Sonic logging analyzes subterranean rocks and soil formations using sound waves. A transmitter sends high-frequency sound pulses into the rock, and a receiver records the returning sound. This technique helps determine the characteristics of a well and is important in oil drilling for fracking. Sonic recording provides a visual representation of the subsurface, aiding drilling operations and reducing environmental concerns.
Sonic logging is a technique used in drilling operations to analyze subterranean rocks and soil formations with sound waves. Exploration and recovery for oil or gas uses a drilling rig that creates a deep hole called a borehole, using a rotary drilling tool attached to long sections of pipe. The drill head creates a hole with a diameter equal to the size of the drill head.
A sound producing instrument is connected to a powered wire and sent down the well to create a sound recording graph. This instrument consists of a sound transmitter and receiver placed together on a long tube that fits into the hole. The transmitter sends out a series of high-frequency sound pulses in all directions which enter the surrounding rock formations and return to the receiver.
To prevent the transmitter and receiver from interfering with each other, several techniques are used. The transmitter and receiver are separated by distance, creating a longer cylindrical shape. Sound absorbing materials and rubber gaskets can help reduce some of the sound from the transmitter reaching the receiver. The most important design element is to turn off the receiver each time the transmitter sends a pulse. This prevents false signals in the sonic recording results and prevents transmitted sounds from damaging the receiver.
The transmitter sends out pulses of sound in short bursts, which enter the rock surrounding the well; some of the sound quickly reflects back to the receiver and some enters the surrounding rock and is diffracted, meaning it changes direction relative to the outgoing sound. When the diffracted sound returns to the receiver, the time difference between the transmitted sound and the returning sound is recorded. Another effect of sound propagation in the ground is attenuation, i.e. a reduction of sound due to absorption. As sound enters the rock around the well, the rock and other materials absorb the sound, reducing the amount of signal returning to the receiver; this in turn can provide information on terrain characteristics.
Sonic recording is effective for determining the characteristics of a well 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 fastest speed or velocity. P waves will travel faster in higher density rock and slower in less dense sand or soil, which are called more porous.
The second type of sound waves to be returned to the receiver are S, or cut, waves. A shear force wants to tear something apart, so these waves are measuring the formation for its ability to cut or break. This is important in oil drilling because the formation containing the oil or gas must be broken up before the product can be recovered; this is called fracking. S waves will provide the information used in this operation.
When the sonic recording instrument is sent down a borehole, it provides a visual representation of the characteristics of the subsurface. Fractures in rocks can aid drilling operations if they occur in the product area, but can cause problems if found elsewhere in the hole, which may need to be sealed with piping or a cement-like sealant to prevent hole leakage. Water can also be a problem for drilling operations, as it will mix with the product; if the water enters the well in large quantities, further processing may be required later to remove it from the oil. Another concern is groundwater contamination with oil, so understanding where water layers exist can reduce environmental concerns.
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