Searching for prehistoric small-sized graves in complex geoarchaeological conditions: Ayios Vasilios North Cemetery (Peloponnese, Greece)

Journal of Archaeological Science: Reports 24 (2019) 1–15

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Searching for prehistoric small-sized graves in complex geoarchaeological conditions: Ayios Vasilios North Cemetery (Peloponnese, Greece)

T

Lazaros Polymenakos (Geological - Geophysical Consultant, Independent Researcher) Athens, Greece

A R T I C LE I N FO

A B S T R A C T

Keywords: Ground-penetrating radar Small-sized graves Prehistoric cemetery Late Bronze Age Mediterranean

Prehistoric cemeteries are important and unique sources of funerary and social practices from the early stages of civilization, but also a highly challenging environment for geophysical investigation. This study evaluates the results of a Ground Penetrating Radar (GPR) survey for the detection and mapping of simple, small-sized, boxshaped graves, placed at small depths at the Late Bronze Age Ayios Vasilios North Cemetery (Laconia, Peloponnese, Greece). Interpreted grave anomalies allowed post-survey excavations to verify and not miss existent graves, but also led to the exploration of several anomalies that were not graves. We highlight and analyze in detail anomalies possibly related to graves in plan- and profile-view, compare them to real structures from pre- and post-survey excavation findings, and search for solutions for an efficient identification of GPR anomalies with graves and increase of the effectiveness and dependability of GPR for grave searching in complex site conditions, which include strong variations between geological and anthropogenic structure. We propose a procedure for refining grave interpretation that utilizes the results of both initial and advanced processing of 2D and 3D GPR data. The results of our study will benefit the use of GPR as a non-invasive method to search for small-sized graves placed at small depths in complex subsurface conditions and extend GPR use to investigate the organization and function of prehistoric cemetery sites in an intensively used space. Our work enhances the literature relative to geophysical surveying of prehistoric cemeteries, while the methodology discussed in this case study provides a useful tool for cemetery site management.

1. Introduction Non-invasive site investigations of prehistoric cemeteries with use of geophysical methods can provide important aid in the understanding of cemetery site organization and grave placement, and consequently in site management (e.g., Conyers, 2006; Jones, 2008; Sarris and Papadopoulos, 2012; Barone, 2017). Prehistoric cemeteries are important and unique sources of funerary and social practices from the early stages of civilization, but also a highly challenging environment for geophysical investigation: signals from generally small-sized and not regularly placed graves may by disturbed by local complex geological and anthropogenic conditions, in addition to the possibility that grave physical features (i.e. walls, floor, ceiling, filling) may not always have a discernible geophysical signature depending on the dimensions, building materials and fill material (e.g., Whiting et al., 2001; Gonçalves et al., 2008; Juerges et al., 2010; Bigman, 2012; Kemp et al., 2014; Lowe et al., 2014). This paper presents the results of a detailed GPR survey at the Middle - Late Bronze Age site (MLBA - early Mycenaean, ca. 1700–1500 BCE) of the North Cemetery at Ayios Vasilios (AVNC;

E-mail address: [email protected]. https://doi.org/10.1016/j.jasrep.2018.12.003 Received 28 November 2018; Accepted 1 December 2018 2352-409X/ © 2018 Elsevier Ltd. All rights reserved.

Moutafi and Voutsaki, 2016) in Laconia (Peloponnese, Southern Greece). The survey attempted to locate small-sized and shallow-placed graves (referred to as ‘simple graves’ in Boyd, 2002), estimate the extents of the cemetery and contribute to the cemetery site organization. Interpreted grave anomalies allowed post-survey excavations to verify and not miss existent graves, but also led to the exploration of several anomalies that were not graves. This result motivated us to highlight and analyze in detail anomalies possibly related to graves in plan- and profile-view, compare them to real structures from post-survey excavation findings, and search for solutions for a more efficient identification of anomalies with graves and increase of the effectiveness and dependability of the method for grave searching in complex site conditions. To this end, we propose a procedure that refines the results of initial processing with advanced 2D and 3D data enhancement techniques. 2. Site description and field surveys The survey site is located on the ridge of the northern part of a hill with olive groves near Xirokambi village to the south of Sparta

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Fig. 1. (a) Location of Ayios Vasilios North Cemetery site (AVNC) in Laconia (Southern Peloponnese, Greece). Map available from http://www.d-maps.com/m/ mediterranean/meditorient/meditorient05.gif (accessed: 10.01.2017). (b) The GPR survey site at the AVNC. MCT: Main Cemetery Trench, the initial excavation trench made prior to the GPR survey. Map coordinates after the GGRS87 geodetic network. Background image available from www.ktimatologio.gr (accessed: 10.11.2014). Fig. 2. Detailed map of the GPR survey area at the AVNC. Solid rectangle in black: MCT. Dotted rectangles in black/red: graves revealed at the excavations made prior of/after the GPR survey (simplified after Moutafi and Voutsaki, 2016: Fig. 1). Green rectangle: sub-area surveyed with additional parallel and perpendicular lines. Red lines/labels: selected GPR profile locations discussed in text; labels refer to Fig. 5a to e. Isolines in purple indicate ground surface relief at the time of GPR survey and before the postsurvey excavations; purple labels show elevation differences in meters, from a local 'zero-elevation' reference point; isoline interval is 0.2 m.

(Laconia, Southern Peloponnese, Greece) (Fig. 1). The ground surface has a small dip to the north in the central part of the survey site, which increases towards the northwest and northeast in the western and

eastern part of the site, respectively (Fig. 2). The survey site encloses the North Cemetery, an extramural cemetery located close to the contemporary settlement of Ayios Vasilios, which developed into the 2

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rock have been subsequently occupied by graves or other artificial structures, and filled by a thick soil cover (Fig. 3d). The GPR survey was made in 2012–2013, after the initial MCT excavation, in an area of 2750 m2 surrounding the trench (Fig. 2) (Polymenakos, 2013). The GPR was used as the primary tool for grave detection and site management, because of the ability of detecting anomalies that may relate to small-sized graves in environmental conditions encountered in the Greek landscape (e.g., Sarris, 1998) and creating accurate maps of graves and cemetery geology (e.g., Dojack, 2012). Additional surveying with magnetic gradiometry, resistance mapping and low frequency electromagnetics, provided some information relative to artificial structures such as ditches and stone walls, but no information relative to graves. Earth resistivity tomography was not considered an option because of the small size of graves and the anticipated negative effect on resistivity of the extensive rock formations at small depths. The strong effect of local geology on resistivity at the cemetery area had been already experienced in former resistivity surveys made in the northern sector of the hill by Tsokas et al. (2012) and Polymenakos (2012b).

palatial center of Laconia in the later Mycenaean period. Excavations made in 2010–2011, prior to the GPR survey, at the initial excavation trench (hereafter mentioned as ‘main cemetery trench’, MCT) revealed mostly box-shaped stone covered and stone lined graves (cists) and few simple or stone-lined pits, found at different depths, in an organized layout comprising two principal directions (N-S and E-W), sometimes divided by wall segments, and forming a dense cluster (Moutafi and Voutsaki, 2016). Post-survey excavations revealed one more cist grave, a burial pit, and one larger built tomb (Fig. 2). Field observations of cist graves made by the author at the MCT reveal plan dimensions (length × width, including possible side-lining) between 1.9 × 0.9 m and 2.5 × 1.3 m, interior height between 0.5 m and 1.0 m, and placement depths between 0.30 m and 1.5 m. The grave shaft sides are vertical, lined at their full height with quartzite, chert, and marble stones, topped with mica schist or phyllite slabs, with an average lining width between 0.25 m and 0.40 m, wider on the long side. The graves are usually covered with single or multiple thick (5–10 cm) phyllite slabs, however there are also non-covered burials. Their interior space is mostly filled to the top of the grave with finegrained clayey-sandy soil with sparse stone fragments. Graves share a NNW-SSE and a WSW-ENE orientation, of which the NNW-SSE prevails. The above orientations follow the principal orientations observed for the built layout of the settlement extending to the south of the cemetery in excavations and geophysical surveys made on the hill (Tsokas et al., 2012; Polymenakos, 2012b). Pit burials are non-side-lined (or have a simple stone outline), shallow (interior height < 0.50 m) and of variable dimensions (length × width: 0.5–1.5 × 0.3–1.2 m, considerably smaller than cist graves) and oriented roughly E-W. Site geology consists of rock formations, namely a near-surface 0.20–0.50 m thick, fine-to-middle grain conglomerate bed, underlain by a > 2 m thick calcareous marl bed which forms the geological bedrock of the site, with inter-beds of 0.5–2 m thick marly limestone (Fig. 3a). The beds strike N-S to NNE-SSW and are sub-horizontal or dip slightly (ca. 10o) to the E-ESE. They outcrop partly along the north crest of the hill and are revealed in the excavation trenches (Fig. 3b; Polymenakos, 2012a). The above formations are overlain by a clayey-sandy soil cover with random gravels, from ground surface to depths of 1–1.5 m. Artificial cuttings of the conglomerate and marl beds are observed at MCT (Fig. 3c). These cuttings are related to rock extraction for setting up the area for occupational use and possible rock supply. Areas of extracted

3. Methods Implementation of the cemetery survey included the following steps: (a) Field collection and initial processing-interpretation of GPR data, considering GPR identification criteria for grave physical features; (b) Evaluation of initial grave interpretation using post-survey excavation findings; (c) Advanced processing of GPR data aiming at reducing complications with grave-related interpretation. The methods are presented in detail in Sections 3.1 and 3.2. Results and interpretation are described in Section 4. 3.1. Field collection and initial processing-interpretation of GPR data 3.1.1. Data collection The GPR field acquisition was made with the NogginPlus250, a compact, fixed-separation shielded transducer system, with a 250 MHz

Fig. 3. Geology at the AVNC site. (a) Simplified geological cross section across the cemetery site (after Polymenakos, 2012b). (b) Soil cover, fine-grained conglomerate and marl. (c) Cuttings in the marl bedrock (view towards the WNW). (d) The GPR unit at the cemetery area; a cist grave found in the main cemetery trench (MCT; bottom left) is shown overlain by soil cover (top left). All photos taken by the author in 2012. 3

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Table 1 Parameters of GPR data collection at the AVNC. Principal frequency Antenna separation Nominal vertical resolution (Jol, 1995) Time-sampling interval Recording window Vertical trace stacking Trace separation Survey line principal layout; additional layouts (local test) Survey line positioning Trace positioning Elevation differences of the grid nodes Weather conditions (time)

250 MHz 0.28 m, fixed 0.15 m 0.321 ns 60 ns 16 times 0.05 m (20 traces/m) Parallel, separated @ 0.50 m; parallel @ 0.25 m; perpendicular @ 0.50 m 10 × 10 m rectangular topographical grid, established with cm accuracy using a DGPS system Odometer, built-in Recorded with reference to a fixed ‘zero-elevation’ point established at the site Dry, warm (May–June)

amplitude envelope of the standard processed profile data (i.e., amplitude with gain applied at later times to compensate for weaker signals at depth). Only parallel profiles spaced 0.5 m apart were used, since additional in-between or perpendicular profiles did not improve the subsurface detail. Profile and time-slice data were not migrated, to preserve original hyperbolas that may pinpoint grave features and avoid artifacts in small-scale anomaly imaging. Our initial approach was to not apply topographical correction to time-slices, considering the rather small elevation differences at the survey area: there is an elevation difference of about 3% (1.5 m over a distance of 50 m) along the N-S direction in the central part of the survey area, and a maximum of 6% (1.8 m over a distance of 30 m) along the NE direction in the eastern part of the survey area (Fig. 2; an elevation difference is considered significant when > 10%; e.g., Conyers, 2013). However, because of the abundance of sub-horizontal geological beds, which might produce reflection artifacts when crossed by non-horizontal time-slices (e.g., Conyers, 2013), we considered worth exploring the effect of local elevation differences on the imaging of possible graves and non-grave structures. Hence, we produced nontopographically and topographically corrected time-slices (hereafter mentioned as surface-parallel slices and horizontal slices, respectively). The topographic correction was applied to the 3D volume of standard processed GPR data using a calculated 0.10 × 0.10 m ground surface relief elevation grid compatible to the GPR data, and a velocity of 0.125 m/ns. Horizontal slices were then extracted from the topographically corrected data volume at selected elevations. Both types of GPR time-slices were produced with use of the Surfer software (Golden Software, 2013).

antenna. Data collection parameters are summarized in Table 1. The survey area was initially covered by a set of parallel reflection profiles (referred to as profiles hereafter) in the N-S direction, which is along the lesser elevation difference of ground surface and the anticipated strike of the geological formations, to reduce the effect of geological structure on artificial structure in GPR profiles. The layout was also perpendicular to one of the grave orientations revealed at the MCT (WSW-ENE). The short profile spacing would allow for graves oriented at any direction relative to the profiles to be adequately imaged. However, considering that graves may follow an irregular placement pattern and have dimensions similar to non-grave artificial or natural structure, we examined the possibility of improving the subsurface images by reducing the profile separation and running profiles in perpendicular directions (e.g., Dionne et al., 2010). Thus, additional sets of profiles were run in the western part of the survey area (Fig. 2): one set of parallel profiles with similar spacing and direction to the initial profiles were run in-between, decreasing profile spacing to 0.25 m, while another set of profiles spaced 0.5 m apart were run in an E-W direction, perpendicular to the initial profiles. 3.1.2. Initial data processing The GPR profiles were initially processed with a standard work-flow using the steps summarized in Table 2. Background removal used all of the data traces of a GPR section allowable in the running average window to ensure as little effect on the bulk of the data (e.g., Dojack, 2012). Temporal band pass filters had limited success with the enhancement of profiles, and were thus not further used in data processing. Radar wave velocity ranged from 0.12 to 0.13 m/ns with an average of 0.125 m/ns. Velocity variations were not significant with depth. The profiles were transformed with use of algorithms written by the author into reflection amplitude time-slices (i.e. amplitude maps at selected time intervals) for a better spatial correlation of reflection anomaly patterns. Time-slices were calculated from 2 ns two-way time intervals (corresponds to a thickness of ca. 0.15 m, using a velocity of 0.125 m/ns) and no overlap between adjoining slices. Final time-slices were calculated with a resolution of 0.1 m, using the equalized

3.1.3. GPR identification of grave physical features We initially evaluated the identification of small-sized graves, by considering the physical features of pre-survey excavated graves that can be identified by GPR, summarized in Table 3 (e.g., Goodman et al., 2009; Dionne et al., 2010; Schultz and Martin, 2011; Conyers, 2013; Lowe et al., 2013): However, identification of particular GPR reflection patterns (hereafter mentioned as GPR anomalies or anomalies) with actual graves

Table 2 Parameters of initial GPR data processing at the AVNC. Profiles

Amplitude time-slices

First reflection correction Removal of a low frequency surge induced by the transmit signal into the ground (dewow filter)

Trial windows 2–16 ns Final slices calculated from 2 ns two-way time intervals (5 samples), using only parallel profiles No overlap between adjoining slices

Exponential gain compensation to enhance deeper reflections (SEC, calculated with parameters: attenuation = 8; threshold = 1; max value = 500) Total (i.e. using all of the data traces) background removal for removing low frequency noise in the earlier times of the recording Equalized amplitude envelope for time-slice and volume evaluation; raw amplitude for profile evaluation Radar wave velocity calculated using the hyperbola fitting procedure

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Amplitude data: non-migrated amplitude envelope Resolution: 0.10 m Produced both non-topographically and topographically corrected slices

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Table 3 Physical features of pre-survey excavated graves at the AVNC that can be identified by GPR. Physical grave feature

GPR anomaly (S: in slices; P: in profiles)

Burial shaft: contrast between the sediment fill of the burial shaft and the surrounding deposits.

S: Low-amplitude or high-amplitude reflections P: Truncations or breaks in the stratigraphy

Burial shaft interior: low compaction or fine grain soil fill

P: Low amplitude gap due to the attenuation of signal strength as it passes through the soil fill

Burial shaft interior: settling and slumping of soil fill; accumulation of other sediments above the shaft and surroundings following the grave filling

P: Shallow (small vertical size) anomalies slightly concave-upward or bowlshaped

Burial pit edges; Grave shaft sides

P: Possible diagonal reflections (or half-hyperbolas) dipping towards one another from the upper edge of the burial feature

Grave floor

P: Possible continuous reflections and/or narrow hyperbolas

Stone cover

P: possible distinct hyperbolas or continuous reflections

Stone lining of shaft sides

P: possible wide and/or multiple hyperbolas

Buried human body and associated artefacts

P: possible discernible hyperbolas within the area of the grave shaft

in a three-dimensional volume, separating data less than the threshold value from data greater than this value (Golden Software, 2016). Isosurface rendering provides a very quick method for reconstructing polygonal surface models from a 3D lattice. The data set is displayed with isosurfaces using different threshold values, expressed as percentage of the maximum GPR trace amplitude (e.g, Zhao et al., 2015). Lower threshold values improve the reconstruction of major anomalies; however, they also increase unwanted incoherent patterns (noise). The threshold value is the most significant parameter of the technique: values between 25% and 30% are considered a good choice for outlining anomalies related to buried architecture, whereas values between 30% and 50% appear to better outline isolated anomalies possibly related to graves (e.g., Leucci and Negri, 2006; Yalciner, 2012). Isosurface rendering was applied to the 3D volume of equalized amplitude envelope data to emphasize possible grave-related anomalies, with use of the Voxler software (Golden Software, 2016).

(i.e. positive interpretations or positives) may be affected by sources of disturbance (such as roots, stones, rock layers, and rodent holes; e.g., Doolittle and Bellantoni, 2010) which result in identifying GPR anomalies with no-grave structure (i.e. false positive interpretations or false positives), or conditions where graves are indistinguishable from the surrounding geology (such as grave fill with similar lithology to the surroundings, absent side lining or grave cover, rock layers overlying burials or tombs; e.g., Davenport, 2001; Dalan et al., 2010) which may result in no identification of existing graves (i.e. false negative interpretations or false negatives). Possible sources of disturbance in this study were considered: (i) the characteristics of small-sized burials encountered in the MLBA in Southern Greece and Peloponnese in particular (i.e. irregular horizontal and vertical positioning, variable dimensions, variable cover and/or side lining, limited or no artefacts; e.g., Cavanagh and Mee, 1998; Boyd, 2002; Voutsaki, 2010; Papadimitriou, 2016); (ii) the local geological and environmental conditions, that is the gravelly/rocky sediments and sedimentary rocks (conglomerates/sandstones/marls), the buried anthropogenic or natural relief, the trees and tree roots, that characterize most of the MLBA sites in Peloponnese (e.g., Bintliff, 1977; Malaperdas and Zacharias, 2018). A further source of complication in grave-related evaluation of GPR data was the inability to compare our results with tests on local graves due to the late start of the survey relative to the initial excavation.

3.2.2. Profiles Edge enhancement filters were applied to profiles to emphasize lateral discontinuities and isolated reflection anomalies, as an aid to the identification of possible graves. Raw-amplitude unmigrated profiles were used in the procedure, to exploit the full range of reflection characteristics (hyperbola tails, changes in reflection pattern, gaps, etc.). The following edge enhancement filters were found effective for our dataset:

3.2. Post-survey excavation and advanced data processing

(i) Difference of Gaussians (DG)

The post-survey excavations made in the GPR surveyed area (east of the MCT in 2012–2013; west and south of the MCT in 2014–2015; Moutafi and Voutsaki, 2016: Fig. 1), verified interpreted grave anomalies and complex subsurface conditions, but also explored several interpreted grave anomalies that were non-grave structures. This condition motivated us to attempt advanced data enhancement of GPR 3D volume and 2D profile data (e.g. Yalciner, 2012; Zhao et al., 2013), to discriminate between natural and anthropogenic subsurface features and reduce false positive grave interpretations. From several tested parameters and techniques, we present below a selection of procedures applied to the initially processed data that we found effective for improving grave-related interpretation in the conditions encountered at the AVNC site.

This is a linear convolution second order derivative filter applied by calculating the difference of two Gaussian-filtered (smoothed) instances of the original data array, g(x, y), with different standard deviation values, σ (Pitas, 2000). The filter was used to emphasize individual reflection anomalies, and suppress horizontal reflections and noise. (ii) Kirsh filter (KH) This is a non-linear convolution filter for edge-detection at different directions, calculated by a convolution of the original data array with the Kirsh kernel (Crane, 1997). The Kirsh is a specially designed convolution kernel, superimposed over the pixel distribution of an image, f (x, y), to calculate the gradient value at every pixel. In order to detect the presence of a gradient discontinuity we calculate the change in gradient at pixel (i, j) (Marshall, 1997). The filter was applied to emphasize lateral discontinuities and horizontal reflections, and the

3.2.1. 3D volume Iso-amplitude surface (isosurface) rendering: An isosurface is a surface on which all points have a constant amplitude value (threshold value) 5

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Fig. 4. Amplitude time-slices representative of the depth range of placement of expected graves at the AVNC (0.3–2.0 m). (a–c) Surface-parallel slices. (d–f) Horizontal slices. GPR processing parameters: unmigrated equalized amplitude envelope; dewow, exponential gain (attenuation parameter: 8) and total background filters applied; resolution 0.1 m; interpolation limit 0.75 m; topographical correction applied to horizontal slices. ‘d’: depth from ground surface; ‘t’: time, ns; ‘e’: elevation difference from ‘zero-elevation’ reference point. Labeled reflection anomalies discussed in Section 4. Yellow arrows outline buried architectural structures. Orange ellipses outline low amplitude areas specifically considered for grave search.

4. Results and interpretation

contrast of materials with different compaction (soil/rock; geological/ anthropogenic). Both of the above filters were applied to profiles with use of the ImLab software (Scuri, 2016).

4.1. Initial interpretation 4.1.1. Site conditions The GPR results reveal complex subsurface conditions defined by

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the focus of grave-related evaluation of GPR data.

local geology and artificial structure in the depth range of expected graves (0.3–2.0 m). Artificial structure abounds in surface-parallel slices with: (a) Stone-built planned architecture shown as middle-to-high amplitude regular geometry anomalies, e.g., a nearly rectangular high amplitude enclosure oriented NE-SW to the east of the MCT, (outlined by yellow arrows in Fig. 4a and b); other rectangular and elongated anomalies oriented WSW-ENE/NNW-SSE and spatially related to the extensive planned layout of a large settlement revealed further to the south, outlined by yellow arrows and labeled BL in Fig. 4b and c). (b) Soil-filled ditches and trenches: ditches are shown as prominent low amplitude elongated anomalies (e.g. the E-W oriented anomalies labeled DH and DH1; DH relates to an artificial ditch dug in the finegrained conglomerate and marl to a depth of 1.0–1.5 m, as revealed at the MCT), while trenches are shown as low amplitude enclosed anomalies (outlined by orange ellipses in Fig. 4b and c). Geology prevails with extensive non-regular geometry, middle-tohigh reflection amplitude areas and patches in surface-parallel slices (labeled RK in Fig. 4a to c). Traces of artificial landscaping of the rock beds abound: the isolated, non-regular geometry, middle and high amplitude patches in the central and northern part of the survey area, are interpreted as relics of the marl bed after artificial extraction (Fig. 4c). The NW-SE orientation of the rock bed relics in the northern and eastern part of the survey area, which shifts to E-W to the west of the MCT (Fig. 4c) in an arcuate shape, is incompatible to the geological strike (N-S to NNE-SSW), follows that of the architectural remains (NNW-SSE) and artificial ditches (E-W), and implies extensive and deep artificial landscaping of the buried relief; the landscaping is more prominent in the central part of the survey area (X: 5–45 m; Y: 18–38 m; Fig. 4c) and spatially associated with the grave cluster at the MCT. The plan-views produce a confusing image relative to isolated high or low amplitude spot anomalies that might relate to local small-sized graves: a large number of spot anomalies is shown, that follow a random geometrical pattern over the survey area throughout the depth range of interest (Fig. 4a to c). Spot anomaly imaging does not improve with the addition of parallel profiles to decrease profile spacing to 0.25 m, or that of perpendicular profiles spaced 0.50 m apart in the area to the west of the MCT. Similarly, topographical correction has no significant effect on spot anomaly imaging: horizontal slices restore the 3D shape of the extensive anomalies related to artificial structure (regular geometry anomalies indicated by yellow arrows and labeled BL in Fig. 4d to f) and local geology (i.e. the conglomerate bed shown with the extensive shallow middle amplitude areas, Fig. 4d and e, and the underlying marl bed shown with the deeper higher amplitude areas and non-regular geometry anomalies, Fig. 4f), but do not enhance the spot anomalies that might relate to possible graves (compare e.g., Fig. 4 panel e with panel b). As such, the results of additional parallel and perpendicular profiles and topographical correction were not further considered in grave-related interpretation of GPR 3D volume data. Subsurface complexity is also revealed in profiles (Fig. 5; for profile location see Fig. 2). There appear well-defined full- or half- hyperbolas related to fragmented competent/stone material, and distinct continuous reflection patterns related to the fine-grained conglomerate and the underlying marl beds (CM and ML, respectively; Fig. 5a, b). Rock bed reflections reveal significant variations in the buried relief of the conglomerate, while strongly contrast with low amplitude areas associated with soil cover (TS; Fig. 5a to 5e) and are locally abruptly truncated (RC; Fig. 5a to c), suggesting artificial cuttings, as is observed at the MCT. Soil cover is generally of limited thickness (< 0.3 m), but reaches depths of 1.0–1.5 m at places (e.g., 20–26 m distance, 0–18 ns time in Fig. 5a). Hyperbolas within the soil cover observed in individual profiles are interpreted as random rock fragments (e.g., 22.5–23 m distance, 12 ns time in Fig. 5d). Prominent, wide aperture hyperbolas that appear in several instances between 20 ns and 30 ns two-way time, originate in trees. Compared to the time-slices, profiles are also affected by disturbing factors, however they carry significant information that could be related to specific grave physical features, and were therefore

4.1.2. Grave-related interpretation In this complex setting, with no prior information on GPR response from local graves, we initially focused on GPR anomalies that could relate to the stone cover slabs, the stone lining of the grave shaft sides or the grave shaft side cuttings, which were the principal physical features of the burials excavated prior to the GPR survey. Following the discussion in Sections 2 and 3.1.3, we searched in time-slices for isolated high reflection amplitude spots measuring between 1 × 0.5 m and 2 × 3 m, that might relate to the grave cover or floor. In profiles, we searched for 1–2 m wide reflection hyperbolas that might relate to grave cover, and 0.2–0.5 m wide full- and half- hyperbolas and truncations, that might relate to the stone-lining or cutting of grave shaft sides and floor; we also searched for concave-upward and intermittent reflections that might be associated with grave fill and buried individuals. Anomalies with the above characteristics appear mainly to the east of the MCT, in the area of the regular geometry high amplitude enclosure and in other high amplitude spots located to the south of the enclosure (Fig. 4a and b). Apart from slab-covered/side-lined graves, field details at the MCT and the post-survey excavations to the east of the trench revealed that there are also uncovered graves and no side-lined simple pit burials, while most graves are not dug directly in rock but in places where rock has been extracted beforehand. Following the above findings, we also evaluated the low reflection amplitude anomalies in profiles and timeslices that appear mainly to the west of the MCT (Fig. 4b and c), as possibly related to the soil fill of graves without side lining or cover slabs, or to soil-filled areas where burials might abound compared to areas where rock prevails and burial expectation is lower. Consequently, the location of an anomaly relative to geology (rock or soil), and its size and shape compared to anomalies related to nongrave artificial structure (e.g., planned architecture, ditches) were considered for grave interpretation, in addition to the reflection pattern. Thus, interpreted grave anomalies are those anomalies that show some or all of the GPR patterns discussed in Section 3.1.3, have dimensions and overall shape compatible with the MCT excavated graves and are primarily located in soil surroundings. We integrated interpreted grave anomalies from the entire depth range of interest (0.3–2 m), other artificial structure and geology into a synthetic interpretation plan (Fig. 6), according to which high amplitude grave anomalies prevail to the east and west-southwest of the MCT, while low amplitude grave anomalies are located to the west of the trench. Grave anomalies are spatially distributed in clusters. Most grave anomalies measure up to 2 × 1 m with only a few exceeding this dimension and are oriented E-W, while some others are oriented NW-SE and a few NE-SW; the first two orientations are also observed in the excavated graves at the MCT. Prominent non-grave structures are the long, E-W trending, artificial ditch in the northern part of the survey area, and the shallow conglomerate bed that extends over the northern and the southwestern part of the survey area, where it shows a distinct ‘bottleneck’ shape. We interpret the rock bed outline and the bed's absence in the south-eastern part of the survey area as the result of artificial shaping. 4.2. Post-survey evaluation of grave-related interpretations In post-survey excavations to the east of the MCT, field observations made by the author identified the high amplitude grave anomaly H1 (red solid-line rectangle in Fig. 6) with a large (2.8 × 1.9 × 1.1 m), boulder-lined-and-covered grave extending in an NW-SE direction, containing many buried individuals (Moutafi and Voutsaki, 2016: Fig. 1, Grave 21). In plan-view, H1 is defined by a high amplitude spot at small and intermediate depths (0.3–0.6 m), measuring ca. 2.7 × 1.8 m and oriented NW-SE (Fig. 4a, b). In profile-view, the anomaly is defined by high amplitude reflections at the top (5–7 ns two7

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Fig. 5. GPR profiles with locations of interpreted reflection anomalies discussed in Section 4. The X and Y location indicated for every profile refers to the coordinates of the topographical map shown in Fig. 2. Processing details: unmigrated raw amplitude; dewow, exponential gain (attenuation parameter: 8) and total background filters applied. Topographical correction applied. Annotations in yellow: interpreted grave anomalies; green: geology; cyan: other interpreted artificial structure.

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Fig. 6. Integration of interpreted GPR results and excavation findings at the MCT and post-survey trenches. Isolines in purple indicate ground surface relief as in Fig. 2. Interpreted grave anomalies H1 to H5 and L1 to L4 are discussed in Sections 4.2 and 4.3.

found at the cemetery. There is an excellent match of GPR anomaly dimensions, orientation and details related to grave physical features, with the excavation findings. However, other high amplitude grave anomalies located to the east and southeast of the MCT were identified with non-grave artificial structures or buried rock surfaces: anomalies located at small depths were identified with simple stone structures of undefined use (blue solid-line rectangles in Fig. 6), while anomalies located at greater depths were identified with parts of the conglomerate bed (black solidline rectangles in Fig. 6). We discuss some of these cases in detail: (i) Anomaly H2 (blue solid-line rectangle in Fig. 6), showing a high amplitude spot in plan-view (Fig. 4a, b) and a concave-downward reflection pattern in profile-view (Fig. 5c), was identified with a near-surface

way time, about 0.25–0.35 m depth) with a concave-downward outline, and several well-defined outer and inner narrow hyperbolas (aperture of about 0.5 m), with the distance between the outer hyperbolas measuring about 2 m (Fig. 5d). The outer reflection hyperbolas (at 27.5 and 29.5 m in Fig. 5d) accurately correlate to the boulder side lining, whereas the internal hyperbolas correlate to boulders found at the top of the grave, possibly also to the many bodies densely buried within. A continuous dipping reflection below the hyperbolas at 24 ns is interpreted as the grave floor (at about 1.3–1.4 m depth, suggesting a grave depth of 1.0 m). The overall shape of the anomaly resembles a barrel vault, with a concave-downward top and nearly vertical sides, and the floor dipping slightly towards the north end of the profile, following the buried relief. This is the largest, deepest and more elaborate grave

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location of an interpreted low amplitude grave anomaly. This result suggests that in surveys with similar parameters to that at the AVNC some of the interpreted low amplitude grave anomalies may be associated with pit graves instead of non-stone lined shaft graves, and are thus worth of further exploration. Other low amplitude grave anomalies, showing similar reflection patterns to L1, were identified with simple soil fills, e.g., anomalies L3 and L4 (blue and black dotted-line rectangles in Fig. 6; for profile-views see Fig. 5c and a). Our evaluation of the post-survey findings shows that grave anomalies H1, L1 were identified with a total of two box-shaped graves found outside of the MCT, while the exploration of anomaly L2 resulted in the discovery of a simple pit grave, which could not be discernible with the design parameters of the particular survey. Thus, all existing graves in the survey area surrounding the MCT were positively interpreted (the location of another grave found just off the east side of the MCT and to the NW of H1, Grave 17 in Moutafi and Voutsaki, 2016: Fig. 1, was not surveyed by GPR). While there were grave anomalies eventually identified with other anthropogenic or geological structure (i.e. false positives), there were no graves found where no interpreted grave anomalies were shown (i.e. there were no false negatives). We explored solutions for reducing false positives with use of advanced processing of GPR data, discussed in Section 4.3.

stone wall oriented WSW-ENE (Fig. 6). Compared to H1, anomaly H2 is less deep and wide, and does not show any hyperbolas in profile-view; (ii) Anomaly H3 (blue solid-line rectangle in Fig. 6), which shows a high amplitude spot in plan-view (Fig. 4a) and narrow hyperbolas similar to H1 and deeper/wider hyperbolas than H2 in profile-view, that is an overall reflection pattern and geometry compatible with a grave similar to H1, was instead identified with an undefined stone structure (Fig. 5e); (iii) Anomalies H4 and H5 (black solid-line rectangles in Fig. 6), were identified with parts of the buried rock bed. While both show high amplitude spots in plan-view (Fig. 4a, b), they show contrasting patterns to each other and to anomaly H1 in profile-view: H4 shows a strong horizontal reflection with no hyperbolas (Fig. 5a), whereas H5 shows truncated wide hyperbolas (aperture of ca. 1 m) (Fig. 5e). Field observations made by the author in the excavation to the west of the MCT verified the interpreted fine-grained conglomerate bed and its ‘bottleneck’ plan-shape in the southwestern part of the survey area, a geometrical feature of undefined use. Other stone structures of undefined use and relation to the burials were also revealed at locations of uninterpreted high amplitude anomalies located at small depths, such as linear stone aggregations to the east and the west of the MCT (gray dashed-dotted lines in Fig. 6; labeled W in plan-view, Fig. 4a), and a cobblestone floor off the southern edge of the MCT surveyed only marginally by GPR (CF in Fig. 6; for the anomaly plan-view, see Fig. 4a). Excavations explored also low amplitude grave anomalies. Anomaly L1 to the west of the MCT (red dotted-line rectangle in Fig. 6) was identified with a N-S oriented, 2.1 × 1.1 × 0.3 m stone-lined grave, containing few individuals (Moutafi and Voutsaki, 2016: Fig. 1, Grave 23). In plan-view, the anomaly is defined at smaller depths (< 0.6 m) by a middle-to-low amplitude spot measuring ca. 1.8 × 1 m and oriented N-S (Fig. 4a); the spot shows higher amplitudes at greater depths (Fig. 4b). In profile-view, L1 shows low-to-middle amplitude reflections without hyperbolas, a ‘shadow-area’ about 2 m long, bordered by high reflections with no hyperbolas on its southern side which may indicate side-lining (Fig. 5b1: 28–28.4 m distance, 4 ns two-way time); other high reflections at a depth of about 0.6 m, which correspond with the higher amplitude spot in plan-view, are interpreted as floor reflections (Fig. 5b1: 28–30 m distance, 10 ns time). In a perpendicular profile across the anomaly, the same low amplitude pattern with high reflections at the floor and on the western side is shown, however with a pair of half- and full hyperbolas just above the interpreted floor, providing additional indication of side-lining (Fig. 5b2: 18 m and 19.2 m distance, 7 ns time). Compared to H1, the simpler stone lining of anomaly L1 produced no or few hyperbolas in profile-view (on the southern and western side of the anomaly), while the small depth, the fewer individuals and the generally simpler construction of the burial, produced an anomaly of lower amplitude and less reflection details. Nonetheless, the match between the interpreted dimensions and orientation with those in the excavation is excellent. Anomaly L2, defined by a low amplitude spot measuring ca. 1 × 0.5 m and oriented E-W in plan-view (Fig. 4b) and a ‘shadow-area’ 0.6 m long with low amplitude reflections without hyperbolas (distance: 28.4–29 m in Fig. 5a), was identified with simple soil fill. However, the excavation for L2 revealed a simple shallow pit grave in the immediate vicinity of the anomaly location, that is ca. 0.5 m to the south (red dotted-line rectangle in Fig. 6; Moutafi and Voutsaki, 2016: Fig. 1, Grave 22). Following field observations made by the author, the pit grave extends to 0.5 m along the E-W direction and 0.3 m across (that is, along the GPR line direction) and is < 0.3 m deep. Because of the simple structure and small dimensions that were below the detection ability of the GPR survey design (i.e. survey line separation, trace sampling interval and principal frequency) and complex subsurface conditions, this pit grave could not have been identified with any low amplitude anomaly produced from the particular survey. However, it is considered a positive interpretation since this grave was found at the

4.3. Grave-related evaluation of advanced processed GPR data 4.3.1. 3D volume Isosurface rendering of a Gaussian-filtered instance of the non-topographically corrected data volume using a threshold value of 43% of the maximum amplitude (i.e. within the range reported as suitable to highlight grave-related anomalies, 30–50%), resulted in isolating a number of high amplitude anomalies, including H1, to the east and southwest of the MCT (Fig. 7a). A threshold value of 60% was necessary to isolate anomaly H1 (the only high amplitude anomaly identified with a grave in the survey area), which also isolated a pair of high amplitude anomalies to the east and southwest of the MCT (Fig. 7b). Since the anomalies to the east have been already identified with rock-relics, and considering that their volume appears greater than H1 and similar to the anomaly to the southwest of the MCT, we interpret the southern anomaly also as non-grave. The powerful image suggests a very low probability for the existence of high amplitude possible graves around the MCT, a picture in accordance with the findings of the post-survey excavation made so far. This information may effectively reduce false positive interpretations, however it should be treated with caution, since it does not constrain all high amplitude anomalies which were identified with non-grave structure; moreover, it suppresses low amplitude possible grave anomalies and may also suppress high amplitude grave anomalies with dimensions smaller than H1. 4.3.2. Profiles The DG filter, applied to the original profile using a 2.5 on 1.0 standard deviation values in two passes, enhances isolated high amplitude anomalies against the surrounding geology and considerably suppresses tree reflections. The KH filter, applied to a 1.5 standard deviation Gaussian filtered instance of the original profile, enhances geology and lateral discontinuities (Fig. 8). Table 4 summarizes the results of DG and KH filtering relative to the anomalies discussed in Section 4.2. The results of data enhancement show that in all cases concerning high or low reflection anomalies identified with non-grave structures, the KH filtered images provide diagnostic hints that may aid to not characterize a reflection anomaly as grave-related. Regarding identified graves, the KH produces the fullest and most accurate shape and dimensions of the grave identified with anomaly H1, emphasizing the grave physical features (concave-downward cover, side walls, floor, soil fill and buried individuals) and sharply outlining the barrel vault shape 10

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Fig. 7. Iso-amplitude surface rendering of the full 3D non-topographically corrected data volume. (a) Threshold value at 43% of the maximum equalized amplitude envelope. (b) Threshold value at 60%. The original data were gridded with a 3D kernel (node spacing X, Y, Z: 0.5 m, 0.5 m, 1 ns) and filtered with a Gaussian 5× kernel for incoherent noise reduction. The outline of the MCT and the location of anomaly H1 (identified with a stone-lined grave) and other high amplitude anomalies are shown. X and Y axis dimension in meters. Z dimension in ns (negative values for presentation purposes); corresponding depth range is 0.0 to 1.5 m. Data from two-way time later than 20 ns omitted for clarity. Initial GPR processing parameters as in Fig. 4. Results discussed in Section 4.3.1.

closure of the excavation and was therefore of no value for the interpretation and the design of subsequent excavations. In spite of the fact that our interpretation resulted in positively identifying and not bypassing existing graves, the exploration of several anomalies eventually identified with non-grave structure required an increased field labor and questioned the effectiveness of GPR. To overcome this condition, increase the effectiveness of GPR in complex site conditions and help to exploit future excavation resources more efficiently, we applied advanced processing and evaluated the results with reference to the post-survey excavation findings. Data enhancement procedures may significantly aid to the reduction of false positive interpretations, in particular relative to profiles: the KH filter discriminates between anomalies related to grave (e.g., shaft sides, floor and lining) and non-grave features (e.g., surrounding geology and soil fill that may not be related to a grave fill), while the DG filter strongly attenuates unwanted signals such as tree reflections and rock bedding, complementing the KH filter. In addition, the KH filter provides a powerful tool for studying local stratigraphy in detail, contributing to site characterization. While the profile-by-profile examination is a rather slow process compared to procedures applied to a data volume, it ensures that anomalies in difficult to assess, complex conditions, are adequately evaluated and characterized. Optimally, the above procedure could be implemented in semi-automatic system that will compare patterns between selected ‘test-windows’ and highlight possible grave anomalies that will be subsequently inspected in more detail by the interpreter. False positives may also be reduced with isosurface rendering applied to the non-topographically corrected data volume. However, the results should be treated only as a general guidance for high amplitude grave search, and must be complemented by profile enhancement for a detailed evaluation of anomalies possibly related to graves. Specialized procedures related to field collection of data such as denser profiling or topographical correction do not seem able to reduce false positives, except of perpendicular profiles, which may locally complement profile-based grave interpretation (as in the case of anomaly L1), if logistics and local conditions allow. The implementation of the data enhancement procedures discussed above to the entire survey area at AVNC result in a substantial decrease of interpreted grave anomalies: the only remaining anomalies are the two anomalies identified with graves (H1, L1) and another two anomalies (one high- and one low-amplitude) located to the west of the MCT (Fig. 9). The refined grave interpretation is very much closer to the actual excavation findings, and reveals that the cemetery is formed of a single, dense cluster of graves, with very few isolated burials outside of

of the grave; it also emphasizes the side lining of the grave identified with anomaly L1. The DG filter enhances isolated anomalies that may relate to graves, while it suppresses geological bedding and certain anomalies that could be falsely interpreted as grave-related (e.g., tree reflections); as such it may complement the KH image and the combined filtered results effectively improve the interpretation of grave anomalies. 5. Discussion and conclusions 5.1. Search for graves In our exploration of the prehistoric AVNC site, we designed the GPR survey for mapping of small-sized stone-lined and stone-covered graves, considering the known patterning, size, geometry, and other physical features of graves encountered in the pre-survey excavations. We interpreted grave anomalies according to criteria related to local grave physical features and environmental conditions (as discussed in Section 4.1.2), to minimize the possibility of bypassing existing graves, since a grave anomaly could be easily misinterpreted or bypassed in the complex conditions of the site. Grave interpretation resulted in both positives and false positives, but not in false negatives. Grave shaft sides, stone lining and cover, and grave fill were positively interpreted, with the interpreted position and size accurately matching excavation data. False grave positives included stone structures or rock bed reliefrelics identified with interpreted stone-lined graves, and simple soil fill identified with interpreted non-stone-lined graves. Most false positives were related to the relief-relics produced from the widespread cutting and extraction of the rock beds, and the placement of graves not in the intact rock beds but in areas occupied with the rock bed relief-relics. Non-grave GPR anomalies showed reflection patterns and dimensions similar to stone-lined graves, and also a clustered spatial distribution, simulating high amplitude grave anomalies in profiles and time-slices. The positive interpretation of anomaly H1 and the false positive interpretation of anomalies H3 and H5 explored in the initial stages of the post-survey excavation, provided some reference reflection patterns, which however were of limited interpretational value, since 85% (13/ 15) of the existent graves were already excavated before the survey. Moreover, the physical features of the grave identified with anomaly H1 were above the average of local graves (ca. 3 × 2 m plan dimensions × 1.1 m depth instead of 2.5 × 1.3 × 1 m of the largest local grave; boulder lining instead of stone lining; 20+ buried individuals instead of maximum 7 in local graves). The only grave with physical features similar to the local average (anomaly L1), was explored at the 11

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(caption on next page)

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Fig. 8. Edge enhancement filtering applied to GPR profiles. Left panels: Difference of Gaussian filter (DG) applied to original profile using a 2.5 on 1.0 standard deviation values in two passes. Right panels: Kirsh filter (KH) applied to a Gaussian (using a 1.5 standard deviation value) filtered instance of an original profile. Initial GPR processing parameters as in Fig. 5. Profile labels (‘a’ to ‘e’) and labeled anomalies as in Fig. 5, discussed in Section 4.3.2.

cultural information may substantially reduce false positive cases and aid in an efficient site management and focusing of subsequent excavations on positively identified grave locations. As such, the proposed procedure may greatly benefit future GPR cemetery surveys in complex subsurface conditions such as those at the AVNC.

it, verifying and expanding the results of the post-survey excavation made so far. The GRP results at the AVNC site remind us that the search for graves remains a challenging exploration task and agree with studies of prehistoric small-sized and box-shaped shallow graves situated in complex subsurface conditions and not necessarily following an organized placement layout (e.g., Lowe et al., 2014), in that time-slices may be affected by signals related to non-grave anthropogenic or natural structure and may not be dependable for grave interpretation and location. In these challenging conditions, the profiles, run in a detailed parallel and possibly perpendicular layout and enhanced with advanced processing techniques, would be the primary source of grave-related information and grave location, with the time-slice and volume data used as source of environmental and complementary information for the profile-based grave interpretation. The identification procedure of grave anomalies at the AVNC was not completed due to excavation closure in 2015. With all graves found in the surveyed area positively interpreted by GPR, and no graves found where interpretation did not suggest a grave anomaly, the GPR survey can be considered as generally successful, given the type of local graves and the complex local conditions. Evidently, the use of data enhancement procedures used interchangeably with geological and other

5.2. Site organization and grave placement The GPR images at the AVNC reveal an area of complex usage within a limited space, comprising rock bed extraction and landscaping, laying out ditches and stone walls, placing graves, etc. Individual graves and the cemetery as a whole do not dominate the site, but are confined within a cluster that occupies only a part of the site. The placement of most graves in areas where the rock bed was extracted beforehand suggests that the majority of graves were placed without specific landscaping, at a locale already in diverse use. Considering that postsurvey excavations confirmed only a few graves outside of the cluster, and that the cluster is located on the highest and flattest sector of the survey area and near the highest part of the hill, the above spatial pattern may reflect on a certain stage of the site usage or relate to certain funerary practices, such as establishing cemeteries according to specific topological concepts, or in areas with specific use (e.g., Boyd,

Table 4 Results of data enhancement filters applied to GPR profiles. Anomaly (identified with); Reference Figure

DG result

KH result

H1

Emphasized internal reflection pattern against reflections of surrounding geology; Sharper outer and inner reflections, side-lining and interior details

Larger contrast with the anomaly's surroundings; Emphasized lateral discontinuity of rock bed reflections and change from continuous to intermittent reflections and hyperbolas in the interior of the anomaly (Fig. 8d: compare 27–30 m with 24–27 m and 30–32 m, 5–20 ns)

Weak internal hyperbolas compared to H1

More coherent reflection pattern compared to the truncated pattern of H1; Suggests a non-grave structure

Well-defined hyperbolas, similar to H1

More coherent reflection pattern compared to the truncated pattern of H1; Suggests a non-grave structure

Isolated strong reflection

Coherent reflection pattern similar to the surrounding conglomerate bed (Fig. 8a: 37–38 m, 5–15 ns; compare with the pattern between 35 and 37 m); Suggests a non-grave structure

Weak reflection pattern

Coherent reflection pattern similar to the surrounding marl bed (Fig. 8e: 34–36 m, 12–20 ns; compare with the pattern between 32 and 34 m); Suggests a non-grave structure

No distinct reflection pattern (Fig. 8b1); Perpendicular profile: Intermittent pattern, emphasizes the hyperbolas pair at both ends of the anomaly (Fig. 8 b2: 18 & 19 m, 10 ns)

Less strong, intermittent reflection pattern, compared to the immediate surroundings (Fig. 8b1); Perpendicular profile: Distinct strong reflection pattern compared to the immediate surroundings (Fig. 8b2); Suggests a side-lined grave

No distinct reflection pattern

Less strong reflection pattern compared to the immediate surroundings, contrary to the more distinct pattern of L1 particularly seen in the perpendicular profile; Suggests a non-side-lined grave

No distinct reflection pattern (“shadow area”) with few similarities to the L1 pattern

More coherent weak reflections compared to the intermittent reflection pattern of L1; Suggests a simple soil fill

No distinct reflection pattern

More coherent weak reflections; Suggests a simple soil fill

(boulder-lined deep grave) Fig. 8d

H2 (stone wall) Fig. 8c H3 (undefined stone structure) Fig. 8e H4 (rock bed) Fig. 8a H5 (rock bed) Fig. 8e L1 (stone-lined shallow grave) Fig. 8b1, b2

L2 (simple pit grave) Fig. 8a

L3 (simple soil-fill) Fig. 8c L4 (simple soil-fill) Fig. 8a

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Fig. 9. Refined interpretation plan after implementation of advanced processing of GPR data to the entire survey area (isoamplitude surface rendering and edge enhancement filters discussed in Section 4.3). Results discussed in Section 5.1. Labels and other features as in Fig. 6.

Fig. 10. Highlighting the artificially landscaped buried relief with isoamplitude surface rendering of the full 3D topographically corrected data volume. (a) View from SE. (b) View from NNE. Threshold value at 38% of the maximum equalized amplitude envelope. The original data were gridded with a 3D kernel (node spacing X, Y, Z: 1 m, 1 m, 0.25 m) and filtered with an average rectangular filter 5× kernel for incoherent noise reduction. The outline of the MCT and the location of anomaly DH (dashed red line; identified with a soil-filled ditch) and other possible ditch/trench anomalies (dashed green lines) are shown. Ellipses show the locations of excavated, identified and interpreted graves after Fig. 9 (gray: excavated; red: grave anomalies identified with graves; green: interpreted grave anomalies). Axes dimension in meters. Z axis shows elevation with respect to a local zero-elevation reference point. Data from two-way time earlier than 10 ns omitted for clarity. Initial GPR processing parameters as in Fig. 4. Results discussed in Section 5.2.

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2002). The extensive and deep artificial landscaping of the buried relief with the arcuate-shaped ditches and stepped surfaces revealed in the GPR time-slices and profiles and highlighted with 3D visualization of the equalized amplitude envelope data (Fig. 10), is an important indication of a specific use of the site where the cemetery was established, deserving further archaeological exploration. This essential output of the GPR survey is equally important to graves for the understanding of the site usage, and for site management when it becomes available prior to excavation. The results on both grave and non-grave related structure show that a systematic use of GPR at prehistoric cemetery sites will bring about the amount of information necessary to improve our understanding of local conditions and allow for experimentation with techniques and parameters to improve survey results and interpretation. In this respect, it is most important that detailed geophysical surveys at cemetery sites are made prior to any excavation, to preserve important site information and be most effective with respect to grave location and mapping.

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Acknowledgments The geophysical survey was made in the framework of the AVNC archaeological project, directed by Prof. S. Voutsaki (University of Groningen, The Netherlands), under the auspices of the Greek Archaeological Society. This project is part of the greater Ayios Vasilios Archaeological Project directed by Ms. A. Vasilogamvrou (Director Emerita of the Directory of Antiquities of Laconia). Financial support was provided by the Ammodo Foundation, The Netherlands. The author thanks Prof. Voutsaki for her support during the field season and the intriguing discussions on GPR results, and to Surveyor Engineer Y. Yaxoglou for his topographical support. Finally, we are grateful to the editors and the two anonymous reviewers for their constructive comments on this text. References Barone, P.M., 2017. Forensic geophysics. In: Di Maggio, R., Barone, P. (Eds.), Geoscientists at Crime Scenes. Soil Forensics. Springer, Cham, pp. 175–190. Bigman, D.P., 2012. The use of electromagnetic induction in locating graves and mapping cemeteries: an example from native North America. Archaeol. Prospect. 19, 31–39. https://doi.org/10.1002/arp.1416. Bintliff, J., 1977. Natural environment and human settlement in prehistoric Greece based on original fieldwork. In: BAR Supplementary Series 28 British Archaeological Reports, Oxford (2 v). Boyd, M.J., 2002. Middle Helladic and Early Mycenaean Mortuary Customs in the Southern and Western Peloponnese. BAR-IS 1009. Archaeopress, Oxford. Cavanagh, W., Mee, C., 1998. A private place: death in prehistoric Greece. In: Studies in Mediterranean Archaeology. 125 Jonsered: Paul Åström Förlag. Conyers, L., 2006. Ground-penetrating radar techniques to discover and map historic graves. Hist. Archaeol. 40 (3), 64–73. https://doi.org/10.1007/BF03376733. Conyers, L.B., 2013. Ground-penetrating radar for archaeology, 3rd ed. Altamira Press, UK. Crane, R., 1997. A simplified approach to image processing: Classical and modern techniques in C. In: Prentice Hall PTR. Upper Saddle River, New Jersey. Dalan, R.A., DeVore, S.L., Clay, R.B., 2010. Geophysical identification of unmarked historic graves. Geoarchaeology 25, 572–601. https://doi.org/10.1002/gea.20325. Davenport, G.C., 2001. Remote sensing applications in forensic investigations. Hist. Archaeol. 35, 87–100. https://doi.org/10.1007/BF03374530. Dionne, C.A., Wardlaw, D.K., Schultz, J.J., 2010. Delineation and resolution of cemetery graves using a conductivity meter and ground-penetrating radar. Tech. Briefs Hist. Archaeol. 5, 20–30. Dojack, L., 2012. Ground Penetrating Radar Theory, Data Collection, Processing and Interpretation: A Guide for Archaeologists (30-Apr-2012). Available online at. https://open.library.ubc.ca/cIRcle/collections/42591/items/1.0086065 (accessed: 22.04.2017). Doolittle, J.A., Bellantoni, N.F., 2010. The search for graves with ground-penetrating radar in Connecticut. J. Archaeol. Sci. 37, 941–949. https://doi.org/10.1016/j.jas. 2009.11.027. Golden Software, 2013. Surfer, Surface Mapping System, Version 11. Golden. Golden Software, 2016. Voxler, 3D Well and Volumetric Data Visualization, Version 4. Golden.

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