Following on from the initial post, I was reading through various email/website/forum talk and the emphasis was very much on the skeptical side of the debate. But that is no surprise since most people do not think there is a large, unidentified creature in Loch Ness and so any story like this is going to lead to "natural" explanations.
Of course, there may be a natural explanation, I would not be so fanatical as to class every story as "monster". Where I diverge is in my view that not all sightings are explained by natural phenomenon.
As it turns out the favoured explanation is a submerged log. It's a theory and it sounds plausible and most people would accept that without any further critical thinking and move on to the next thing. I prefer to persist with some critical thinking.
Aside from the rather important question as to whether a submerged piece of tree debris could produce the strength of such a sonar trace, there are some other things to consider.
One skeptical forum summed it up (link here):
"Page 61 of Radford and Nickell’s Lake Monster Mysteries. Jerry Monk, a British hydrographic surveyor notes that when a piece of wood is immersed in water, over time, it sinks. If there is a thermocline, it is possible for the log to float in mid-water on the denser layer of cold water. Or, the log may sink, decay and form methane, which makes it rise again. The log idea, sinking and rising, degassing and sinking again was used to explain the Mansi sighting on Lake Champlain."
The thermocline is an area below the loch surface where the temperature drops rapidly creating a kind of partition between the upper and lower layers of water. For Loch Ness, it is usually stated to be at a depth of 40 meters but could vary between 30m and 60m depending on the season. Interestingly, the thermocline can be seen on sonar scans as well, which leads to the question as to why it is not visible on this particular scan if it is claimed to be buoying up the supposed piece of tree.
This is an open question since it may be a matter of sonar calibration and sensitivity but it does suggest that the "log" (which is 25 metres below the surface) is well above the thermocline.
Note that the quote mentions that the theoretical log would "float" on the thermocline. However, this target is moving at such a speed that the trace soon disappears off the screen. Can underwater currents achieve this rapidity? Yet another open question which suggests a log would tend to drift rather than speed.
Finally, the quote mentions vertical ascent and descent of a log due to water saturation and then gassification and renewed buoyancy from methane gas decay. Aside from the fact that the object moved horizontally, such a scenario is unlikely at Loch Ness. Decay of organic matter happens at very slow rates at the bottom and indeed tests have shown little gas production to stimulate ascent.
At this point, I have tried to contact the witness for his view on the matter and whether the tree debris he has seen in the past matches this. I suspect the answer would be "No". For me, talking to witnesses is important - unlike too many a number of critics who completely disregard verbal testimony (unless to find inconsistencies to discredit them)!
Another "natural" explanation often put forward are echo effects from sonar beams bouncing off the sides and bottom of the loch. These do produce strange effects such as vertical lines but it is unclear whether they would be capable of producing the trace in question and from the moored, stationary position he was at. Regular users of sonar equipment would be able to recognise such a pattern.
Other causes of unusual patterns are boat wakes. However, this would require a boat to pass fairly close by and again would be recognisable to a regular user (though as with side echoes it is unclear that they could even produce the pattern in question).