Have a look at the Brekke and Solberg (2005) paper - slicks can dampen the water anywhere from 0.6dB to 13.0dB, natural seeps are similar.
Finding the right threshold will depend on content, age, weather conditions - it really is a complex issue. What we find is we need weather data and then increase or decrease the threshold accordingly. If the wind speed is too low (i.e. calm), everything will look like a slick, too high (a gale) nothing will look like a slick.