Figure 5.1. Photograph of Vincent Murphy conducting a magnetic survey at a known tomb entrance in the Valley of the Kings, Egypt.
Here, it will be shown how the concepts introduced in Chapter 4 have been applied to some real problems. Three case studies are presented here: two at archaeological sites and the third at the site of buried unexploded ordnance.
The building of great pyramids began during the IVth dynasty (ca. 2700 BCE) in Giza near modern day Cairo. Unfortunately, these great monuments to the Pharaohs also served as billboards to grave robbers announcing the location of valuable treasures. While these pyramids have provided a great deal of information for modern scholars, a great deal more information has been lost through looting that has occurred during the many millennia since their abandonment. For this reason, and perhaps because construction of great pyramids was bankrupting the empire, an era of tomb building into the rocky cliffs in the Valley of the Kings began about 1500 BCE: and persisted through the remainder of the Egyptian
Ramses II, also known as Ramses the Great, ruled Egypt for 66 years (ca. 1250 BCE) during the XlXth dynasty. Ramses is one of the most important figures in history because Egypt reached the height of its political power during his rule. Ramses had numerous wives and even more children. Because he substantially outlived the live expectancy of the time, he was pre-deceased by many of his children. Ramses constructed a large tomb for the interment of his children. It can be expected, based on the length of his rule and his stature during life, that the tomb of the descendents of Ramses will be large, opulent, and provide a great deal of information about his life. It is likely that the tomb of his descendents has been looted. In spite of this, the tomb may still yield historical treasures, such as sarcophagi, which were of little monetary value. This was the motivation for a geophysical study conducted by Vincent Murphy (President, Weston Geophysical) as part of an archaeological expedition, under the direction of Kent R. Weeks (University of California at Berkeley), in 1987. Wiile a number of geophysical tools were tried, it was magnetometry that ultimately located the entrance to the tomb of the descendents of Ramses II.
Natural forces, such as wind and rain, had overcome the intermittent actions of grave robbers and caused the entrance of this tomb to be completely filled with limestone rubble. Murphy, recognizing that the relatively recent deposition would not have the paleomagnetism of the undisturbed limestone (Sect. 4.11, Figs. 4.21 and 4.22), selected magnetometry as one of the prime methods for this search. He tested this hypothesis by making magnetometer measurements alongside an excavated tomb entrance (Fig. 5.1) and this produced the expected magnetic low adjacent to this entrance. With this encouraging result, Murphy began his search for the entrance to the tomb of the descendents of Ramses II. This effort yielded a measured magnetic low similar to that produced by the measurement at the known tomb entrance. Figure 5.2 shows a composite of the magnetic transects along the known tomb entrance and a newly discovered anomaly. Excavations at the location identified by Murphy as a magnetic low uncovered steps carved out of limestone leading through a portal and passageway and into a 30 meter square room supported by 16 pillars. Beyond this chamber, archaeologists discovered the tomb of the descendents of Ramses.
Figure 5.1. Photograph of Vincent Murphy conducting a magnetic survey at a known tomb entrance in the Valley of the Kings, Egypt.
Figure 5.2. Composite of magnetic measurements around a known tomb entrance and a similar anomaly above what proved to be a tomb entrance (Source: The New York Times, February 24, 1987).
For more than 50 years, the United States military has used vast amounts of its property as practice bombing and artillery ranges. Some of the ordnance used in these exercises penetrated the ground rather than detonating on impact and so they remain live to this day. The United States Department of Defense is in the process of relinquishing some of this land for private use under the directive of the Base Realignment and Closure Act.
As part of the base closure activities, the military must ensure that all released land is free of both environmental and safety hazards. The detection and location of buried unexploded ordnance (UXO) has significantly complicated many base closures. The problem is severe because thousands of square miles have been used as practice ranges, for a wide variety of ordnance types and depths. Explosive targets can be as small as a hand grenade, or even smaller, and as large as a 500 pound bomb buried to depths of up to three meters.
Figure 5.3. Contour plot of magnetometer data acquired at the Naval EOD Center.
Figure 5.3 is a contour plot of magnetometer data acquired at the Naval Explosive Ordnance Detection (EOD) Center's Magnetic Test Range at Indian Head, Maryland. A variety of inert ordnance are buried at this range which is used to train the military's explosive ordnance disposal specialists and as a test site for evaluating the efficacy of new methods to locate buried UXO. Each significant magnetic anomaly shown in Fig. 5.3 is labeled and each label is 'keyed' to Table 5.1 which describes each target by type and depth.
Table 5.1. Description and depth of magnetic targets shown in Fig. 5.3.
| target no. | target type | target depth (m) |
| A-1 | 500 lb bomb | 5 |
| A-2 | 500 lb bomb | 5 |
| B-1 | 250 lb bomb | 3.3 |
| B-2 | 250 lb bomb | 3.3 |
| C-1 | 155 mm projectile | 1.6 |
| D-1 | 81 mm mortar | 0.6 |
| E-2 | 60 mm mortar | 0.3 |
The interpretation procedure described in Sect. 4.10.2 becomes clear when viewing Fig. 5.3 in comparison to Table 5.1. The contours above targets A-1 and A-2 exhibit a coarser spacing than those above targets B-2 and C-l and, consequently, targets A-l and A-2 are interpreted as being deeper than other targets. This interpretation is confirmed in Table 5.1. Target orientation can also be inferred by inspection of Fig. 5.3. Target B-2 exhibits a characteristic dipolar nature (Fig. 4.16) with a negative (north) lobe to the north and a positive (south) lobe to the south suggesting that this target is buried with its long axis aligned horizontally and in the north-south direction. All of the other targets shown in Fig. 5.3 are monopolar, with a single positive (south) lobe indicating that these ordnance are buried with their long axes aligned vertically. From the magnetic data shown in Fig. 5.3, it is difficult to establish if ordnance B-2 is a permanent or induced magnet. Figure 5.4 displays two contour plots of measured magnetic anomaly over a 60 mm mortar shell placed on the ground surface. The same target is used in both elements of this figure. However, the axis of the mortar shell is aligned in the north-south direction in Fig. 5.4a and in the east-west direction in Fig. 5.4b. Since the dipolar character of the magnetic anomaly rotates with the long axis of the target, it is expected that this, and probably all other, ordnance behave as permanent magnets.
Figure 5.4. Contour plot of the magnetic anomaly produced by a 60 mm mortar shell with its long axis oriented (a) north-south and (b) east-west. The orientation of the mortar shell is annotated.
To illustrate how the use of a horizontal gradiometer (Sect. 4.9) can complicate the pattern of magnetic measurements (Sect. 4.12.3), measurements were repeated over the east-west oriented mortar shell (Fig. 5.4b) with both an east-west and north-south horizontal gradiometer (Fig. 5.5).
The east-west horizontal gradiometer configuration is shown in Fig. 5.5a. Comparing this figure to the magnetometer measurement (Fig. 5.4b), it is obvious that, as expected (Figs. 4.34-36), a dipole in the magnetometer data has become a tripole in the east-west gradiometer data. The situation is even more complex for the north-south horizontal gradiometer (Fig. 5.5b), where each pole of the dipole itself becomes a dipole leading to the four poles shown in this data.
Figure 5.5. Contour plot of the horizontal gradiometer anomaly produced by a 60 mm mortar shell with its long axis oriented east-west for (a) an east-west horizontal gradiometer and (b) an north-south horizontal gradiometer.
Many archaeological excavations focus on the so called 'elite' sectors of sites such as public and religious centers and the residences of the most important individuals and families. The reason for limiting excavations to such 'high payoff areas is that archaeological excavation is a slow and tedious process that is not financially well supported. It is, therefore, necessary for archaeologists to focus their limited resources on areas that can yield the most information. However, this manner of study often results in an incomplete understanding of the society as a whole. While it is quite important to the overall knowledge of ancient societies to investigate domestic sectors of ancient cities in order to characterize both their spatial and cultural relationships to the elite sectors, in many cases this is a luxury archaeologists cannot afford.
In certain parts of the world, such as Central America, cities were occupied for a limited time and then abandoned without any later occupation. In these areas, the ruins of ancient civilizations exist at, or near, the surface so that both elite and domestic architecture can be studied with little or no excavation. Near Eastern cities, however, were typically occupied for thousands of years so that their initial occupations could be buried under meters of later construction. This fact significantly increases the complexity of excavation and presents further justification for limiting these excavations to high payoff sectors of a site. Because of the climate and nature of Near Eastern construction techniques, based largely on mudbrick construction, ancient settlements commonly appear as formless mounds leaving archaeologists to rely only on their instincts to identify the most promising areas within a site for excavation.
Titriş Höyük was a small, Early Bronze Age (EBA) city located on the Euphrates river basin of southeastern Turkey. This former city was under excavation by University of California archaeologist Guillermo Algaze and others between 1990 and 1999. The city consisted of three main areas: the Outer Town, the largest single sector of the site; the Lower Town, extending along a now nearly dry tributary of the Euphrates river; and the Acropolis or High Mound area. What makes Titriş Höyük unique in the Near East is that it was occupied between 2500 and 2200 BCE and then the Lower and Outer Towns, which together comprised about 90% of the city's 125 acres, were abandoned. This left archaeologists with a Near Eastern EBA city almost completely free of shallower strata resulting from later occupations. From a geophysical perspective, Titriş Höyük is important because it has become a 'test bed' for the use of magnetometry in characterizing an archaeological site.
The physical conditions at Titriş Höyük make it an ideal site to study both the elite and domestic culture of the EBA in the Near East. Titriş Höyük was one of many fortified cities in the region that emerged around 2500 BCE from farming villages in the northern part of the ancient Near East. The reason for this rapid urbanization is the subject of considerable speculation. To date, three hypotheses have been suggested for the development of these cities:
Increased rainfall could have occurred at this time in the northern reaches of the fertile crescent and boosted the agricultural productivity of region,
Agricultural villages consolidated into fortified cities as a protective measure against military incursions, and
These cities became sites of intersecting trade routes believed to have been developed throughout the Near East during this period. Thus the rapid rise of these cities was stimulated by a boom in trade-based economy.
With all of the potential benefits offered to archaeologists at Titriş Höyük, the site remains too large for a complete excavation. The benefit of the application of geophysics at this site is the potential that such efforts could differentiate various components of the culture, for example areas of thicker wall construction, larger rooms, or wider roads. In turn, this information could be used to select limited areas of excavation such that all cultural components would be sampled. Figures 5.6 and 5.7 show the Outer and Lower Towns, respectively.
While these figures look remarkably like airborne imagery, they are, in fact, a display of magnetic data. This data was acquired by Lewis Somers of Geoscan Research Incorporated. At Titriş Höyük, the data were acquired in a manner similar to that described in Sect. 2.9.2 by first establishing a measurement grid (Fig. 2.23), here one meter by one meter grid cells, and then acquiring data at each grid cell using a flux-gate gradiometer (Sect. 4.9). In Figs. 5.6 and 5.7, the measured value from the gradiometer at each grid cell is assigned a gray level, where white is assigned to the lowest measured values, black is assigned to highest measured values, and the intermediate values are assigned various shades of gray. By filling each grid cell with the assigned value, the display of the magnetic data has the appearance of a black and white photograph. In order to interpret the information displayed in these figures, it is useful to review how magnetic anomalies are detected. As established in Sect. 4.10.2, various subsurface features have different magnetic signatures. Iron-bearing metals might appear as relative magnetic highs or lows (monopoles), appearing as black or white, respectively, in the gray-level presentations. They can also manifest a dipole response appearing as an adjacent black and white feature in Figs. 5.6 and 5.7. Limestone walls would displace some volume of iron-bearing soil and, because of the paleomagnetism of this soil, would be manifested as a magnetic low (Sect. 4.11, Figs. 4.21 and 4.23). These features would appear as white areas in Figs. 5.6 and 5.7.
Returning to the interpretation of Fig. 5.6, it is clear that there are many areas of the Outer Town that are displayed in near uniform gray tones. This suggests that these areas contain no detectable cultural information. Other features, as noted, are modern and appear on the ground surface. These are a Wadi (a dried up river bed) and a road. The remaining features are all buried. The sharp linear features are ancient roads that appear as magnetic highs because they are composed of broken compacted firedclay pottery sherds. The 'fuzzier' broken linear feature to the east is a fortified wall. The small, more finely grained black and white features are presumed to be walls and similar structural elements. The Lower Town magnetic data (Fig. 5.7) suggests a denser development. Almost the entire region shows strong evidence of construction with more roads.
Figure 5.6. Magnetic data, displayed as gray levels, over the Outer Town of Titriş Höyük.
Figure 5.7. Magnetic data, displayed as gray levels, over the Lower Town of Titriş Höyük.
The interpretation of Algaze and Somers was tested by means of two large-area excavations of more than 500 square meters and several small-scale excavations. Figures 5.8 and 5.9 are photographs of the Outer Town and Lower Town excavations, respectively.
Figure 5.8. Photograph of the Titriş Höyük Outer Town excavation.
Figure 5.9. Photograph of the Titriş Höyük Lower Town excavation.
Both of these photographs clearly show the barren landscape of the site. There is no surface evidence of the buried architecture revealed by the magnetic survey.
Excavations in the Outer Town revealed this area to be more domestic with numerous storage pits, a grain silo, parts of eight structures, and the city wall in the northeastern corner of the site. These excavations also uncovered a massive building complex with walls in excess of one meter thick. This complex is believed to be an administration building. Excavation at the fortification wall established this wall to be constructed of mud brick over a stone foundation. A moat was also discovered alongside the wall. The Lower Town excavations established this area to have similar architecture to the Outer Town but artifacts found in the Lower Town suggest that this area was wealthier. In the words of Guillermo Algaze, 'It was the high rent district.'