National Parks Service, Title 36......
[Code of Federal Regulations]
[Title 36, Volume 1]
[Revised as of July 1, 2001]
From the U.S. Government Printing Office via GPO Access
[CITE: 36CFR2.1]
[Page 17-18]TITLE 36--PARKS, FORESTS, AND PUBLIC PROPERTY
CHAPTER I--NATIONAL PARK SERVICE, DEPARTMENT OF THE INTERIORPART 2--RESOURCE PROTECTION, PUBLIC USE AND RECREATION--Table of Contents
Sec. 2.1 Preservation of natural, cultural and archeological resources.
(a) Except as otherwise provided in this chapter, the following is
prohibited:(7) Possessing or using a mineral or metal detector, magnetometer,
side scan sonar, other metal detecting device, or subbottom profiler.This paragraph does not apply to:
(i) A device broken down and stored or packed to prevent its use while in park areas.
http://www.access.gpo.gov/nara/cfr/waisidx_01/36cfrv1_01.html
Good luck, and if it seems for real, make sure you carry a copy of the permission at all times.
follow this link for full regulations.