Knudsen Sub-bottom Profiler Processing


            The Knudsen sub-bottom profiler was selected over the Odec profiler after a brief comparison (see  While the Knudsen was used by Jacobsson and Darby previously, it is a poorly understood instrument and it’s capabilities were extended on HLY0503.  Many of the settings for the Knudsen remain “historical”.

            The Knudsen must be operated in the LF (Low Frequency or 3-6kHz) option since the HF (High Frequency or 10.5-13.5kHz) option interferes with the 12kHz. SeaBeam system.  All Knudsen post-processing was done from SEG-Y files.

            The first big breakthrough was when Steve Roberts read in Lee Ellett’s setup instructions for SIO ships stating that TVG (time varing gain) is an essential setting.  Lee recommends always using TVG referenced to the water bottom.  The setting is found on the pull down “Sounder” menu.  TVG improves the displayed data and is included in the KEB files but not in the SEG-Y files.  Lee’s guide may be downloaded from

            A discussion of chirp systems and chirp signal processing may be found at   The processing done by Jacobsson on HLY0503 consisted of forming the envelope from the correlates in the SEG-Y files, performing a 25mil AGC, and grayscale plotting.

            Several idiosyncrasies were discovered and reported to Knudsen Engineering:

1)    Not enough data is recorded and the sub-bottom events are truncated, especially in areas of rapid bathymetric deepening.  The penetration from the Knudsen exceeds the data recording length.

2)    The min/max depths for searching seem to be honored in manual mode, whereas the manual says they are used in auto-phase mode only.

3)    Auto-phase mode triggers lots of small SEG-Y files and also requires frequent operator intervention because of file name conflicts.