Sonic Log - Engineering Geology - Lecture Slides, Slides of Genetic Engineering

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THEORY OF MEASUREMENT:

• Transmitter emits sound waves into the formation and measures the time taken to detect at a receiver of known distance from the transmitter.
• The Sonic tool operates at 20 cycles per second as sounds similar to a pedestrian crossing at a set of traffic lights.
• The first arrival is the compressional or 'p' waves, which travel adjacent to the borehole
• It is this arrival that is used to measure the individual travel times T1, T2, T3, and T4. Two receivers for each transmitter eliminates the borehole signal.
• The transit time DT is computed from these travel times as shown in the equation below.
• This particular arrangement of sonic tool transmitters and receivers is known as the standard BHC Sonic tool and compensates for borehole washouts and also for tool tilting in real time while logging.

DTLOG = [(T1 - T2) + (T3 - T4)] / 2

APPLICATION:

• Porosity PHIS
• Volume of clay V (^) S
• Lithology
• Time-depth relationship
• Reflection coefficients
• Mechanical properties
• VDL/CBL
• By combining sonic and Checkshot data we can calibrate down hole log data with surface seismic data.
• Mechanical properties can be determined from the shear and compressional waves, fracture identification from shear and Stoneley waves and permeability indication from Stoneley waves

Presentation

• Presentation is usually 140-40 us/ft (or 500-100 us/m) across tracks 5 & 6