Abstract：
A cased well model consists of a coaxial tank and casing, which houses coaxially installed transmitting and receiving coils. The transmitting coil is excited by the current produced by the transmitting circuit, and transient electromagnetic responses occur in the casing, including direct coupling and casing responses. As the range between the transmitting and receiving coils increases, direct coupling responses decay rapidly, are less than the casing response at 0.3 m, and disappear at 0.7 m. By contrast, a casing response increases rapidly and then declines slowly after reaching a peak and changes little within a specific range. The peak decreases slowly with range. The continuous addition of water to the tank causes slight changes in transient electromagnetic responses, so the difference which are subtracted from the response without water is used. Moreover, the differences at the time of rapid increase in response and the time of rapid decrease in response are large, forming a peak and a trough. Given that the conductivity of water in a full tank changes after the addition of salt, the difference in the peak is linear with the increase in the conductivity of water. This result provides an experimental basis for the design of a transient electromagnetic logging instrument that measures the conductivity of formation in cased well
2025, Vol. 22 
