108. The initial survey
When I dive a site for the first time, my goal is to assess its general characteristics, with a particular focus upon the identification of any potential hazards. Finding bottles is only a secondary consideration at this preliminary stage. It may take two or three dives, or more, before I am satisfied that I understand a site well enough to commence serious work on it.
These early dives comprise what I refer to as an “initial survey”. They are conducted at a fairly high level, both literally and figuratively speaking. I will be finning three or four feet above the bottom, following a defined course generally parallel to the shoreline, looking for evidence of two things:
1) Hazards that may cause me to reassess the risk assessment that I have made for the dive.
2) Bottles
Primary hazards include obstacles such as large pieces of junk, sunken trees or branches, moorings for boats such as riser chains and sinkers, and any kind of fishing line or areas where discarded line may have accumulated. All possible sources of entrapment must be marked so that they can be avoided during working dives. Another type of primary hazard is a current - even a slow one of 2 or 3 knots may be faster than a diver can reasonably hope to fin. The direction of a current should be noted since it may lead towards, and not away from, other hazards. The direction of a current is not always obvious and it is easy to make completely the wrong assumption about it. A case in point is where a river meets a lake, a place where communities tend to cluster and good bottle diving is often to be had. It is natural to assume that the river will be flowing into the lake, much as a river flows into the sea, but it could very well be flowing out of it. The diver who gets this wrong could be in for an interesting ride.
Secondary hazards include discarded containers, equipment, or other artifacts that suggest an increased pollution risk, or biological indicators such as dead fish. They are secondary because they are not likely to cause immediate physical danger, but may cause illness later.
During an initial survey, if I find any bottles lying on the bottom then I may choose to retrieve them, but if they are more than half buried in silt I will merely note their position for later recovery. What I want to avoid at this stage is stirring up silt. I prefer to be either neutrally buoyant or slightly positive for an initial survey so the thrust of the fins will not be directed towards the bottom. If there is one thing that differentiates an initial survey from any later dives it is the need to avoid blackwater conditions on what still amounts to an unknown site. For similar reasons I will not lay any line during an initial survey; I will merely follow a compass heading while noting depth. Also, I will be checking overhead to make sure that I can see daylight above me. The last thing I want is to blunder into a cave or cavern, which is a serious possibility with certain sinkholes. If the water is too turbid for me to see daylight overhead then I will abort the dive. There are other techniques for dealing with unassessed blackwater conditions, such as tender directed diving.
I typically employ a classical U-search pattern for initial surveys. Each leg will follow the shoreline, will be about 20 metres long, and will be separated from the others by an “arm” of one or two metres, depending on visibility. I will initially assume that the search area should extend about 10 metres from shore (throwing distance), making for about 5 or 10 legs across the sweet spot. If the finds are good further out then I may extend the range to 15 metres or so.
I will begin an initial survey close in to shore and then move outwards. This way, if I encounter any hazards they are not going to be between myself and the shore. Also it allows me to acclimatise to a site gradually and get the feel of it, if I start in the shallowest water and only gradually move deeper. This is contrary to the normal practice of multilevel diving but it should be remembered that bottle dives are typically shallow – usually no more than twenty feet – and so there is little chance of my seriously abusing a dive profile by this technique.
If the area contains a good deal of scattered debris, as in the case of a dump, then it may be necessary to reduce the length of the legs to 10 metres or less, especially if visibility is poor, and do several smaller surveys that cover the projected area of the site. Personally I like to scale each search area so that it can be covered in one dive of 20 – 30 minutes. I don't like having to abort a survey part way through and then pick it up later, I'd rather just work with a grid of smaller search legs and do multiple surveys. Even so, it is generally possible to make good progress during an initial survey and to cover 100 metres or so in total. This can feel quite exhilarating at times, because nothing remotely like that sort of ground will be covered in the serious working dives that follow.