Geophysical Methods

Geophysical Methods

Archaeological geophysics includes a suite of techniques that enable detection of buried archaeological features and related deposits. The most common and arguably successful techniques include ground-penetrating radar (GPR), electrical resistance, magnetometry, magnetic susceptibility, and induced electrical conductivity. The latter two datasets can be obtained from an electromagnetic induction (EMI) instrument. We have employed all of these at Tiwanaku with considerable success, though electrical resistance survey was abandoned after initial tests because it was too difficult to insert the electrodes into the dry soils that characterize the altiplano during winter. The following is a brief explanation of GPR, EMI, and magnetometry (for more details see Clark 1996; Conyers 2004; Gaffney and Gater 2003; Johnson 2006; Witten 2006).

Ground-penetrating Radar

Electromagnetic waves are sent into the subsurface from an antenna that is pulled along the ground. When the waves encounter an interface, such as the boundary between site matrix and rock architecture, some of them are reflected back to the surface and recorded (Conyers 2004). At the same time, portions of the signal travel deeper and reflect from other buried interfaces. This continues until the signal deteriorates and cannot be distinguished from noise. The time it takes for each reflection to be recorded by the receiving antenna is measured and therefore depth can be approximated. The types of interfaces that can be detected include changes in moisture, sediment size, and compaction. Thus GPR is well suited for detecting rock and mud-brick architecture, and many other types of features including pathways, ditches, and graves. Depth penetration varies widely depending on the frequency used and the electrical properties of local soils and sediments (Conyers 2004). A GSSI SIR2000 system with 400 MHz antenna was used at Tiwanaku (Figure 2) with relatively limited depth penetration (about one meter) due to high percentages of clay, but this was still sufficient to detect most of the cultural layers at Tiwanaku. Data were collected in rectangular sub-regions using half-meter line spacing. Processing included gaining, band-pass filtering, background removal, and position correction. Time slices representing 3-6, 6-9, and 9-12 nanoseconds (ns) were created representing approximate depth intervals of 27-40, 40-54, and 54-67 cm. For display and interpretation these slices were combined into one layer capturing the majority of anomalies using principal components analysis (Kvamme 2006a, 2007). Original slice maps and reflection profiles were consulted whenever depth or other details were needed.

Electromagnetic Induction (EMI): Conductivity

Electromagnetic Induction (EMI) is a method that creates electromagnetic fields which are induced into the ground. These frequencies are much lower than those used for GPR, and the method is fundamentally different (Witten 2006). Electromagnetic (EM) fields emanate outward in all directions from the transmitter, but most importantly into the ground. This primary field induces eddy currents in the ground in response to its electrical and magnetic properties, which collectively create a secondary field that is measured by a receiver (Witten 2006). The secondary field is used to approximate the electrical (conductivity) and magnetic (susceptibility) properties of the subsurface within the range of the instrument. Conductivity is useful for detecting differences in ground moisture and grain size, where high conductivity often indicates moisture retention or relatively high percentages of clay (McNeill 1980a). Thus conductivity is useful for mapping sedimentology, but also archaeological features if they have moisture or sediment size contrast with the surrounding site matrix. Sometimes rock or brick walls can be detected because they are drier than surrounding materials. A Geonics EM38 (Figure 3) was used at Tiwanaku for separate conductivity and magnetic susceptibility surveys. Conductivity data represent a weighted average of conductivity for approximately the upper 1.5 meters of ground (McNeill 1980b). In both cases lines of data were collected every half meter, with four readings per meter taken in the survey direction. Data were downloaded and processed to remove drift, then integrated with other geophysical layers in GIS.

Figure 2: Geophsycial Survey Systems Inc. (GSSI) SIR-2000 GPR system with 400 MHz antenna and survey wheel.

Figure 3: Geonics EM38 EMI instrument.

Electromagnetic Induction (EMI): Magnetic Susceptibility

Magnetic susceptibility is a measure of a material’s ability to become magnetized in the presence of a magnetizing field (Clark 1996; Dalan 2006). Materials that are susceptible include anything containing relatively high amounts magnetic minerals such as magnetite, maghemite, and many others (Dalan 2006). Topsoil is often much more magnetic than subsoil due to soil formation processes (Dalan 2006). In addition, human activity tends to enhance the magnetic susceptibility of soils by disturbance, using fire, and depositing waste (Dalan 2006). As a result soils at archaeological sites are often more magnetic than nearby soils (Clark 1996), and this provides additional contrast for archaeological features created by the accumulation or removal of topsoil such as graves, pits, burials, ditches, and middens. The depth of penetration for magnetic susceptibility is limited to about 0.5 m with the EM38 (and much less with other sensors). Data were collected with the EM38 (Figure 3) using the same sampling densities as conductivity (see above) and were processed in the same way.

Magnetometry

Unlike GPR and EMI, which actively generate EM fields, magnetometers passively measure subtle variations the earth’s magnetic field without imposing an artificial field (Clark 1996). Magnetometers are sensitive to two types of magnetism: induced and remnant. Induced magnetism includes magnetic fields that are created by and external magnetic field, in this case Earth’s magnetic field. If the Earth’s magnetic field could be “turned off” this type of magnetism would also cease. Induced magnetic fields indicate a material’s magnetic susceptibility, so this component of magnetometry data is similar to magnetic susceptibility measured by EMI (the only difference is the way it is measured). Remnant magnetic fields include all magnetic fields that are fixed and do not rely on external magnetizing fields. Objects that possess remnant magnetic fields usually have a history of extreme heating. When objects are heated to a high enough temperature their magnetic minerals align to earth’s magnetic field, and then are “frozen” that way when cooled (Clark 1996). Igneous rocks, which form from molten rock, have remnant magnetic fields. Other fired items, such as pottery, fired brick, kilns, hearths, and burned foundations also have remnant magnetic fields. The depth of penetration of a magnetometer is generally about 1.5 meters (Kvamme 2006b), but strongly magnetic features such as the large andesite blocks at Tiwanaku can be detected at greater depths. Depth estimation is very difficult, but at Tiwanaku there are places where large magnetic anomalies suggest andesite blocks that are not visible in GPR, suggesting they are deeper than the maximum depth penetration of GPR, or beyond about one meter. Magnetometry data were collected using a Geometrics G-858 Cesium gradiometer in 2005 and a Bartington fluxgate dual gradiometer system (Figure 4) in lines spaced .5 m apart with 8 samples per meter in the survey direction. Data were downloaded and processed to remove striping errors, then assembled into mosaics and integrated with the other data in GIS.

Figure 4. Magnetometry instruments used for this project. (a) Geometrics G-858 cesium magnetic gradiometer; (b) Bartingtion dual-fluxgate gradiometer.