I was wondering this for a school paper I'm writing.
This is one of key questions that NASA's Mars InSight probe is trying to answer but it has only recently arrived and starting taking data and it will be a while before the data is analyzed and published in peer reviewed journals. The "mole" which would burrow below the surface and make the temperature measurements encountered some initial issues and NASA and the instrument's builders were running further tests and looking at those data before proceeding according to this mission press release.
For a term-paper kind of answer you don't actually need to wait for Mars InSight to tell you. 15 feet is deep enough for you to able to assume that the temperature will be the average of temperatures on the surface.
So first find a graph of the temperature on a typical Martian spring or autumn day, then work out the average temperature over the 25 hours. That is a good first guess.
I think that 15 feet is deep enough for variations across the year, between summer and winter, to be averaged out fairly effectively as well, though that is something you may want to discuss a little in your paper. This Wikipedia article about permafrost is worth reading to see how the Earth averages its own temperatures at depth over the course of a year.
On a planetary scale 15 feet is very shallow indeed, so the amount of heat flowing from the inside of Mars towards the surface will be tiny (this is what that InSight probe is measuring) and I would suggest mentioning it only in order to say that you are ignoring it.