DRI Update: 28 July 1999

The Site Survey for SAX99 was completed last week. The summary given here will provide an overview on our progress in converging to a specific site for SAX99. A cruise report being prepared by Kevin Briggs, Mike Richardson, and others involved in the survey will give more details on day-by-day activities, and cover other aspects of the survey.

During the site survey, four candidate areas were examined: sites at Panama City Beach, Inlet Beach, Grayton Beach, and Fort Walton Beach. By the end of the survey, the site at Fort Walton Beach had emerged, in my opinion, as the most promising site, with Panama City Beach a close second. As discussed below, another set of vibracores is being arranged for both sites, and the final decision between these two sites will be made after the new vibracores have been done.

Recall that prior to the site survey, results were available for a set of 11 vibracores taken during March by investigators from USGS at St. Petersburg. Of these, 8 were taken in water of approximately 60 ft. depth (the others shallower), and only two (Core ID NRL-6 and NRL-9) gave results close to those desired; that is, nearly uniform sand sediment with no important layering, no mud inclusions, and without high concentrations of shell fragments.

During Leg 1 of the site survey, Steve Schock and his group carried out chirp sonar surveys of all four sites, while NRL simultaneously recorded side-scan sonar data. Both indicated that Inlet Beach and Grayton Beach seafloors had only thin sediment over harder ground that emerged in numerous outcroppings. The Inlet Beach and Grayton Beach areas were removed from further consideration. Based on chirp sonar results a region was located at Fort Walton Beach that appeared promising, i.e., without important sub-bottom returns in the top meter or more. At the Panama City Beach site a relatively large region (1 km by 1 km) was found without important sub-bottom returns down to depths of 2-3 m. While at this point the Panama City Beach site appeared very promising, there were also signs that all might not be well. Chirp sonar lines were run through the vibracore sites, which should provide important ground truth information for data interpretation (though perfect coincidence can never be assumed, so some assumptions about the spatial scale of sub-bottom variation needs to be made here). It was found that while mud or clay layers do appear to show up in the chirp sonar data, layers of concentrated shell fragments (shell hash) do not; apparently the impedance contrast for shell layers is insufficient. Thus, more information was needed on the occurrence of these layers than could be provided by the chirp sonar.

Leg 2 was to be devoted almost exclusively to working with Paul Johnson's tripod measurements (sound speed, density, and permeability). Unfortunately, almost unbelievable shipping problems (with UPS) delayed delivery of a critical item by more than a week, and in the end only about one day of measurement time (spread over two days) was available. The permeability unit never did arrive. Some diving was also done during Leg 2, but more on diving results later.

Paul's tripod was used first at two separate places in the promising 1 km by 1 km region of the Panama City Beach site. In both places the probe ran into a relatively high concentration of shell fragments at about 30-40 cm depth. The shell could be seen directly with a video camera mounted on the tripod. It shows the region where sediment flows up onto the original interface as a result of the hydro-jetting used to advance the probe. As mentioned above, these concentrations of broken shell did not show up in the chirp sonar data, though these data did show plenty of other sub-bottom layers and structures. These shell hash layers are a concern because they could make it difficult to insert acoustic equipment to the proper depths. And, of course, we would prefer uniform sand with little shell content regardless of the ease of inserting the acoustic equipment.

Some of the vibracores taken in March at the Panama City site also showed high concentrations of shell fragments below a sand layer near the surface, and these dense layers of shell fragments may have been responsible for limiting the depths reached by the vibracores in some of the cases.

After the shell was encountered, Paul Johnson's tripod was moved to the best looking vibracore site (Core ID NRL-9), which showed little shell and nearly uniform sand down to about 90 cm. Using Paul's probe in a penetrometer mode it was confirmed there was little shell in this region, but we also saw qualitative evidence of silt and mud inclusions from the fines that were brought back up into the water. Unfortunately, because of the late start with Paul's tripod, we were able to go to just one other vibracore site, and then had to stop without taking his unit to Fort Walton Beach. In addition to the visual observations and penetration behavior, some sound speed and density data were taken, but results were not available on site.

During Leg 3 more extensive diver examination was made of the sites. Mike Richardson bought a shovel to see how far down he could dig at various sites (not easy underwater). In regions without shell he could get down 60-80 cm. Mike, Kevin Briggs, and Rob Wheatcroft examined both Fort Walton Beach and Panama City Beach sites. The divers all prefer Fort Walton Beach: the sand is coarser, they do not kick up clouds of fines when they dig or stir up the bottom, and they have seen no evidence of mud inclusions. The visibility is also much better even before the bottom is disturbed. A site at Fort Walton Beach was found with no shell layer down the about 60 cm, but it is not clear how large the region is. (At a nearby site Kevin Briggs ran into thick shell deposits at 20-30 cm depth during Leg 2.) The size of the region relatively free of shell will need to be determined using vibracores. At Panama City sites, clouds of fines were sometimes generated when the divers worked on the bottom.

Aside from layers of shell fragments, occasional whole sand dollars (dead) were found at both sites (deeper than about 20 cm), and more were observed at Fort Walton Beach. Sand ripple left over from last summer's hurricanes can still be seen at both sites, but is decaying rapidly, and may well be gone by October. The ripple amplitude is larger at Fort Walton Beach, but even there it is down to about 5 cm. Ripple amplitudes and wavelengths were measured by Mike Richardson and Kevin Briggs. The biological activity was roughly the same at both sites; effects include mounds (typically 5-10 cm high, but as high as 20 cm), and holes (up to 1 cm diameter).

In summary, we may have regions at both sites where the experiment could be done, but we still need to confirm that the region is large enough at Fort Walton Beach. At Panama City the region without significant shell layers appears to extend over several hundred meters, and thus is likely to be large enough, but this also needs to be confirmed with vibracores. If the Fort Walton Beach region free from shell layers turns out to be large enough, that would appear to be the best site. Mike Richardson is coordinating with USGS in St. Petersburg to arrange for a set of 10 vibracores to be taken at each of the two sites.

In addition to the work described, Rob Wheatcroft, Dan Albert, and Dick Bennett pursued other investigations as well, but these will not be reviewed here.

Following the site survey, I went to the Destin/Fort Walton Beach area to look over options for shore site housing in case that turns out to be the experiment site. I did find a place that should work well. The Fort Walton Beach site (near 30 deg 22.83 min N, 86 deg 38.69 min W) is about 86 km WNW of the Panama City Beach site (30 deg 7.58 min N, 85 deg 47.80 min W; vibracore site #9). Whichever site is chosen, the other will remain a backup in the event that a tropical storm should affect the primary site, but less so the secondary site.