Abstract: By taking one of the analyzing wavelet as basic wavelet, which best matching seismic wavelets, a new method of calculating the instantaneous spectrum bandwidth (ISB) is given (called WT method).For a comparison between WT method and the commonly used Hibert Transform (HT method), the ISB maps of a noisy synthetic data are calculated using both methods.The results demonstrated the advantages of WT method over HT method both in precision and in anti-noise performance.The WT method was used for calculating the ISB map of a seismic recording from an oilfield;the map of the ISB distinctly displyed the position and pattern of an ancient subsurface ditch.