Too bad the lady was such a bad sport and that this had to happen to you. If I really wanted to go back and hunt that site (and I would!), the first thing I would do is write them a handwritten, sincere letter apologizing for the whole situation. Take full responsibility for everything that happened. Yes, I know the husband already gave his permission and yes, she was a jerk. If I want to get back to that yard, I must win her over. I would try to put myself in the lady's shoes, perhaps thinking that she had had a very bad day before she saw me in her yard. The letter would sound something like this:
Dear Mr. and Mrs. Johnson, We only met for the first time last week when I was metal detecting at your home, and I'm afraid I left a very poor impression. I am so sorry that I put you both through this uncomfortable situation, as this was never my intention. Mrs. Johnson, it must have been quite unsettling for you to come home and find me unexpectedly in your yard. I have a passion for learning about our local history through the hobby of metal detecting, and I should have spoken with both of you before detecting in your yard. Please accept my apologies and my best wishes.
I would sign the letter and leave them my phone number. Hopefully the lady has a change of heart and they welcome me back with a phone call. If I get a call inviting a return trip, I would make sure she is home when I return and I would surprise her with some small gift ( like flowers or vegetables from my garden). Even if I do not get an invitation to return, I have tried to do right be her, and maybe in the process have made life a little easier for her husband.