properties of the medium through which it is moving. The amplitude of the waveform, which is a measurement of the height from top to bottom peaks decreases with distance. Though the original disturbance has ceased, the acoustic wave continues to propagate for a short time. This property is called persistence.

WAVE TYPES:

Particle propagation and displacement are perpendicular causing shear waves and compression waves.

In acoustic logging, a waveform is transmitted from a tool centered in a borehole. The wave passes through the borehole fluid to the wall of the borehole. Waveform velocities are greater in the formation than through the fluid. Both compression (P) and Shear (S) head waves are propagated through the interface back to an acoustic receiver. The first waveform arrivals at the receiver are the P and lower velocity S waves.

The composite receiver waveform is the sum of low amplitude head waves, and higher amplitude normal mode waves. Tube waves and normal mode waves are often referred to as fluid waves. Stoneley waves and tube waves have lower velocity and occur later in the composite waveform.

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