MAD71, page-12

There are several reasons you can't drill during a DHEM survey. The obvious reason is you need to put the Downhole probe down the hole (assuming you want to drill the same one as surveying). More likely however is the nearby proximity. That's interesting because it tells us that they probably want to drill some holes within 50-200m of the discovery hole. That tells a story too.

Now there are a couple of reasons for not drilling simultaneously.

1) A large metal item (drill rig and support vehicles) in the middle of a Fixed Loop (to create the EM field) or nearby would also induce an EM response in the vehicles. This EM field would interfere with the data and create unwanted noise also referred to as cultural noise (think fences, powerlines, vehicles etc). Probably not a problem if we are talking a deep target, but for something <50m or so, it definitely is a problem.

2) A drill rig operating (grinding, spinning, cutting the rock) creates an associated electrical field and also the power running through the rig and generators creates an electromagnetic field.

Long story short - lot's of EM interference that is unwanted to create the best model possible.