Migration and bar-holing. Episode thirty-nine of the Gas Service Animated series.

An underground leak almost never surfaces right over the pipe. Gas takes the path of least resistance and can travel a long way before it comes up — or comes up inside a building. Where you smell it is a clue, not the answer.

Underground, gas follows whatever is loosest — backfill along the pipe, dry soil, a sewer trench, a conduit. Natural gas is buoyant, so once it finds an open path it tends to rise and travel uphill, and it can surface or enter a structure well up-slope of the leak. The plume has a shape, and your job is to map it.

Now the winter killer. Frozen ground acts like an impermeable lid — a frost cap. Gas that would normally vent harmlessly to the surface gets sealed under the frost and forced sideways, straight toward building foundations. Pavement does the same thing year-round, and saturated or snow-crusted soil too. When frost gets deep, operators go to heightened surveillance for exactly this reason.

To pinpoint it, you bar-hole. Drive a series of test holes along the pipe route, offset about a pipe diameter plus a foot to one side and angled away from the pipe, every hole to the same depth — pipe depth — so the readings compare. A shallow hole under-reads and lies to you. Read each hole with the meter and note the percent gas.

The pattern tightens as you close in. Start wide — six to ten feet apart — to map where the gas is. Then bracket between your two highest holes and tighten toward one foot. A five-hole diamond — four around a point and one in the center — lets you triangulate the peak. The hole with the highest sustained reading points at the source.

One word decides it: sustained. A momentary spike when you open a hole is only an indication, not a grade. Force-vent the holes with an exhauster, let them settle, and read the value that holds. Watch for holes full of water — saturated soil both caps the surface and floods your sample, so a flooded hole under-reports. Handle those conservatively.

Then you draw the edges. Bar out in every direction until the meter reads zero, north, south, east, and west, and note the distances. Those zero holes are the boundary of the plume — proof of how far the gas has traveled and, more important, whether it's heading for a building.

And check the highways. Sewers, storm drains, and electric and telephone conduits carry gas straight into basements — migration into structures is the leading cause of gas explosions in buildings. Any reading within five feet of a wall sends you inside; a reading in a manhole at four percent or more is Grade one on its own.

Migration in one breath. Gas travels far and often uphill — the surface clue isn't the source. Frost, pavement, and wet ground cap the surface and drive gas into buildings. Bar-hole equal-depth, tighten the pattern, read the sustained peak, and zero out the plume. Watch the sewers and conduits. Next: making it safe.