A geologist was a scientist that studied the natural materials which formed a planet.
Professor Kyle, a geologist, was the only survivor of an ill-fated group after the rest of her expedition was wiped out by the Cybermen's androids in 2526. She later shared their fate, resulting in the total loss of the expedition crew. (TV: Earthshock)
Tanya Webster worked as a geologist with the Oil Exploration Group of the Webster Corporation. (AUDIO: Red Dawn)
Dr Nasreen Chaudhry was a geologist doing deep drilling in Cwmtaff in 2020. This awoke some Silurians that had been hibernating beneath the area for millennia. (TV: The Hungry Earth)