The United States Geological Survey has a website that shows ground level EMF activity changes that are substantial. Of course, this map is made for more nominal uses such as predicting power grid and other potential interference. However, as paranormal researchers, if you believe spirits use that energy to help manifest themselves in some manner, then keeping this website close by for research before and after an investigation could help add anecdotal evidence to any investigation you are doing (or even potentially offer some level of evidence to debunk activity). Scroll down the map to see what is current in the United States in real time. You can also view the site on it's own by going to http://geomag.usgs.gov/map/.