Well integrity analysis using sonic measurements over depth interval

A system comprising:

• a sonic logging tool configured to perform sonic measurements along a length in a multi-string wellbore to obtain sonic data for multiple locations along the length in the wellbore;
• and
• a processor configured to (i) process the sonic data to obtain phase slowness dispersions for the multiple locations in the wellbore;
• (ii) aggregate the phase slowness dispersions to obtain aggregated phase slowness dispersions;
• (iii) compare the aggregated phase slowness dispersions with a plurality of cut-off mode templates in order to identify the presence of cut-off modes or the lack thereof in the aggregated phase slowness dispersions;
• and (iv) identify features of the multi-string wellbore based on presence of the cut-off modes or lack thereof.

A method of investigating a multi-string wellbore having a plurality of casings and a plurality of annuli, the method comprising:

• performing sonic measurements along a depth interval in the wellbore using a sonic tool to obtain sonic data for multiple locations along the depth interval in the wellbore;
• processing the sonic data to obtain indications as a function of depth in the depth interval of at least one of an energy spectrum, an attenuation dispersion projection, a slowness dispersion projection, and a wavenumber dispersion projection;
• and
• identifying a shift in at least one of the indications at a particular depth in the depth interval, wherein the shift represents a transition in the makeup of material in at least one annulus of the plurality of annuli.