Mark Bessey
Forum Enthusiast
The D50 has an option for "Noise reduction for long exposures" which should help with just these sorts of long-exposure thermal noise problems. Does the D70 have that feature, and did you use it in your example?
Failing that, I bet you could remove much of that noise by taking a 30-second exposure in total darkness, and subtracting it from the cave photo in your favorite picture editing program. This is a pretty common noise-reduction technique. Ideally, you'd want your "dark frame" to be taken under as close as possible to the same conditions, but a surprising amount of that noise is probably repeatable.
Failing that, I bet you could remove much of that noise by taking a 30-second exposure in total darkness, and subtracting it from the cave photo in your favorite picture editing program. This is a pretty common noise-reduction technique. Ideally, you'd want your "dark frame" to be taken under as close as possible to the same conditions, but a surprising amount of that noise is probably repeatable.