In this study, we present a direct detection distributed acoustic sensor based on phase-sensitive optical time domain reflectometer (phi-OTDR) with long sensing range and high signal-to-noise ratio (SNR), which is field-tested over a 50 km-long fiber. Due to the random nature of Rayleigh backscattered light and fading phenomena, it is hard to characterize the performance of the system. For this reason, the performance of our sensor is specified in a statistical manner in which the mean SNR is determined using the histograms of the SNR. The SNR values are measured for identical acoustic signals in five different days, total of 48 hours and the SNR histograms are obtained for fiber distances of 100 m, 12 km, 21 km, 30 km, 40 km and 50 km. The system is field-tested using external disturbances that are generated from a 50-Hz vibrator. The SNR values are extracted from the power spectral density (psd) of the collected data over the monitored fiber span. Our results show that the phi-OTDR system exhibits a mean SNR of 22.5 dB at 50 km distance.