Multi-casing evaluation using multi-frequency, non-collocated, induction measurements

A system comprising:

• a plurality of nested casings disposed in a well;
• an electromagnetic (EM) inspection tool disposed in the plurality of nested casings, wherein the EM inspection tool comprises a transmitter and a plurality of non-collocated receivers, each of the plurality of non-collocated receivers being located at a different spacing with respect to the transmitter;
• and
• a control system communicatively coupled to the EM inspection tool, the control system comprising:
• one or more memories collectively storing instructions;
• and
• one or more processors coupled to the one or more memories, the one or more processors being collectively configured to execute the instructions to cause the control system to perform an operation comprising:
• emitting, from the transmitter of the EM inspection tool, a primary time-varying magnetic field signal towards the plurality of nested casings, the primary time-varying magnetic field signal inducing a corresponding one or more secondary time-varying magnetic field signals in the plurality of nested casings, and the one or more secondary time-varying magnetic field signals being detected by one or more of the plurality of non-collocated receivers;
• obtaining, using the EM inspection tool, a set of induction, multi-spacing, multi-frequency measurements of the plurality of nested casings;
• and
• determining at least one of a number of the plurality of nested casings or a respective size of each casing of the plurality of nested casings, based on the set of induction, multi-spacing, multi-frequency measurements,
• wherein the set of induction, multi-spacing, multi-frequency measurements comprise, for each of the plurality of non-collocated receivers, a respective set of frequency domain responses to the primary time-varying magnetic field signal and the one or more secondary time-varying magnetic field signals,
• wherein determining the respective size of each casing of the plurality of nested casings comprises determining the size of a first casing of the plurality of nested casings based on the respective set of frequency domain responses for at least one first non-collocated receiver of the plurality of non-collocated receivers, and
• wherein the spacing of the at least one first non-collocated receiver is such that the respective set of frequency domain responses for the at least one first non-collocated receiver is based on a section of a frequency band where the primary time-varying magnetic field signal is greater than the one or more secondary time-varying magnetic field signals.