Austin Aguirre
Archaeology
2012
James L. Dykman
Geophysical Survey
Geophysical is a rather newer method used in archaeology today. They have been used
since 1946 and have been increasingly used since then, thus still considered a newer method of
surveying. The earliest method was first used earlier than this, much earlier. Every method is
different and somewhat unique and they each have their positives and their negatives. There are
methods used more commonly and others used not so much. The most common methods are
often argued over which method is the best. The techniques are rounded up as so; airphotos and
modern digital imaging systems, seismic methods, and magnetic methods. I will also be going
over a few of the lesser known methods.
Archaeologists or classical archaeologists surveyed using shovels to trench and used
trowels and brushes. This requires an enormous amount of human labor and often provided by
unpaid student labor. Enthusiasm can only provide payment up to a certain point, which is also
hard labor. (Wynn, 2012) Geophysical methods can survey an area of 10,000 square meters per
day. (Wynn, 2012) As a result the enormous expenditure of human hands and knees effort can
now be used much more efficiently. The reason for the growing nature of this method is that it
can provide an extremely rapid, three dimensional reconnaissance of a site. They can also
provide a synoptic view of the potential human cultural resources of a target area. Surface
geophysical methods currently in use can usually detect soils disturbed by burials. (Wynn, 2012)
They can identify hollows and voids in structures such as pyramids and ancient fortifications,
and can map buried stone foundations. (Wynn, 2012) All this can be done rapidly, without
disturbing the ground. This ability to explore without damaging a site probably does not seem
important to a geologist but to an archaeologist, however, excavation opens the way for the
inevitable destruction of preserved remains by weather and vandals. (Wynn, 2012) Excavation is,
in fact, destruction. “Archaeology is the one science that destroys its own lab-no repeated
experiments, it better be done right the first time, or not at all.” J. W. Weymouth remarks. In
short geophysical survey methods are rapid, non-destructive techniques for identifying structure
in the ground. Because of their effectiveness and speed, the emphasis is now on high speed, high
density survey techniques. Density being the number of readings per unit area. (Wynn, 2012)
This is why geophysical survey methods are becoming more and more common today, or are
now common today; depending in the continent you live in. (Clay, 2001)
Probably the first instrument-based prospection method applied to archaeological sites,
first used in England just after World War 1, was aerial photography. (Wynn, 2012)
Archaeologists have frequently used it since then because it gives a synoptic view, something
especially useful before a site is excavated. (Clay, 2001) Aerial photogrammetric methods are
particularly useful because they help identify soil marks and crop marks. (Wynn, 2012) These
subtle differences in soil color, moisture content, and texture indicate buried structures such as
walls, or past human agricultural activity. A typical application of this method is to photograph a
site from a low-flying airplane or a tethered balloon. (Wynn, 2012)Fine-scale contour base maps
are then constructed from photos taken in stereo pairs. Photos are also used to look for any
features not readily visible on the ground that may give the archaeologists some place from
which to start their excavation efforts. (Clay, 2001)
Airborne and space-borne digital imagery has been used experimentally since the 1960s with
only limited success. (Wynn, 2012)The resolution of the imagery, especially LANDSAT, is too
low to allow effective use in intra-site archaeological prospection. (Wynn, 2012) A good
summary of the modern state-of the-art is found in Ebert, who points out that such imagery can
sometimes identify land forms and vegetative types conducive to human occupation. (Clay,
2001) Berlin et al. Have successfully used Landsat imagery to map areas in northern Arizona that
had been cultivated and then abandoned almost 700 years ago. (Wynn, 2012)
The U.S. Geological Survey and NASA have experimented with aircraft digital imagery as a tool
for mapping mineral resources. (Wynn, 2012) This kind of imagery has considerably higher
resolution than LANDSAT data, and is limited only partly by the height above ground. It also
covers a much wider spectral range than even the space-borne LANDSAT Thematic Mapper
system. (Wynn, 2012) To the author’s knowledge, this kind of imagery has not yet been used in
an archaeological application, but could prove extremely useful. Extensive bibliographies on airand space-borne remote sensing applications in archaeology are available in Lyons et al. (Wynn,
2012)
Thermal methods, both ground and airborne, have great potential for use in the
archaeological sciences. (Clay, 2001) These methods include temperature probes placed in the
ground and thermal infrared imagery, used to map subtle temperature differences in exposed
surfaces and soils. (Wynn, 2012) Thermal-inertia methods, using aircraft thermal imaging
systems flown at two different times, are also included in this category; buried stone blocks, for
example, lose thermal energy at different rates than the surrounding topsoil, resulting in
anomalous day time night time thermal ratios. (Wynn, 2012) Specific applications could include
mapping voids and other structures in ancient fortifications such as those that exist in Peru,
Egypt, and Jerusalem. Perisset and Tabbagh have demonstrated that digital imagery in the
infrared can be used in archaeological applications. (Clay, 2001)
Historically, bosing has proven to be a useful but nonquantitative exploration tool. (Clay,
2001) Bosing refers to thumping the ground with a heavy rammer- to detect different sounds
caused by hollows, structures, and soils of different compaction. (Wynn, 2012) A sonic
spectroscope was used to test stone and brick walls for voids and variations in thickness. The
method uses transmitted sonic energy in the frequency range of about 20-3000 Hz. (Wynn, 2012)
Resonance effects are detected by means of a wide bandwidth accelerometer or geophone
attached to a wall. Typically, the low natural resonant frequency of a solid wall will increase in
the vicinity of a void. (Wynn, 2012) This is the seismic method.
Geophysicists have experimented with refraction seismic methods in archaeological applications
with relatively little success. (Wynn, 2012) Refraction methods work best in mapping
undisturbed layers that have velocities increasing with depth. (Clay, 2001) The method becomes
less useful and interpretation becomes very qualitative and difficult when there are velocity
inversions representative of human cultural disturbance, or highly three-dimensional objects such
as burial sites or stone foundations. (Wynn, 2012)
Seismic-reflection methods are known to work well in marine applications. (Wynn, 2012)
Shipwrecks buried in sediments in the Mediterranean stand out clearly by means of this kind of
technique. Seismic-reflection methods have also been used to detect cavities in otherwise
homogeneous rock masses or in ancient stone structures. (Wynn, 2012) One particular example
was the detection of a cavity beneath the ancient mosque/synagogue at Machpelah in modern
Israel, where tradition suggests that Abraham, the ancient Hebrew prophet, was buried. (Clay,
2001) Seismic-reflection methods were also used to map faults and cavities in Chephren’s
pyramid at Giza in Egypt. (Clay, 2001)
High-resolution structure mapping technologies are the most recent application of seismic
reflection in archaeology. (Wynn, 2012) These methods are the same as those used to investigate
sites for ocean-drilling platforms. Recently, they were used to map sites of potential human
occupation on ancient shorelines beneath the sediments of the Gulf of Mexico. (Clay, 2001)
These surveys have resolution capabilities sufficient to permit the identification of maddens and
perhaps even dwellings. (Wynn, 2012) They hold out the promise of making major contributions
to the study of the early human migration across the Bering Straits, where most potential sites are
now underwater. (Wynn, 2012)
Magnetic methods were first used in the 1950s, and have since become the backbone of
archaeological prospection. (Wynn, 2012) They are now used even more frequently than
electrical prospection methods. (Wynn, 2012) Typically, soils that have had a campfire
maintained over them develop an increased magnetic susceptibility resulting from the
consequent reducing environment. (Wynn, 2012) This reducing environment causes the
formation of magnetite if even moderate amounts of iron are present. Magnetometers can easily
detect variations of less than 0.1 percent magnetite content in the soil. (Clay, 2001) Soils
compacted by human occupation or disturbed by a burial will also show a variation from
background values of magnetic susceptibility. (Wynn, 2012) Burials frequently cause localized
oxidation, creating a void in magnetite content. These phenomena have been exploited by
geophysicists to produce large-scale maps of aboriginal occupation sites in the Great Plains of
North America. (Wynn, 2012) They have also been used to map Roman-age occupation sites in
Europe. Archaeologists now even use susceptibility meters to map soils modified by human
activity. (Wynn, 2012)
Field procedures for the magnetic prospection method have been refined considerably in the last
two decades. (Clay, 2001) Some archaeologists now use magnetic gradiometers and
microcomputer-controlled automatic data gathering systems to make low-noise, highproductivity surveys. (Wynn, 2012) It is not unusual for a field crew to acquire as much as a
third of a hectare of 1-m-grid data in a single day. (Clay, 2001) Some archaeologists have
reintroduced fluxgate magnetometers to speed up the data gathering process even further.
(Wynn, 2012) Experiments have also been carried out to physically model the effects of
campfires and human occupation. This results in better interpretation of field acquired data.
Radiometric data have been experimented with at least since 1967. (Wynn, 2012) Their
use stems from the fact that soils normally have greater uranium, thorium, and potassium content
than underlying sedimentary rock. Consequently one would suspect that soil-filled pits would
give anomalously high radioactivity. Human remains and middens also have high levels of
phosphate in them along with attendant radiogenic isotopes. (Wynn, 2012) However, soils only a
few centimeters thick can mask any radioactivity generated by sources buried beneath them.
Measurement times with a small portable crystal detector are also on the order of a minute per
reading. These limitations will probably prevent the method from ever being widely used.
(Wynn, 2012)
Neutron scattering can detect moisture content in anthropogenic soils. It is theoretically
possible to detect stone structures beneath a porous soil, or voids behind a rock or brick wall.
Penetration limits of 10-20 cm, however, make it hardly worth serious consideration. This
method could detect a void behind one brick thickness of a wall. (Wynn, 2012)
Gravity methods were tried in archaeology several times since 1965. Anthropogenic
gravity signatures are typically extremely small, however, and usually close to the instrumental
noise threshold. (Wynn, 2012) Centimeter-precision accuracy in elevation control is absolutely
necessary when applying the gravity method in archaeology. (Clay, 2001) This accuracy
requirement, and similar precision requirements in calculating terrain corrections, have
significantly dampened the interest of geophysicists who might have considered applying this
method. (Wynn, 2012)
Geophysical survey methods are ever-growing and continue to grow as technology
expands. The main methods all have their advantages but also as everything in life has its
disadvantages. The best method that people argue about is nonexistent. Every method works
together or does what it does by itself. Methods do not compete they complement each other.
When one methods weakness is another’s strength they balance each other out, thus making the
best combination to use. Geophysical survey methods will soon be all around the globe widely
used and all be productive.
Works Cited
Clay, Berle R. "Complementary Geophysical Survey Techniques." 2001.
Crai-ky. 2012. www.crai-ky.com.
Wynn, Jeffrey C. Terraplus . 2012. www.terraplus.ca/case.