Mapping Anthropogenic Fill with GPR for Unmarked

CSIRO PUBLISHING Exploration Geophysics, 2011, 42, 249–257 http://dx.doi.org/10.1071/EG11018 Mapping anthropogenic fill with GPR for unmarked grave detection: a case study from a possible location of Mokare’s grave, Albany, Western Australia Paul Bladon1,5 Ian Moffat1,2 David Guilfoyle3 Alice Beale4 Jennifer Milani1 1Department of Archaeology, Flinders University, GPO Box 2100, Adelaide, SA 5001, Australia. 2Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. 3Applied Archaeology, PO Box 4025, Albany, WA 6330, Australia. 4Anthropology and Archaeology Department, Western Australian Museum, Locked Bag 49, Welshpool DC, WA 6986, Australia. 5Corresponding author. Email: [email protected] Abstract. Geophysical techniques are a commonly used, non-invasive method for the location of unmarked graves. Contrary to popular perception, most studies rely not on directly imaging skeletal material but instead on locating the subsurface disturbance created by grave digging. This approach is effective only when sufficient contrast exists between detectable properties (such as structure, mineralogy or porosity) of the grave fill and the surrounding sediment. Resolving these features can be particularly problematic in disturbed areas where other anthropogenic fill is in place, as it is often complex in character and lacks a natural stratigraphy. In many cultural heritage projects, it is often more important to ensure that burials are not disturbed rather than to specifically locate them. Under these circumstances, ground penetrating radar (GPR) can be used to locate modern anthropogenic fill. This may show which areas of the site are younger than the targeted graves and therefore of no archaeological interest. This approach is trialled on a site thought to contain the grave of Mokare, a significant historical figure in the colonial settlement of the Albany area in Western Australia. The delineation of a package of modern fill in the shallow subsurface in the context of the probable history of earthworks on the site demonstrates that Mokare is not buried in the surveyed location. This approach, applied to suitable sites, could contribute to culturally sensitive non-invasive investigation of burial sites in other locations. Key words: anthropogenic stratigraphy, ground penetrating radar, Mokare, unmarked graves. Received 8 April 2011, accepted 7 October 2011, published online 23 November 2011 Introduction successful in homogeneous regolith such as some lacustrine Geophysics has wide application in archaeological, and aeolian soil materials (Doolittle and Bellantoni, 2010). geotechnical and environmental investigations. Application Another potentially fruitful approach is to locate metallic of geophysical techniques enables exploration of the objects associated with the burials, such as metallic coffins or subsurface in a non-invasive and non-destructive manner grave goods, which will likely result in high-frequency signatures and in turn provides a culturally appropriate approach to in GPR data (Bevan, 1991; Novo et al., 2010). Additionally the surveying sensitive locations. Many geophysical techniques void inside a coffin can be detectable with GPR (Bevan, 1991; are applicable for burial detection; however, ground Doolittle and Bellantoni, 2010), however untreated wooden penetrating radar (GPR) is a particularly effective and coffins often decompose and collapse within a decade widely applied technique. GPR has been used for (Doolittle and Bellantoni, 2010). Metallic coffins will be most geological applications since the 1960s, and since the mid- easily imaged based on their high contrast with soil and resistance 1980s has been increasingly adopted within the engineering to collapse (Doolittle and Bellantoni, 2010; Owsley and and archaeological communities (e.g. Bevan, 1991; Cook, Compton, 1997). 1960;Imaietal.,1987; Gracia et al., 2000;Reynolds,1997; To locate burials using GPR there must be minimal post- Watters and Hunter, 2004). interment disturbance to the grave fill and surrounding GPR has been widely applied to the detection of graves, stratigraphy (Conyers, 2006). This disturbance can be caused although the direct detection of skeletal material is difficult by animal, plant or human intrusions that alter the contact (Bevan, 1991; Schultz, 2003). Burials are most easily interface between the vertical shaft backfill and the substrate recognised in GPR by grave cuts, particularly where the soil (Conyers, 2006) or introduce scattering (Bruzewicz et al., 1986; has horizontal stratification (Bevan, 1991; Conyers, 2006). Grave King et al., 1993; Nobes, 2000; Vaughan, 1986). Another shafts are visible because they cause the truncation of soil impediment to locating graves may occur when a cluster of horizons and stratigraphic layers, and are then backfilled with graves cause overlapping diffractions that are not individually mixed soil materials which differ in dielectric properties (Bevan, identifiable (Nobes, 1999). This effect may be minimised by the 1991; Miller, 1996; Vaughan, 1986). This approach is less choice of a higher frequency antenna (i.e. Rial et al., 2009)or Journal compilation Ó ASEG 2011 www.publish.csiro.au/journals/eg 250 Exploration Geophysics P. Bladon et al. through appropriate migration during data processing (Barton three and a half feet (Shellam, 2009). During the ceremony and Montagu, 2004) Mokare’s spear, meara, and kangaroo skin cap were placed on the surface of the burial mound (Goode, 2001). GPR and stratigraphic investigations On the 8th of November 1835 Alexander Collie died in Albany most likely from tuberculosis (Shellam, 2009). GPR has great application to the delineation of geological or Collie’s dying wish was to be ‘laid beside the Native Chief anthropogenic features in the subsurface (as summarised by Morkew, whom he himself buried, when Resident of that part Bristow and Jol (2003a)). GPR can be used for a range of of the colony’, a request which was respected with his interment applications including sedimentological investigations, beside Mokare’s grave (Shellam, 2009). However Collie’s dying (summarised in edited volumes by Baker and Jol, 2007, wish was reneged five years later when Governor Hutt Bristow and Jol, 2003b), detection of kimberlite pipes (Power requested his burial be moved from the side of Mokare and re- et al., 2004), fault detection (Dentith et al., 2010), landfill studies interred in the new Albany cemetery on Middleton Road. The (Splajt et al., 2003), concrete testing (Grandjean et al., 2000) and exact location of Mokare’s grave has been lost, although some a variety of other targets of economic importance (Francke, evidence for its location is presented in the following section of 2010). this paper. Variations in GPR wave behaviour in sediment are predominately caused by variation in water content Case study area (Huggenberger, 1993; McCann et al., 1988). On a small scale, this variation reflects sedimentary structures and hence these The area for the GPR case study investigation measures may be visible with GPR (Van Dam, 2002). As a result, GPR ~10 Â 6 m and is located in Albany Town Lot S112 on the corner has been extensively applied to the delineation of stratigraphy of Collie Street and Grey Street West. This block serves as a car including mapping sedimentary facies and stratal geometry park for the library and town hall located immediately to the east. (Baker and Jol, 2007; Conyers and Goodman, 1997). For The study area includes a small section (~6 Â 3 m) of bitumen example, the characteristic steeply dipping parallel reflectors in surface located in the south-east corner of the middle level car aeolian sediments can be effectively imaged with GPR (Bristow park and a larger (contiguous) section located on the eastern end et al., 1996). of a grassy slope which connects the middle car park with the GPR data can also be interpreted to calculate energy velocity lower car park (see Figures 1, 2). through the subsurface stratigraphy, which not only establishes Although Collie (1834) provides a detailed record of true depths, but can also assist in determining lithology (e.g. Mokare’s burial ceremony, there are no historical records Pedley et al., 2000; Pringle et al., 2003; Smith and Jol, 1992; which indicate the exact location of the gravesite. The Whiting et al., 2001) although this can be compromised by small geophysical survey was undertaken in April 2009 with the scale variations in soil moisture (Whiting et al., 2001). Smith and study area being chosen based on guidance and advice from Jol (1992) demonstrate how GPR can be used to determine the local community. The chosen location conflicts with some stratigraphic parameters including bed thickness, dip, and oral histories and an early historical map by Chauncy (1851) who internal structure. was the first town surveyor (an excerpt of this map is provided in Figure 3). An alternative site was suggested in an area of trees A history of Mokare in the north-west corner of the lower car park (see Figure 1) where an oval shaped grave can be seen in the map drawn by Mokare (also known as Mokaré, Mokkare, Mawcurrie, Mokoré Chauncy in 1851 (see Figure 3); unfortunately due the dense fi fi and Mokare (Mulvaney and Green, 1992)) is a signi cant gure accumulation of trees GPR was not deemed a suitable method in the colonial history of the Albany area in south-western for investigating this location. Western Australia. Mokare was a Noongar man who lived in the area immediately surrounding Albany who, following an encounter with Dumont d’Urville in October 1826, became an informant, guide, intermediary and close friend to many members of the European community (Shellam, 2009). European records of Mokare’s dialogue and actions provide some insights into the functioning of Indigenous society in the area both before the British arrival and during the garrison period (Shellam, 2009). Mokare is also significant to the joint narrative Middle car park – Upper car park of Indigenous European relations in his role as liaison and GPR survey proponent of peace, which, along with other contributing factors, led to a uniquely harmonious co-existence between the Indigenous people and Europeans during this period.

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