1. A method for extracting intermittent, randomly occurring transient target signals of known type from a raw data source signal containing an unknown number of the target signals in addition to interfering signals or intrinsic noise signals, the method comprising: a) obtaining the raw data source signal from a sensor for detecting the passage of particles or bubbles in a fluid flow, wherein a target signal is generated by the sensor when detecting a particle or bubble; b) defining quantifiable signatures or characteristics that respectively and independently represent the target signals, the interfering signals or the intrinsic noise signals, wherein the signatures comprise a frequency range of the target signals and a kurtosis value range of wavelet coefficients of respective wavelet scales of interest that reflect a transient nature of the target signals; c) applying a Time-Invariant Wavelet Transform (TIWT) to decompose the raw data source signal into distinct data sets in a form of wavelet coefficients of the respective wavelet scales; d) processing said data sets to identify a first group of data sets which display the signatures or characteristics representing the interfering signals or intrinsic noise signals and a second group of data sets which display the signatures or characteristics representing the target signals; e) setting the data sets of the first group to zero; and f) applying an inverse transform to the processed data sets in order to reconstruct a processed output signal.