The refraction survey Robert Schoch ran with the geophysicist Thomas Dobecki in April 1991 around the body of the Great Sphinx, published as Dobecki and Schoch, “Seismic Investigations in the Vicinity of the Great Sphinx of Giza, Egypt”, Geoarchaeology 7(6), 1992. It is the only instrumented measurement in the whole dispute, and the fullest account of it in print is chapter two of Origins of the Sphinx.
Method and result
Four lines, a five-kilogram sledgehammer on a steel plate and twenty-four geophones, all four on the same limestone member that floors the enclosure. On the north, south and east sides the weathered layer under the floor runs 1.8 to 2.5 metres deep; behind the rump it runs about 1 to 1.2 metres. Schoch’s own comment on the asymmetry is that “these results were completely unexpected.” The reading he offers is that the body was not freed from the bedrock in one campaign: flanks and front cut down first, the rear still merged with the rock, the rump finished later. The same survey found a rectangular low-velocity anomaly under the left paw about twelve metres by nine, and a buried cliff east of the Sphinx Temple.
The hinge
The 1992 paper converted the depth ratio into a date by simple proportion and got 7000 to 5000 BCE; the 2016 bracket inside the reprint says “I now believe I severely underestimated the age of the original structure,” and the recalibration reaches circa 10,000 BCE. Both conversions assume that the floor behind the rump was first exposed around 2500 BCE, which is a Khafre date borrowed from the chronology the book is attacking, and both assume a weathering rate Schoch calls crude and then treats as roughly linear after arguing that it is not.
The core nobody has drilled
The layer has never been sampled. Mark Lehner’s colleague James Harrell put it plainly in 2000, in a line the book quotes against itself: “He has not dug or drilled into this layer and so has no idea of what is really down there.” Schoch’s own 2016 bracket, at the end of the list of tests he wanted run, reads: “Unfortunately, as of 2016, none of this work has taken place.” It still has not.
Connections
Robert Schoch · Great Sphinx · Origins of the Sphinx · Khafre · Sphinx Water Erosion Hypothesis · Mark Lehner · Colin Reader · Valley Temple and Sphinx Temple · Edgar Cayce · Thomas Dobecki