Would it be possible to use the Ultrasonic sensor without the Grove Base Hat, connecting it directly to a Raspberry Pi ?
Are you talking about the HCSR-04 SONAR sensor? If yes, then you can use the ultrasonic sensor without the Grove-base hat. You have to use four female to female jumpers.
I was actually referring to the ultrasonic Grove Ranger sensor. Are they one and the same ?
This one,right? Grove - Ultrasonic Ranger - Seeed Wiki
This one comes with a 4-pin connector cable. You will have to connect 4 Male to Female jumpers with the connecting cable. The female sides with go to Raspberry Pi.