Seeing Beneath the Surface at Giza

by Glen Dash

Archaeological excavation is slow, painstaking work. Remote sensing can be a helpful adjunct with its indirect look below the surface based on geophysics. In the fall of 2003 we put some remote sensing techniques to the test in a pilot season of the Giza geophysical survey, sponsored by the National Geographic Society with AERA (Ancient Egypt Research Associates), the University of Birmingham’s Institute of Archaeology, and the California firm of Tremaine and Associates. Here, Glen Dash, an AERA board member and Research Fellow at Boston University’s Center for Remote Sensing, describes the work carried out by Tremaine and Associates. – Mark Lehner

The basic technology needed to see beneath the soil has been around for decades. “Remote sensing” tools such as magnetometers, surface penetrating radars and electromagnetic induction machines are routinely used in geological, environmental, and criminal investigations. But their use in archaeology has been relatively limited. Part of the reason is their complexity. In order to do remote sensing well, the operator needs to know something about electromagnetism, geology, and archaeology all at the same time.

One goal of our 2003 geophysical pilot season was to bring together people from all these disciplines to explore unexcavated areas of the Giza Plateau with remote sensing tools. With National Geographic sponsorship and in collaboration with Dr. Zahi Hawass and the Supreme Council of Antiquities, AERA assembled a multi-disciplinary team that included AERA crew members and staff from Tremaine and the University of Birmingham. The Birmingham group carried out radar and magnetometry surveys of part of the areas covered by Tremaine plus a survey of the area north of the Khafre Causeway. Glynn Barrett did a survey of the Menkaure Pyramid and Queen’s Pyramid GI-C using LiDar, a laser scanning technique. In this installment we report only on the Tremaine survey using electromagnetic conductivity.

Tremaine’s Survey

Tremaine uses equipment that “sees” beneath the surface with “electromagnetic induction” (EM). In the simplest terms it works as follows. A transmitting device is suspended over the ground, which generates an electrical current that flows into the soil. A receiving device, located nearby, “reads” the current flowing in the ground. The ground current varies depending on the materials in the soil. Some materials conduct electrical currents readily, such as clay; others impede it. An abrupt change in the current, called an “anomaly,” may signal the presence of a structure or a wall.

We selected a number of areas across the Giza Plateau for remote sensing (Figure 2). Some of these were of special interest to our project, or of interest for understanding cultural features on the plateau. The team chose some locations as test sites. Only a portion of the surveyed area is discussed here since the data are still being analyzed.

The Khentkawes and Menkaure Pyramid Valley Temple Towns

In 1932 and 1933, Selim Hassan found an orderly collection of houses that lined the causeway running east from the tomb of Khentkawes, a queen who lived at the end of the 4th Dynasty. (See AERAGRAM 5/2, Spring 2002.) When Hassan uncovered the walls, they stood chest high and still bore traces of red, white, and black paint. These features were not backfilled and over the intervening 70 years they deflated, leaving little remaining today except for dark patches on the ground.

Nearby is the Menkaure Pyramid Valley Temple. George Reisner began excavations here in the summer of 1908 and found a temple that had been in service from the 4th to 6th Dynasty. During that time it had been destroyed by flooding, was rebuilt, and then invaded from the east by small irregular houses and granaries. Sand layers intervene between the 4th and 6th Dynasty levels testifying to a period of neglect or abandonment. Because these two sites had been previously excavated, they served as ready test beds for the Tremaine technology.

In Figure 3 Tremaine’s data are superimposed over Hassan’s 1932-’33 map. The features found by remote sensing correspond well to the structures Hassan mapped in the Khentkawes town. The northern and eastern walls of the Menkaure Pyramid Valley Temple also show clearly. Within the rectangle lie possible Valley Temple structures or perhaps artifacts resulting from Reisner’s backfilling. He did not excavate the whole temple at one time but rather, excavated the western part first, then the southern, backfilling the western area as he worked. He cleared the court in the east and the northeast sections last, perhaps accounting for the smoothness of the data in that area.

The Soccer Field

It is certain that the large Royal Administrative Building found in 2002 extends under the Abu Hol sports club (Area B in Figure 2). Excavations to date have mapped the northern end of this building, exposing an area 25 meters in length and close to 50 meters wide. The structure could be a royal palace dating to the reigns of Khafre and Menkaure.

Tremaine’s remote sensing results are shown in Figure 4. One obvious feature is the L-shaped anomaly, which seems to align with the Royal Administrative Building. If the L is the juncture of the building's south and west walls, then the structure would extend almost 100 meters from north to south. The feature is approximately one meter below the surface, about the same height as the remaining exterior walls of the building.

The Workers’ Cemetery

The “Workers’ Cemetery” (Areas C-1 and C-2 in Figure 2) sits on the eastern slope of the Maadi formation. Currently being excavated by Dr. Zahi Hawass, the cemetery is thought to be the final resting place for some of the workers who built or serviced the Giza necropolis in the 4th and 5th Dynasties.

The remote sensing results are shown in the maps of C-1 (Figure 5) and C-2 (Figure 6). Rectilinear anomalies dot the areas surveyed. The darkened anomaly to the northeast in Figure 5 could be an outcrop of bedrock or a spoils pile. As one moves to the south and west, the conductivity rises from a lower (yellow) to a higher level (red). This could be a result of bedrock (higher conductivity) dipping under a layer of sand (lower conductivity). The several rectilinear features in the diagram may reflect tomb shafts or voids. In addition, there is an abrupt transition from yellow to red forming a distinctive 90° angle, possible evidence of quarrying.

At the southern end of the Workers’ Cemetery (Area C-2 in Figure 2) Tremaine’s surveys detected a number of features along with one truly huge anomaly, approximately 50 x 15 meters, oriented to the northeast-southwest. Areas of high conductivity probably reflect bedrock close to the surface. Lower conductivity rectilinear anomalies may indicate air or sand filled voids in the bedrock.

The Wadi, the Sphinx, and the Harbor

The Central Wadi (D in Figure 2), lying between the Mokattam and Maadi formations, probably served as the principle conduit for non-local materials used in pyramid building. The sandy wadi surface could hide hauling tracks or even walls built to control flash floods. The eastern end of the wadi may have emptied into an ancient harbor which could have extended from an area to the east of the Khafre Pyramid Valley Temple, up along the eastern edge of the Menkaure Pyramid Valley Temple, and down along the north side of the Wall of the Crow—a distance of more than 250 meters.

Tremaine surveyed the eastern and western sections of the Central Wadi. The results are shown in Figure 7. The eastern section was largely devoid of changes in conductivity which could indicate a relatively uniform layer of sand overlying the wadi floor. To the west, more variation in conductivity was recorded. The L-shaped anomaly could be due to quarrying.

The Sphinx, Sphinx Temple, and adjacent Khafre Valley Temple (E in Figure 2) have buried features that Dr. Zahi Hawass is still discovering. Remote sensing could be useful in detecting these. During excavations in 1980, Dr. Hawass found that a relatively smooth bedrock terrace extends some 35 meters east of the Sphinx Temple. This terrace probably runs no more than 10 or 15 meters beyond the excavated area, based on core sampling done in 1980 by the Egyptian Institute of Underground Water and the Ministry of Irrigation. One coring taken 50 meters east of the Sphinx Temple revealed deposits 16 meters deep: a thick blanket of sand, then dense, dark clay, and finally a hard surface from which the core drill pulled up red granite.

What is visible on the surface in the area today is shown in Figure 8. Further excavations by Dr. Hawass and Mansour Bureik revealed more of the eastern extension of two limestone ramps at least 26 meters in length leading from a terrace in front of the Khafre Pyramid Valley Temple eastward. Part of a group of enigmatic structures here, the ramps slope downward, with one disappearing under more than 3 meters of overburden at an elevation less than 14 meters above sea level. Tunnels pass under the ramps.

In surveying the area directly to the east of the Sphinx, Tremaine found several distinct anomalies. Number 1 in Figure 8 does not correlate with any features visible on the surface and seems to align with the northern end of the Sphinx ditch. This could be an ancient wall. Anomaly 2 is likely to be the remnant of a low wall which once bordered the eastern edge of a stage built in 1969 and removed in 1996. Number 3 is a short, linear anomaly of an unknown nature, probably a void. Drilling and bore scope inspection may be appropriate here. The anomalies labeled 4 are likely evidence of modern road building, perhaps pipes or cables laid parallel to the road edge.

The Golf Course

The golf course (F on map) is twenty green, grassy hectares of archeological terra incognita. The 9th and 18th holes lie at the very foot of the Great Pyramid of Khufu. Thus, there is every reason to expect that features associated with pyramid building will be found here.

Before urban Cairo encroached on this area, the golf course was part of a sandy plain that spread out along the base of the Giza Plateau. Today it sits approximately 18 meters above sea level. Early in the last century this area was inundated by Nile floods. However, in ancient times flood levels were lower. Thus it is possible that Old Kingdom occupation could be found beneath these 20th century flood deposits. Indeed, basalt paving stones associated with the Khufu Pyramid Valley Temple were found reportedly more than 2 meters below the alluvial layer in sewage trenches dug by the British American consortium AMBRIC in the late 1980s.

Geophysically, the golf course is a difficult target. In addition to the flood layers, the golf course has been continuously loamed, seeded, irrigated, and landscaped. All these factors create layers of high conductivity which can mask features below.

Tremaine’s golf course survey data are shown in Figures 9 and 10. Most of what was detected was modern, such as the dark, linear features, which are irrigation pipes. But the nature of the vertical feature in Figure 10 is yet to be determined.

The areas bordered in red are places where the Tremaine device could not go. What shows up within these rectangles are mostly artifacts of computer processing. However, a computer derived 3-D section reveals a distinct layer 2 to 3 meters below the surface. This is the elevation at which the archaeological horizon from the 4th Dynasty would be expected—about 15 or 16 meters above sea level. (Fifteen meters above sea level is one of the “bench marks” for the 4th Dynasty in this area.) By way of comparison, the base of the Wall of the Crow is approximately 15.4 meters above sea level and the base of the basalt pavement of the Khufu Pyramid Valley Temple, about 14.5 meters.

Remote sensing will not take the place of intensive excavation but it is a very useful tool. Tremaine’s work demonstrates that it can provide a head start by locating buried features and offering clues to their nature. We look forward to more insights when Tremaine completes its analysis.

Figure 2

Figure 3

Figure 4: Results from the Soccer Field

Figure 5: Results from Area C-1, a portion of the "Workers' Cemetery."

Figure 6: Results from Area C-2 in the Workers' Cemetery. The areas indicated may be tombs or related structures.

Figure 7: The Wadi itself displays relatively uniform conductivity. It is probably filled with a uniform bed of sand. Around its edges, however, the ancient builders may have quarried for stone. The dotted yellow lines demark the tracks of Tremaine's vehicle.

Figure 8: Surveys near the Sphinx. We detected four distinct sets of anomalies. Anomaly 1 may be a wall which served as an extension of the "Sphinx Ditch" which bounds the Great Sphinx on the north. Anomaly 2 may be the remnant of a stage built in 1969 and removed in 1996. Anomaly 3 is of an unknown nature, perhaps a void beneath the surface. Anomaly 4 is caused by the modern road which runs through the area. The dotted lines show where Tremaine's vehicle ran in performing the survey.

Figure 9: The Mena House golf course lies at the very foot of the Great Pyramid. Tremaine's data is shown at the bottom. The red squares denote areas that the Tremaine vehicle was unable to navigate. Most of the features detected match the surface features and are of modern origin. Further analysis, however, detected more. See Figure 9.

Figure 10: A 3D analysis of the golf course data shows a band of features at approximately the same elevation as the City of the Pyramid Workers AERA has been excavating to the south. The vertical feature may be an ancient wall.