A method for detection and monitoring changes in an elongated metallic structure having a wall and an exterior surface, comprising: placing at least one acoustic transmitting transducer in vibrational communication with the exterior surface of said metallic structure; placing at least one receiving transducer in vibrational communication with the exterior surface of said metallic structure and spaced apart a chosen length from said at least one transmitting transducer; generating acoustic frequency chirp signals having a selected signal strength, spectral content, and duration; directing the chirp signals to said at least one transmitting transducer; producing a baseline signal by: receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer; averaging a chosen number of received vibrational signals; and removing any DC component from the averaged received signals; producing a monitoring signal by: receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer; averaging a chosen number of received vibrational signals; and removing any DC component from the averaged received signals; normalizing the monitoring signal to the baseline signal, whereby a maximum value of each of the baseline signal and the monitoring signal is equal to a selected value; taking the difference between the monitoring signal and the baseline signal, forming a difference signal; performing short-time Fourier transform of the difference signal using chosen time and frequency window sizes, and time steps, forming thereby a two-dimensional array in time and frequency; calculating the standard deviation of the short-time Fourier transform array along the time-axis for each frequency; and summing the standard deviations as a function of frequency. placing at least one acoustic transmitting transducer in vibrational communication with the exterior surface of said metallic structure; placing at least one receiving transducer in vibrational communication with the exterior surface of said metallic structure and spaced apart a chosen length from said at least one transmitting transducer; generating acoustic frequency chirp signals having a selected signal strength, spectral content, and duration; directing the chirp signals to said at least one transmitting transducer; producing a baseline signal by: receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer; averaging a chosen number of received vibrational signals; and removing any DC component from the averaged received signals; receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer; averaging a chosen number of received vibrational signals; and removing any DC component from the averaged received signals; producing a monitoring signal by: receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer; averaging a chosen number of received vibrational signals; and removing any DC component from the averaged received signals; receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer; averaging a chosen number of received vibrational signals; and removing any DC component from the averaged received signals; normalizing the monitoring signal to the baseline signal, whereby a maximum value of each of the baseline signal and the monitoring signal is equal to a selected value; taking the difference between the monitoring signal and the baseline signal, forming a difference signal; performing short-time Fourier transform of the difference signal using chosen time and frequency window sizes, and time steps, forming thereby a two-dimensional array in time and frequency; calculating the standard deviation of the short-time Fourier transform array along the time-axis for each frequency; and summing the standard deviations as a function of frequency. taking the difference between the monitoring signal and the baseline signal, forming a difference signal; performing short-time Fourier transform of the difference signal using chosen time and frequency window sizes, and time steps, forming thereby a two-dimensional array in time and frequency; calculating the standard deviation of the short-time Fourier transform array along the time-axis for each frequency; and summing the standard deviations as a function of frequency.