Underground Surveying.

A primer in coal mine surveying and mapping.

When someone learns that I survey in underground mines, they very often ask, "What's it like to survey underground?" This article will discuss the similarities and differences between surveying in the sunlight and surveying deep underground.

In Alabama, where my coal mining experience has been, the seams are fairly level and range in thickness from around 2' to as much as 15'. If the coal seam or strata is thick enough to stand on, it is usually called high coal; if not, it is called low coal. In the low coal mines, coal is moved by low profile conveyor belts. In transportation areas, the roof is blasted higher than the seam, and at conveyor belt headers and other key locations, rock is removed to provide room to stand and work. The ideal height of a coal seam is around 7', which permits walking upright while being able to reach the roof. Anything much less or much more presents problems for the workers.

One of the first differences a new surveyor underground notices is the fact that all the survey points are overhead. Because of the mud and the constant traffic of mining equipment, it is not practical to set points in the bottom of the seam. Surveys are generally run on the centerline of the mine areas. Points are set by first drilling a small hole in the roof about 3/4" deep, into which a round wooden plug is driven. Then the survey point is marked by driving a metal spad into the plug. A spad looks like a small flat key. It has a round head with a hole through it and a straight shaft with a rounded point. Once the spad has been put on line, a plumb bob cord is threaded through the spad and tied with a slip knot. Distance measurements are made to the cord with a surveyor's chain or with an EDM. If electronic measurements are made, the prism must be mounted on a special holder to hang it from the spad. In high coal, a 32-ounce bob is used, since the high volume of air moving to the working faces makes it di fficult to steady a lighter one. In low coal it is sometimes necessary to use an extremely short bob, since there is so little headroom.

A Survey Day in the Mines

Surveyors are used to carrying a lot of stuff. To do their work efficiently they need plumb bobs, prisms and plumbing poles, measuring tapes, paint, flagging, nails, tacks, stakes, hammers, field books, calculators and data collectors, transit, EDM, tripod, radios... Underground mine surveyors need most of the same stuff, but that is only the beginning. Every surveyor underground wears a miner's hardhat and a wide leather belt that holds a self-rescuer to supply oxygen in case of emergency, plus a heavy battery connected by a cable to an electric lamp on the hardhat. Each crew carries a hand brace and drill bits as well as a heavy-duty, battery-powered hand drill. Instead of stakes and hubs, they carry wooden plugs and spads. They also carry a methane detector to avoid setting off an explosion. Since parts of most coal mines are wet, everyone wears 16"-high rubber safety boots. In thin coal seams the miners and surveyors must work on their knees in the mud, so heavy duty knee pads are standard equipment. In h igh seams the crew must carry a tall step ladder to reach the roof.

Another difference in underground surveys is that there often must be very short sights. In vertical shafts, it is often necessary to have two control points less than 20' apart since the points are transferred down the vertical shaft by wires or optical devices. And mine entries are often driven around 50' apart. Instrument setups are done with utmost care. The transit is leveled under the plumb bob (there is a mark on the top of the instrument), then the scope set on a 90 degree zenith angle. Then the position under the bob is checked again to ensure that there is no eccentricity. The string is hung from the hole through the spad, making sure it is tied so as to hang from the lowest point in the hole.