C182 pilot reported while on an search and rescue training mission descending below 500 feet AGL resulting in flight towards terrain.

Narrative

During a search and rescue training mission; I believe my aircraft may have gone below 500 AGL. After having completed a contour search around a mountain; we completed a set of drainage searches down the side of the mountain. While I was attempting to maintain a minimum of 500 ft seperation from the ground; my instructor wanted me to push even lower during the searches. I believe we had gone below 500 AGL. ADS-B data claims we got much lower than I would believe myself; but I do believe we were as close as 300 ft off the ground. There would be no immediate safety risk; as the area is sparsely populated (no buildings for miles) and no one was around; but I believe it could be an issue in the future.I believe the issue is that we started too low to begin with. We only started about 500 ft AGL to begin with and then began a much more aggressive descent than I believe was necessary.In the future; I believe this could better be prepared for by better entry planning (i.e figuring the field elevations in a pre-determined way) and then executing them with more caution.