Transient electromagnetic flaw detection and casing wall thickness measurements in multi-string wells

The transient electromagnetic method is the most effective in the studying of multi-string wells. First, the non-stationary field is measured after the current is turned off (without the primary field) that reduces the coil spacing to zero, i.e. combine the generator and receiver coils. This significantly improves the vertical characteristic of the probe, allows the probe to register small defects. Second, the non-stationary mode allows separating signals from the first, second, third, etc. strings much easier than the harmonic mode. It improves the accuracy of thickness estimation and defect detection. The paper proposes an algorithm for interpreting the measurement results.