Abstract: With the range-bearing resolution of sonar image becoming higher and higher, the probability density function of the oceanic reverberation envelope is no longer Rayleigh distribution, but a non-Rayleigh probability density function with heavier tails. The non-Rayleigh probability density function is complicated, and it is difficult to estimate the function parameters accurately. With CFAR (Constant False Alarm Rate) processing of the reverberation envelope data, which attenuates the heavier tails data values to the background value, the data are transformed from non-Rayleigh distribution to being approximately Rayleigh distribution. Since it gets rid of the interferences which affect the estimation of background power, the detection can estimate the oceanic background power level more accurately and its capability is better and robust. With processing of high resolution sonar data, it is shown that the method is efficient.