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Sonic logging is a technique used in drilling operations to analyze underground rock and soil formations with sound waves. Oil or gas exploration and recovery uses a drilling rig that creates a deep hole called a borehole, using a rotating drilling tool attached to long sections of pipe. The drilling head creates a hole with a diameter equal to the drill head size.
A sound-producing tool is attached to a powered wire and sent down the borehole to create a sonic logging graph. This tool consists of a sound transmitter and receiver co-located on a long tube that fits in the borehole. The transmitter sends out a series of high-frequency sound pulses in all directions that enter the surrounding rock formations and return to the receiver.
To prevent the transmitter and receiver from interfering with each other, a number of different techniques are used. The transmitter and receiver are separated by distance, creating a longer cylinder shape. Sound-absorbent materials and rubber gaskets can help reduce some of the sound from the transmitter reaching the receiver. The most important design element is based on shutting off the receiver each time the transmitter sends a pulse. This prevents false signals in the sonic logging results, and prevents the transmitted sounds from damaging the receiver.
The transmitter sends sound pulses in short bursts, which enter the rock surrounding the borehole; some of the sound reflects quickly back to the receiver, and some enters the surrounding rock and is diffracted, which means it changes direction from the outgoing sound. As the diffracted sound returns to the receiver, the time difference between the transmitted and returning sound is recorded. Another effect of sound travel in the ground is attenuation, which is a reduction of sound due to absorption. As the sound enters the rock around the borehole, the rock and other materials absorb the sound, reducing the amount of signal returning to the receiver; this in turn can provide information about the characteristics of the ground.
Sonic logging is effective for determining the characteristics of a borehole because sound travels differently depending on the rock or soil surrounding the transmitter. The first sounds to return to the receiver are p-waves, or pressure waves, because they typically have the highest velocity, or speed. P-waves will travel faster in higher density rock, and slower in less dense sand or soil, which is called more porous.
The second type of sound waves to return to the receiver is S-waves, or shear. A shear force wants to tear something apart, so these waves are measuring the formation for its ability to shear or break. This is important in petroleum drilling, because the formation containing the oil or gas must be broken apart before the product can be recovered; this is called fracking. The S-waves will provide information used in this operation.
When the sonic logging tool is sent down a borehole, it is providing a visual representation of the sub-surface characteristics. Fractures in the rocks can help drilling operations if they occur in the area of the product, but can cause problems if found elsewhere in the borehole, which may have to be sealed with piping or a concrete-like sealant to prevent leakage from the hole. Water can also be a problem for drilling operations, since it will mix with the product; if water enters the borehole in any large quantities, it may require additional processing later to remove it from the petroleum. Another concern is contamination of ground water with petroleum, so understanding where water layers exist can reduce environmental concerns.
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