Machine Dug trial pits
What could be simpler? Hire yourself a JCB or something similar and dig a hole. You can probably dig about 8 pits a day (and log them properly), down to depths of 2.5 to 3 m. You can go deeper but it becomes more difficult to work out what’s actually happening at the bottom of the pit. Trial pits are great for understanding the distribution of near surface soils, but hopeless in water bearing sands and gravels.