Below are the HB depictions for the 455kHz SI sonar beam vs the 800kHz SI sonar beam...
The lower the frequency number, the deeper the sonar beam can be used..but with less detail
The higher the frequency number, the more detail the image will produce...
Now, deeper is a relative term...as there as several things in the sonar world that can affect the effectiveness of the sonar beam...
Water depth, water clarity/water borne particles, bottom composition...are things that can absorb the sonar beam...(deeper water is going to absorb more sonar than shallower water - giving a darker image.... a hard rock bottom is going to reflect sonar better than a soft mud bottom - giving a brighter image...etc...)
I find myself using the higher detail 800kHz SI beam most often except when water conditions just will not let me get enough brightness into the SI image with the highest "20 SI Sensitivity" setting of the 800kHz SI beam...
Rickie