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In acoustic logging, acoustic (sound) energy is transmitted by an acoustic transmitter. The transmitted acoustic energy is received by an acoustic receiver. Acoustic transducers that convert electrical energy into mechanical energy and mechanical energy back into electrical energy are used for this purpose. Acoustic transducers in current use are piezoelectric and magneto-strictive types.
The acoustic transmitter is briefly pulsed by a pulse of electrical energy and in turn, creates an acoustic impulse. The pulsing continues at rate called the repetition rate or “rep” rate. Repetition rates may be once or twice per second. A transmitted acoustic waveform travels in all directions from the transmitter. After a short period of time related to the formation materials and distance traveled, the acoustic waveform arrives at a receiver. The receiver converts the arriving acoustic (mechanical) energy into electrical energy that is measured within the probe or transmitted electrically up hole to the surface electronics for processing.
Two primary acoustic methods are in use today. The two methods are referred to as the “reflection method” and the “transmission method”.
Systems that use the reflection method direct acoustic energy horizontally toward the borehole wall. The reflected signal is measured by the same transducer. Borehole Acoustic Televiewer systems fall into this category.
Systems using the transmission method transmit acoustic energy in all directions into the surrounding formation. One or more receivers at fixed distances and acoustically isolated from the transmitter(s) measure the arriving acoustic signal.
Note: Acoustic isolation is used in the probe design to prevent coupling of acoustic energy through the tool.
Several types of probes are in use today. The most basic is a single transmitter, single receiver probe. Increasing complexity is seen in the single transmitter, dual receiver probe and finally, the dual transmitter, dual receiver probe.
Single transmitter, single receiver probe: View probe
This probe has a single receiver at 3 ft. or 5 ft. spacing from the transmitter. The single receiver probe has a disadvantage in rugose holes or when not properly centralized in the borehole. It is often used in amplitude measurement in cement bond logging operations.
Single transmitter, dual receiver probe: View probe
The dual receiver probe offers an advantage over the single receiver probe in that formation velocities are more accurately obtained. Accuracy is increased by virtue of the two receiver velocities or delta-time measurements when subtracted reveal the actual formation travel time. Given the time for acoustic energy to travel from transmitter thru borehole fluid, to the far receiver including travel thru the borehole fluid a second time and a second shorter path to the near receiver, when the two travel times are subtracted, the result is the actual formation travel time over the distance between the two receivers. The borehole travel times are cancelled arithmetically by subtraction.
Dual transmitter, dual receiver probe: View probe
A further improvement involves adding an additional transmitter below the two receivers. Measuring and canceling borehole effects in both directions and having an average of two measurements helps eliminate errors due to borehole rugosity and tool eccentricity in the borehole. This system is referred to as Borehole Compensated (BHC) in the literature.
WAVES OF INTEREST:
The transmitter transducer generates an alternating expansion and contraction that propagates into the surrounding borehole and formation in all directions. The propagated waveform is predominately a 20 KHz harmonic wave.
Formation wave: Formation waveforms
As the waveform travels into the surrounding formation, the compression and shear components traveling at different velocities are changed in relative time. The compression (P) waves move ahead in advance of the shear (S) waves and later (L) borehole and mud waves. Ultimately these waveform components arrive at the receiver and are measured.
Composite wave: Composite waveform
A composite waveform representing the “acoustic wave train” arrives at the receiver. The first arrival of the Compression wave (P) arrives followed by the remaining compression waves. Before the end of the compression wave arrivals, the shear waves arrive. Overlapping occurs resulting in interference. Finally the later (L) waves arrive before the end of the shear wave arrivals again causing interference.
Casing waveform: Casing waveform
When logging in free casing, the transmitted pulse causes ringing of the pipe resulting in a relatively high amplitude persistent dampened waveform.
CASING BOND LOG:
When used for casing bond logging operations, a time gate is generated that tracks the amplitude of the first arrival of the casing waveform. The amplitude is much greater in free casing. This amplitude establishes a reference for no cement bond referred to a zero percent bond. The tool is lowered to a position in the well where 100 percent bond is assured and the lower amplitude is referenced as indicating 100 percent bond
ACOUSTIC LOGGING SYSTEMS:
Using any of the tools mentioned above, a timing sequence is initiated by a transmitter pulse. Time is measured using electronic circuitry from the transmitter pulse (referred to as T0) to the “first arrival” of an acoustic waveform at the receiver. The first arrival is usually considered to be the first negative half-cycle of the receiver waveform. Delta-t is determined by division of the elapsed time by the number of feet from transmitter to receiver. Borehole compensated systems use two transmitters and two receivers that perform a subtraction of delta-t that cancels the effect of borehole size and rugosity and results in formation travel time only.
INTEGRATED TRAVEL TIME:
Delta-t measurements can be integrated (added with depth) to provide geologists with cumulative formation travel times. Integrated travel time data can be used to improve the accuracy of seismic systems.
MEASUREMENT OF AMPLITUDE:
Again, using any of the acoustic tools mentioned previously, a sequence of transmitting and receiving an acoustic signal is initiated by an acoustic pulse transmission. Amplitude of a desired portion of the acoustic waveform is measured using a timing gate that is placed at the appropriate point in time during the received waveform. Applications of amplitude measurement include cement bond logging in cased holes.
RADIAL BOND TOOLS:
Bond logging tools that have segmented receivers arranged around the radius of the tool give improved accuracy. A tool having eight receivers can measure the signals reflecting from 45 degree radial arcs. This increases the accuracy of cement bond logging.
ULTRASONIC IMAGER TOOL (USIT):
One service company provides a tool having a 200 – 700 KHz Ultrasonic capability. The receiver sensor is rotated 360 degrees as tool is pulled uphole. A log of reflected acoustic energy is created that measures the quality of casing to cement bond, and casing condition.