Question regarding water availability in an old borewell

There is a borewell which is almost 20yrs old, which has not been used till date, as there was little water source. I do not know, how deep it is, but my relatives tell me its more than 500 feet deep. I recently lowered a rope with brick to check for water, these are my findings: Water present at 100 ft to 130 ft. Remaining is filled. Is it worth to desilt or clean the borewell? How do I know the correct depth of the borewell? What is best solution to find whether water will be sufficient for usage. Kindly advise.

Prabhakar, Bangalore