Investigation of broadband acoustic to resolve and identify nearby targets using different pulse durations

Frequency modulated broadband signals provide frequency response over a frequency interval and improve range resolution by pulse compression. These features have made broadband acoustics a favorable tool in several applications in fisheries. However, being able to resolve nearby targets does not assure the signals of the targets are unaffected of each other. To collect correct target strength (TS) using broadband echosounders, several parameters should be assigned properly. These parameters include, among others, frequency range, transmit power, pulse duration, signal modulation, and tapering. Here, we have studied the effects of pulse duration on compressed pulse signal of two adjacent targets and Fourier window length on the TS frequency response estimation using modelling and measurements. Calibration spheres are used as target since their backscattering is known and orientation independent. Acoustic backscattering of two similar (WC38.1 mm) and two different (WC20 and WC57.2 mm) calibration spheres with different spacing were measured. Acoustic interaction between the targets is not included in the modelling while it is inherent in the measurements. Using both measurements and modelling enables decoupling the effects of for example side-lobe overlaps and the acoustic interaction - such as shadowing and multiple scattering - on the TS estimation.