If you are just starting out with the SH, I would hunt in Discrete Trash Elimination at first with Elimination set no higher than 3 and threshold just barley audible. Swing slow to where there is no falsing. Listen for faint deep tones.
After you are comfortable with the machine, you can switch to standard mode, but it will be noisier. Slow your swing down and play with the settings until it is as quite as you can get it without taking elimination above 3.
If you are hearing only surface targets...possible your threshold it to loud, or swinging to fast. Slow down and listen for faint signals, or just a blank spot in your threshold tone. If the threshold is too loud you will not hear the drop off (blank spot).