If you are shy about asking or knocking on a door, here is a letter than i use sometimes after i knock and nobody answers or when i'm just in a hurry. You can roll the paper up and put it in their front door handle or mail it to them. I also have one that is a permission to metal detect and relieves them of any liability issues should i twist an ankle, get cut, etc...
Dear Property Owner,
I would like to introduce myself. My name is (insert your name here) and I have lived in (city or state) most of my life. Normally, I would prefer to meet you in person, but I feel that a letter beforehand is a better approach than just knocking on your door at an inconvenient time and invading your privacy.
I have been engaged in the hobby of metal detecting for almost a year and find it a healthful, enjoyable pastime that allows me to meet many interesting people. The thrill of finding old coins, buttons, etc. is very rewarding to me.
Recently, I have researched information that your property and the area surrounding goes back many years. I would like to obtain permission for myself and a friend to detect your property.
I want you to understand that I always respect the property that I search and leave it in the same condition that I found it. Normally the targets are no deeper than 5 or 6 inches and I retrieve them such that the ground appears not to be disturbed. Also all trash that I find is disposed of and not put back into or on the ground. In addition, I would like to offer my services to you.
Perhaps you or someone you know has lost a valuable jewelry item, keys or a farm tool. I would be willing to assist in finding the lost item, for the joy of seeing it reunited with its owner. I would be happy to meet if further discussion is required, or you could call my phone number, whichever is more convenient for you.
Thank you for your time and consideration.
Sincerely,
**SIGN HERE**
(Your name here)
(Your phone number here)