100 to 150 meter holes are more than enough to provide evidence of the presence or absence of ore that can be reached via strip mining

Actually, no. Often there is little or no near surface indication of ore at depth. Alternately, surface indications are ambiguous and must be tested. There are uncounted thousands of prospects which might be ore bodies based on surface indications, but are subsequently drilled out and found to contain little or no ore grade material.

I believe that all of the significant recent gold production in Northern Nevada has been extracted via strip mining

Again, no. That the majority of Nevada production has been from open pit mines is true. Most new mines in Nevada are underground. Indeed, many big open pit mines go underground from the bottom of the pit as pit-wall laybacks become hopelessly uneconomic. Most exploration today is focused on targets which are only mineable via underground techniques. It is not that we dont want open-pitable deposits; it's that we can't find many. Even in the Carlin Trend underground has been the basis for most new production over the past decade.

Going deeper would appear to make sense if an underground mine was contemplated, or if the shallow stuff had already been proved up

Unfortunately, going deep makes sense because it is where we are finding most of our "new" ounces. Shallow orebodies are welcome...I have been on the waiting list for a long time.

Thanks for the thoughtful question R W. I mostly work in the "finding" side of mining rather than the "digging" side. Mine geologists and mine engineers may have a different take on this.