Sonar Image Analysis
As a result of the recent Associated Press article there have been a number of responses expressing doubt that the sonar image represented the Grunion. I would like to present the facts behind our optimism.
This is not an attempt to persuade the doubters.
Instead the goal is to present the whole story so that skeptics can provide a balanced and intelligent analysis. We would very much appreciate such responses. Moving forward on false assumptions is not going to be helpful to anybody.
There are several points:
To start with sonar images canâ€™t be read like photographic images. If you go to http://skinnersquared.com/grunion/ you will see a set of 3d images viewed (either light or sound pings) from different directions. In addition note that it makes a difference whether the sub is tipped to the side (possible 5 degree port list with 7 degree down pitch) and or whether you are looking at it perpendicularly. In most cases we werenâ€™t. There is enough bandwidth in a photographic image to determine these factors; with a sonar image that is not the case. It is dangerous to make a judgment solely on the sonar image. The one shown in the AP article was particularly misleading. Below are a couple that are better
The length of the image (285 ft) is consistent with that of a Gato class sub The sub is actually 312. The bow appears to have been buried or blown off which would explain the missing 27 ft. (Two of the images appear to show a buried bow section) A destroyer is 375 ft and it appears that we have located the only destroyer sunk in that area, the Japanese destroyer Arare almost exactly where predicted and several miles away. Sub chasers are 165 ft and although two were sunk they, along with the Arare, were a number of miles distant from the sub target. The Japanese sub I-9 which is 375 ft was sunk west of Kiska far from the target. That combination of location and size rule it out also. On the other hand the National Park Service found a sub in Kiska Harbor 243 ft long in 80 feet of water. Measuring length of a sonar image is a function of speed of the towfish which is measure by GPS at the surface. Just how accurate and reliable is that speed unknown.
The sub appears to have slid about 1200 meters down a slope which because of the topography and the fact that the volcano on Kiska has had at least two major eruptions since 1942, may be volcanic ash. There is a reasonable possibility that some of this volcanic ash or silt has built up on the sub distorting the image.
Prop guards: When enhanced one of the sonar images shows what appears to be a prop guard, a rectangular protuberance that protects the propellers when docking and also protects against mines.
Some navy personnel believed that prop guards were removed when a sub went into action. However, according to John Alden â€œ I believe it can be positively stated that she did. [have prop guards] A photo taken 20 March 1942 clearly shows them and the directive to remove the guards for existing and new subs was issued on 1 September 1942. GRUNION was lost before that [July 30, 1942] and had never been modified from her initial configuration.â€
The presence of prop guards would seem to eliminate the hypothesis that the target was a surface ship. However, the images do not indicate with absolute certainty the presence of prop guards. See the photos below.
Prop Guard annotated
Finally we did a great of work on determining the location and ended up with some fairly definitive coordinates. The sub was almost exactly where we expected.
All of these points seem consistent with the hypothesis that it is the Grunion. As yet, we have not seen any points that are clearly inconsistent with that hypothesis.
Again, we would appreciate learning of inconsistencies with the Grunion hypothesis.