Today I got some root cores of a nearby Bermuda grass lawn. Yes, we have a few spots of it in Michigan. I found a similar situation. There dense roots were about 2 inches deep, there were a few very fine roots up to about 4 inches deep, but...many times the cores just fell apart, as the roots were sparse and very thin.
I am not sure if this is a good method to determine the rooting depth. Temperatures were above 90 most of last week. We have had occasional showers.