Figure 3.1. Map of an area in downtown Raleigh, North Carolina showing the gravity measurement points (dotted lines) and the Museum of History that has an underground parking garage.
In Chapter 2, it was shown how gravity measurements can be used to detect subsurface features characterized by densities that differ from that of their surroundings. Here, the gravity method will be further illustrated through the description of two case studies. The first case study involves the use of gravity to detect and locate an underground structure that might be typical of a secret military facility. The second case study deals with the location of a massive object that might be relevant to the cause of dinosaur extinction 65 million years ago.
The political situation in the Mid- and Far-East has resulted in numerous news items concerning the possible existence of secret underground facilities for the production of nuclear, chemical, and biological weapons. In order to keep such facilities secret, it is logical that they be located underground. Initially, it might be thought that the best location for a secret underground facility is in some remote part of the country. It would, in fact, be quite easy to detect a facility that is so located because there would necessarily be electrical power lines apparently going nowhere (even underground power lines in rural areas are easily detected) and because of the necessary release to the environment of effluent by-products of the production process, Hiding a secret facility is like hiding a tree in a forest. The best possible site is within an industrial complex where the presence of electrical power and chemical effluents are expected. The potential for on-sight inspection and surveillance suggests that secret facilities be developed underground within an industrial complex and even as secret basements below surface structures.
The detection and identification of an underground feature beneath a surface structure is one of the most challenging problems in geophysics. The problem has been encountered in a number of applications such as the early detection of leaks in the bottom of large petroleum storage tanks, the presence of chambers beneath the great pyramids of Egypt, and, most recently, the detection of secret underground facilities. The factors that make this particular problem so challenging are directly related to both the surface activities and the constraints on measurement geometry.
Gravity promises to be the best method for detecting a secret underground facility. In this section, we will examine the reasons why this is so and review a field study conducted to prove this hypothesis. The potential for gravity to be useful in this problem may not be immediately obvious. In Sect. 2.6 it was established that the measured gravity anomaly is proportional to the volume of the feature, and the difference between the density of the feature and its surroundings. It might seem that a massive industrial building is a much larger gravity anomaly than an underground structure. This is not the case! This can be established by introducing some densities. The density of air is so low that, for the purpose of gravity measurements, it can be considered zero. The density of rock is about 2.7 grams per cubic centimeter. Any structure that is suitable for human occupation consists of about 95% air by volume. For simplicity, it is assume that all of the surface structure that is not air filled will have the density of rock. The average density of the surface structure is then
| 0.95 | (air fraction) | |
| X | 0 | (density of air) |
| + | 0.05 | (solid fraction) |
| X | 2.7 | (density of solid material) |
| = | 0.135 | grams per cubic centimeter |
The surface structure is embedded in the air so that the difference in density between the surface structure and its surroundings is
| 0.135 | (density of surface structure) | |
| - | 0 | (density of air) |
| = | 0.135 | grams per cubic centimeter |
The underground structure must also be 95% air and, therefore, its density is also 0.135 grams per cubic centimeter. The underground structure is embedded in rock rather than air so that the density difference between the underground structure and its surroundings is
| 0.135 | (density of underground structure) | |
| - | 2.7 | (density of rock) |
| = | -2.565 | grams per cubic centimeter |
It is now seen that the density difference of the underground structure is almost 20 times greater than the density difference of the surface structure. This implies that, if the surface and underground structures were the same size and the same distance from the measurement point, the measured gravity anomaly from the underground structure would be 20 times that of the surface structure. Thus, the underground structure could be smaller and further away from the measurement point (deep, for example) and still be detected in the presence of the surface structure. Referring to Figs. 2.21a and 2.25, it can be observed that, although the magnitude of the measured gravitational anomaly is greatest over the top of the buried sphere, there is still a significant gravitational anomaly measured on the ground surface but away from the buried feature. This suggests that the underground structure can potentially be detected by measurements made around the perimeter of an overlying surface structure.
In order to test the hypothesis that gravity measurements can be used to detect a secret underground structure, a gravity field study was conducted in an area of downtown Raleigh, North Carolina. The study area is shown in Fig. 3.1. The target of interest here is the Museum of History, a three story surface structure on the corner of East Edenton and Wilmington Streets.
The Museum has an underground parking garage consisting of one level below and around part of the building and two levels below and around another part of the building. This site was selected for the study because the museum has an underground component and because the two unidentified buildings to the west of the museum are three story structures of similar size but with no underground components. This allows a direct comparison of measured gravity anomalies around buildings of a similar size but with and without underground basements. Another building of importance in these measurements is the North Carolina State Legislative Building.
The acquired gravity data is plotted in Figs. 3.2 through 3.5. Figure 3.2 shows the gravity data along East Edenton Street. This is the most important measurement line since it passes beside the museum and two similarly sized buildings without basements.
Figure 3.1. Map of an area in downtown Raleigh, North Carolina showing the gravity measurement points (dotted lines) and the Museum of History that has an underground parking garage.
Figure 3.2. Annotated graph of measured gravity anomaly as a function of position along East Edenton Street.
Figure 3.3. Annotated graph of measured gravity anomaly as a function of position along the Plaza line.
Figure 3.4. Annotated graph of measured gravity anomaly as a function of position along Jones Street.
Figure 3.5. Annotated graph of measured gravity anomaly as a function of position along Wilmington Street.
It is clear that the only significant negative gravity anomaly (negative because of the negative density difference between the underground structure and its surroundings) is centered between the limits of the underground garage, rather than centered with respect to the surface structure. The Plaza line is also quite interesting because this line begins against the North Carolina State Legislative Building and passes over a portion of the museum's underground garage. Note that there is no significant gravity anomaly adjacent to the State Legislative Building, a purely surface structure. However, there is a quite strong (about -500 μGals) anomaly over the top of the garage. Also note that there is a relatively flat spot in this low (about -400 μGals) when the measurements are over the one-level underground structure and the anomaly becomes more negative when measurements are over the two-level underground structure. The Jones (Fig. 3.4) and Wilmington (Fig. 3.5) Street lines also clearly show the underground structure with minimal evidence of any surface structures.
At least for underground structures of the size and depth present in this study, it appears that gravity measurements are a viable method for detecting the presence of underground structures even in the presence of surface structures.
Fossil records show that the entire dinosaur population, as well as many other species, became extinct at the end of the Cretaceous period about 65 million years ago. While this mass extinction was tragic for the species living at the time, it was quite fortuitous for modern mammals, including humans, since it is unlikely that higher forms of mammals could have evolved in the dominating presence of the dinosaurs.
It is generally accepted that a planetary climate change is responsible for the mass extinction at the end of the Cretaceous—beginning of the Tertiary periods, known as the K-T boundary. However, the cause of this climatic change is hotly debated by scientists. Furthermore, it has been suggested that either global warming or global cooling could produce an extinction of the dinosaurs. While most scientists believe that the climatic change occurred rather suddenly, some paleontologists claim that fossil records indicate that dinosaur extinction occurred over millions of years. It is known that the gender of some modern reptiles is determined by the incubation temperature of the eggs. Consequently, a relatively slow increase or decrease in the global temperature could result in all individuals in the dinosaur or reptile populations being either male or female. Such a single gender situation would result in the extinction of the species. A gradual change in global temperature would not explain the extinction of warm blooded species that could better adapt to this changing environment. For this reason, it is generally accepted that the mass extinction at the K-T boundary occurred as a result of one or more short-term catastrophic events.
The current scientific argument over mass extinction focuses on two possible causes—a meteoric impact or volcanic activity. For the impact of a single meteor to cause a substantial climatic change, the meteor must have been extremely large. On the other hand, it is unlikely that a single volcanic eruption could produce a climatic change of sufficient magnitude to cause a mass extinction. Therefore, the volcanic eruption hypothesis requires numerous volcanic eruptions to occur over a relatively short time. This leads to a third hypothesis that a meteoric impact caused near-simultaneous or sequential volcanic eruptions.
Figure 3.6. False-color, wire frame plot of gravity data acquired over the Chicxulub Crater (reprinted courtesy of the Lunar and Planetary Institute and Virgil L. Sharpton, University of Alaska, Fairbanks)
In 1980, Alvarez1 used the presence of an Iridium layer found at many places in the K-T boundary to support the meteoric impact hypothesis because Iridium is found in extra-terrestrial bodies. However, Iridium deposition is also associated with volcanic eruptions. An important piece of evidence in support of the meteoric impact hypothesis is a crater associated with the impact of large meteor. Because of the geologic forces acting over the 65 million years since the postulated occurrence of this event, such an impact may not be manifested on the ground surface. In 1981, Penfield and Camargo2 reported measurements of concentric magnetic and gravity anomalies in northernmost Yucatan, Mexico. Intense study of this region has established that this feature is, in fact, an impact crater and is now known as the Chicxulub Crater. This crater is approximately 170 km in diameter and lies buried below 300 to 1100 m of rock. Figure 3.6 is a false-color wire frame plot of recent gravity data acquired over the Chicxulub Crater, Although this feature is now completely buried, the presence of a crater is dramatically illustrated in this figure. The crater represents a 20 to 30 milliGal low as compared to the regional gravity values with a 15 to 20 milliGal relative high in the center.
Numerous studies, including computer simulations, suggest that an impact crater the size of Chicxulub would require the meteor to be approximately 10 km in diameter and enter the Earth's atmosphere with a speed of about 20 km per second. The energy yielded by the impact of this meteor would be 10,000 times greater than the simultaneous detonation of all existing nuclear devices. The immediate consequences of this impact would include fire storms, tsunamis, shockwaves, and the ejection of huge quantities of dust into the upper atmosphere. An asteroid 200 m to 1 km in diameter impacting anywhere in the Atlantic Ocean would destroy coastal areas on both side sides of the Atlantic and would reach the foothills of the Appalachian mountains in the northern Unites States.3 The shock wave from an 80 m diameter asteroid or meteor impact would destroy buildings and trees over a 2000 square km area. The dust ejected into the upper atmosphere would block a substantial amount of sunlight which, in turn, would cause climatic changes lasting for about 10,000 years. The Chicxulub Grater is the largest crater yet found on Earth and it may be the smoking gun in the case for mass extinction by meteoric impact.
1. L.W. Alvarez et al., 'Extraterrestrial cause for Cretaceous-Tertiary extinction', Science 208 (1980): 1095-1108 (1095).
2. G.T. Penfield and Z.A. Camargo, 'Definition of a major igneous zone in the Central Yucatan Platform with aeromagnetics and gravity', Society of Exploration Geophysicists 51 (1981): 38-39 (37).
3. J.G. Hills, 'Consequences of impacts of small asteroids and comets with Earth', in New Developments Regarding the KT Event and Other Catastrophes in Earth History. Houston: Lunar and Planetary Institute, 1994, p. 50.