Hi AM,
You should be able to do this with Agisoft.
1. Digitize a polygon around your gravel pile
2. Right click on it and choose MEASURE
3. In the Volume tab choose Custom level as Base plane and enter a value.
4. Click Update
For example, if you want to know the volume from 400 to 420m, then first get a volume (above) measurement for 400m, another volume (above) measurement for 420m and then calculate the difference. This should give you the volume for the range 400-420m. Let me know if it works.
All the best.
Regards,
SAV