A method, comprising: conveying a tool string into a wellbore, the wellbore at least partially lined with a first casing and a second casing concentrically overlapping a portion of the first casing, wherein a first annulus is defined between the first and second casings and filled with a first cement, and wherein a second annulus is defined between the second casing and the wellbore and filled with a second cement; obtaining wellbore data from a pulsed neutron sensor (124; 702) included in the tool string and optical fiber measurements from one or more optical fibers included in a cable positioned in the wellbore; determining a gas presence in a flow path located at a cement interface in the wellbore and a first distance of the flow path from the tool string using a pulsed neutron log borehole model and the wellbore data obtained from the pulsed neutron sensor; calculating a second distance of the flow path from the tool string and a first velocity of the gas flow in the flow path using a distributed acoustic sensor borehole model and distributed acoustic sensing measurements obtained from the one or more optical fibers; correlating the first distance and the gas presence with the second distance and the first velocity and thereby obtaining a first calculated distance of the flow path and a first calculated velocity of the gas flow; calculating a third distance of the flow path from the tool string and a second velocity of the gas flow in the flow path using a distributed Doppler sensor borehole model and distributed Doppler sensing measurements obtained from the one or more optical fibers; comparing the third distance and the second velocity with the first calculated distance and the first calculated velocity, respectively, to obtain a second calculated distance of the flow path and a second calculated velocity of the gas flow; calculating a fourth distance of the flow path from the tool string and a third velocity of the gas flow in the flow path using a distributed temperature sensor borehole model and distributed temperature sensing measurements obtained from the one or more optical fibers; and comparing the fourth distance and the third velocity with the second calculated distance and the second calculated velocity, respectively, to determine a width of the cement interface, a distance of the cement interface from the tool string, and a velocity of a gas flow in the cement interface. conveying a tool string into a wellbore, the wellbore at least partially lined with a first casing and a second casing concentrically overlapping a portion of the first casing, wherein a first annulus is defined between the first and second casings and filled with a first cement, and wherein a second annulus is defined between the second casing and the wellbore and filled with a second cement; obtaining wellbore data from a pulsed neutron sensor (124; 702) included in the tool string and optical fiber measurements from one or more optical fibers included in a cable positioned in the wellbore; determining a gas presence in a flow path located at a cement interface in the wellbore and a first distance of the flow path from the tool string using a pulsed neutron log borehole model and the wellbore data obtained from the pulsed neutron sensor; calculating a second distance of the flow path from the tool string and a first velocity of the gas flow in the flow path using a distributed acoustic sensor borehole model and distributed acoustic sensing measurements obtained from the one or more optical fibers; correlating the first distance and the gas presence with the second distance and the first velocity and thereby obtaining a first calculated distance of the flow path and a first calculated velocity of the gas flow; calculating a third distance of the flow path from the tool string and a second velocity of the gas flow in the flow path using a distributed Doppler sensor borehole model and distributed Doppler sensing measurements obtained from the one or more optical fibers; comparing the third distance and the second velocity with the first calculated distance and the first calculated velocity, respectively, to obtain a second calculated distance of the flow path and a second calculated velocity of the gas flow; calculating a fourth distance of the flow path from the tool string and a third velocity of the gas flow in the flow path using a distributed temperature sensor borehole model and distributed temperature sensing measurements obtained from the one or more optical fibers; and comparing the fourth distance and the third velocity with the second calculated distance and the second calculated velocity, respectively, to determine a width of the cement interface, a distance of the cement interface from the tool string, and a velocity of a gas flow in the cement interface.