topsoil and ground balancing questions

This one house site from the mid 1700s I have been hunting...It appears to have had a load of different topsoil dumped on it sometime after the 1940s, and I have not found anything older than that in large portions of the yard, while at some of the edges, I have found a few older items.

The first 6 to 8 inches is a very fine, dark black powdery soil that GB on my F75 around 52 with the 5" coil, usually.

The soil below that is much more of a light brown heavy clay soil.

I have not dug any large holes (it is a yard) to try to get a ground balance over just the old soil to see how different it might be.

I have been clearing out a whole lot of stuff from the top layer, in an attempt to find deeper targets in the older layer, regardless of what the ID might be. Not really much luck at all so far.

Any ideas, recommendations, or things to try?

In the spring of the 17th year of a bike probability of the beginning of sales of the detector which will be able to punch a surface carpet.
At the moment there is its finishing and testing.
Many treasure-hunters in Russia wait for this model.
(AKA Intronik)

Tell the owners your detector reads a toxic layer about 12" deep and you would be glad to remove it if they rent a skid steer and a few dumpsters, your labor would be free. Think you could pull that off? I know the answer........as my dad would say to me "you're too honest'.