What is a ranging run?

I have been following the BP Gulf of Mexico "incident" carefully, reading their status on their web site a couple of times a day in addition to checking out the real-time video from their remotely operated vehicles (ROVs). Here's their latest status update:

The DDIII relief well is currently at 17909 ft and is preparing to drill ahead 30 ft followed by another ranging run. Intersection of the MC252 well annulus is expected towards the end of the week.

A lot of jargon in there, most of which makes sense, but what on earth is a "ranging run"? I tried Google and Wikipedia but came up empty. So, today I contacted BP's press office and here's what they told me:

We use electro magnetic sensors at the drill bit to seek out the existing well by following its steel casing, then drilling a bit more to edge towards the target. Very clever stuff, 18,000 ft from the nearest person.

What is really happening is that normally they drill and then line the hole with casing, but with a ranging run they drill a little and then run the drill bit through that new part of the hole again with the sensors to detect where the new part of the hole is with respect to the other well's steel casing that they are trying to intercept, then they line the new hole and then drill some more with their direction updated to reflect what the sensors told them about where the new well is relative to the existing well.

-- Jack Krupansky