1. A method for extracting a target signal from measured signal including the steps of: a) measuring a target signal and background noise to obtain measurement data; b) using a computer to extract coherent structure from the measurement data using wavelets and corresponding wavelet coefficients; c) comparing the wavelet coefficients of the wavelets with a threshold with the computer; and d) eliminating the wavelet coefficients of the wavelets that are less than the threshold with the computer.