After discussions with John Wyngaard over the last 2 days (and a wake-up call this morning), we decided to try to lower at least the high frequency fluctuations in Pref by trying to add a sleeve around the tubing. John had the great idea to use foam pipe insulation. We found some in Hanford and added it to the horizontal sections of the array when it was down for sonic leveling today. Unfortunately, I had only purchased enough for a test this morning, so we were one piece short (between 7t and 8t) and only did the vertical run between 5b and 5t. I now have more insulation and will add it first thing tomorrow. I'll try to add a P.S. tonight to see if there is any effect on the spectra.
Note that at 1140, I opened up the reference tubing to re-route a piece near 5b.
P.S. Spectra from yesterday (7/30) and today (7/31) at 17:00 and 21:00 show that there is some modest improvement even with only this partial tubing. (In these plots, 11b is Pref and 6b is p-Pref.) We'll see tomorrow when the rest are done. 