I just found this: https://nsarchive2.gwu.edu/NSAEBB/NSAEBB372/docs/Underground-GoingDeep.pdf

"Finding deeply buried facilities stands out as one of the toughest technical challenges in the Air Force’s efforts to find, fix, target, track, engage and assess targets of interest anywhere on earth. Use of deeply buried facilities is prevalent among potential enemies of the United States such as Iraq, Libya and North Korea because they realize they cannot confidently defend their airspace against United States airpower. Such nations have located potential high-value, high- interest targets such as weapons of mass destruction (WMD), WMD manufacturing plants and storage areas, missile garrisons, fuel storage areas and command and control nodes underground. All of these targets are critical to an enemy nation’s ability to project power beyond its borders and threaten United States allies, friends and interests abroad. Deeply buried facilities, whether they are bunkers buried under many meters of earth or tunnels dug into mountains, are hard find. In many cases only a door or an air vent reveal the presence of what could be a deeply buried facility with thousands of square meters of floor 1 space. These small aboveground signatures can be easily masked from visible, infrared and other sensors making detection by existing intelligence assets very challenging. Furthermore, most deeply buried facilities of interest are located in denied areas, meaning any sensors must operate from standoff ranges beyond the reach of an adversary’s armed forces. There is no “silver bullet” intelligence solution to the problem of detecting deeply buried facilities. The solution comes from combining several sources of intelligence, including imagery intelligence, e.g. photographs from satellites or aircraft; measurement and signature intelligence, e.g. detecting heat, sound, vibration or chemical exhaust; signals intelligence, e.g. analyzing radio or radar signals, and human intelligence, the use of spies and informants, to locate and characterize a deeply buried facility. Effectively integrating these intelligence sources to find a deeply buried facility becomes easier when the approximate location of the targeted facility is known. Obtaining the approximate location of the facility requires an intelligence capability that can cover wide search areas, determine the rough location and size of a deeply buried facility from stand-off ranges and do so using technology that is difficult to mask or counter by an enemy. This capability could then be used to cue other intelligence assets to focus in on a specific area and collect more detailed information about the facility. To find such a capability it is necessary to go outside the realm of traditional intelligence systems and look at ways that scientists or other specialists may use to understand what is going on beneath the surface of the earth. Geophysicists and geologists used gravity field measurements for over 100 years to analyze geologic structures beneath the earth’s surface. Gravity field measurement is useful in understanding the types of soil, rocks and minerals beneath the earth because variations in the density of underground soil and mineral deposits cause minute but measurable variations in the force of gravity. Geologists, for example, can 2

locate oil by looking for variations in gravity field strength caused by the low-density underground salt domes that contain oil. Similarly, it may be possible to detect deeply buried facilities by measuring the gravity field variations caused by the void created in the rock or soil where the facility is built. Intelligence officers could look for gravity field variations caused by deeply buried facilities that are less dense than the earth around them in the same way oil geologists look for salt domes that are less dense than the rock around them. Since it is very difficult to alter the earth’s gravitational field, it would be equally difficult to mask the gravity field variations caused by a deeply buried facility, making gravity field measurement a reliable source of intelligence for deeply buried facility detection. Gravity measurement technology in use for geophysics applications today could be advanced and integrated into an effective system for locating deeply buried facilities from space. Experience gained from current space borne, ship borne and airborne gravity field measurements, combined with ongoing research, provides a solid technical foundation to create a space system capable of deeply buried target detection. Data gathered by such a system could then cue other types of sensors to the targeted facility. Working together, these sensors would characterize the construction and functions of the facility in a manner that is very difficult to spoof. Information about the construction and functions of the facility will be necessary for those planning to destroy or neutralize the facility. This paper will focus on the development and application of gravity field sensors for deeply buried target detection. It will begin with an explanation of what deeply buried facilities are and how their construction and use has evolved over the years. Next, the discussion will look at the weapons available today to counter deeply buried facilities and the intelligence information needed to make these weapons effective. The information needed to properly target these 3

weapons will directly influence the capability of the sensors needed to detect and characterize deeply buried facilities. The focus of the paper will then shift to intelligence sources, starting with the intelligence capabilities available today and the ability of a potential enemy to counter those intelligence assets, then shift to exploring intelligence sources not in use today and their potential application in the hunt for deeply buried facilities. Finally, this paper will investigate gravity measurement technology to address the problem of deeply buried facility detection and characterization. The examination will include the history of gravity measurement technology, current uses for geology and earth science, ongoing laboratory developments and the applicability of this technology to the search of deeply buried targets. The discussion will conclude with desired system capabilities and potential system concepts. Gravity field measurement has proven its worth in over 100 years of oil geology and earth science applications. It has enormous potential to help with the search for deeply buried targets. The effort to determine how much gravity field measurement can help in the search for deeply buried targets must start with understanding deeply buried targets themselves."

No patience to read the rest right now. File for later. This guy's more than just a prepper it seems.