Thera Island with the location of the investigated area. (Greece map from http://www.culture.gr, Thera map from http://www.greecetravel.com/aegean/map_of_santorini_island.htm)

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... excavations at Akrotiri on Thera (Santorini) island ( Fig. 1) have shown evidence that the first habitation at the site dates from late Neolithic times. During the early Bronze a sizable settlement was founded, extended and gradually developed into one of the main urban centers and ports of the Aegean. The town's life came to an abrupt end in the late 17th century B.C. when the inhabitants were ...

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... common convergence criterion in an inversion procedure is when the misfit value reaches the picking error. Figures 10a and b show plots of the mean absolute errors calculated for both experiments during the iterative procedure. For the cross-shaft case (Fig. 10a) it is obvious that the mean misfit drops quickly during the first five iterations reaching asymptotically the value of 0.1ms right after the 10 th iteration, which is considerably below the effective picking error (0.25ms). ...

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... deviations (<0.3 ms) ensure very good fits between observed and modeled travel time curves. A common convergence criterion in an inversion procedure is when the misfit value reaches the picking error. Figures 10a and b show plots of the mean absolute errors calculated for both experiments during the iterative procedure. For the cross-shaft case (Fig. 10a) it is obvious that the mean misfit drops quickly during the first five iterations reaching asymptotically the value of 0.1ms right after the 10 th iteration, which is considerably below the effective picking error (0.25ms). In the up-hole case, the mean absolute value (Fig. 10b) rapidly drops after the first iteration, stabilizing ...

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... experiments during the iterative procedure. For the cross-shaft case (Fig. 10a) it is obvious that the mean misfit drops quickly during the first five iterations reaching asymptotically the value of 0.1ms right after the 10 th iteration, which is considerably below the effective picking error (0.25ms). In the up-hole case, the mean absolute value (Fig. 10b) rapidly drops after the first iteration, stabilizing around 0.6ms for the remaining iterations. The higher residual value in this layout indicates that the unknown parameters are not significantly constrained due to the quite limited angular coverage of ...

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... this procedure the complexity functions for both the experiments were computed. Figures 11a and b show the complexity graphs for both acquisition layouts, where the oscillation of the model parameters in the cross-shaft case (Fig. 11a) stabilizes after 7 iterations indicating in a way that the model subspace is confined. In the up-hole experiment (Fig. 11b) the oscillations in model perturbations drop significally after about 11 iterations. ...

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... complexity function should increase asymptotically to a plateau representing the misfit of a best-fitted model. Following this procedure the complexity functions for both the experiments were computed. Figures 11a and b show the complexity graphs for both acquisition layouts, where the oscillation of the model parameters in the cross-shaft case (Fig. 11a) stabilizes after 7 iterations indicating in a way that the model subspace is confined. In the up-hole experiment (Fig. 11b) the oscillations in model perturbations drop significally after about 11 iterations. In both tomographic procedures a steady trend is achieved in the last 3 to 4 iterations, indicating in some way the uniqueness ...

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... this procedure the complexity functions for both the experiments were computed. Figures 11a and b show the complexity graphs for both acquisition layouts, where the oscillation of the model parameters in the cross-shaft case (Fig. 11a) stabilizes after 7 iterations indicating in a way that the model subspace is confined. In the up-hole experiment (Fig. 11b) the oscillations in model perturbations drop significally after about 11 iterations. In both tomographic procedures a steady trend is achieved in the last 3 to 4 iterations, indicating in some way the uniqueness of the solution. Criteria of travel time misfit and stability between solution updates were used to understand and ...

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... known natural cavity, unveiled during the excavation of shaft 22, was used to be the target to be detected with the seismic tomography technique. The image of the reconstructed velocity field shown in Figure 12a can be divided in three main velocity zones: 1) the low-velocity (<500 m/s) zone A at the top, attributed to the volcanic tephra layer, which was produced by the volcanic eruption and covered the entire island and the prehistoric settlement itself. The layer was removed during the systematic excavations by professor Marinatos in 1967, and zone A is a visible today remainder of that layer; 2) the zone B consisting of medium to high-velocity (600-1000 m/s) structures, which is attributed to a layer of older in age ruins underlying the existing prehistoric settlement. ...

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... underlying the existing prehistoric settlement. That layer was verified during the shafts excavations; 3) the lower zone C consisting of low to high-velocity structures, which is attributed to the pyroclastic formation basement rock. The geologic report (Alexiadou, et al., 2001) for shaft 22 reports a zone of compacted volcanic tuff with stones (Fig. 12b) between 1 and 2.2m depth, a less compacted zone of the same material between 2.2 and 3.1m depth, a tephra layer with a natural void 1.2m aperture between 3.1 and 4.3m depth and a zone of compacted tuff below 4.3m depth. Figure 12. Image of the reconstructed velocity field (a) with the geologic record for shaft 22 and the cavity (c), ...

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... geologic report (Alexiadou, et al., 2001) for shaft 22 reports a zone of compacted volcanic tuff with stones (Fig. 12b) between 1 and 2.2m depth, a less compacted zone of the same material between 2.2 and 3.1m depth, a tephra layer with a natural void 1.2m aperture between 3.1 and 4.3m depth and a zone of compacted tuff below 4.3m depth. Figure 12. Image of the reconstructed velocity field (a) with the geologic record for shaft 22 and the cavity (c), unveiled during the shaft excavation. ...

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... velocity field (a) with the geologic record for shaft 22 and the cavity (c), unveiled during the shaft excavation. (Photo provided by Alexiadou, 2001) In the zone of interest C, we can distinguish: 1) a high-velocity structure next to the top of the shaft, which is in a very good correlation with the reported zone of tuff with stones (Fig. 12c); 2) a low-velocity (<400 m/s) structure LV (Fig. 12a) located exactly in the same position with the reported natural cavity (Figs. 12b and 12c) and 3) a medium-velocity structure observed below LV, which is also in a very good agreement with the reported zone of compacted tuff with ...

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... for shaft 22 and the cavity (c), unveiled during the shaft excavation. (Photo provided by Alexiadou, 2001) In the zone of interest C, we can distinguish: 1) a high-velocity structure next to the top of the shaft, which is in a very good correlation with the reported zone of tuff with stones (Fig. 12c); 2) a low-velocity (<400 m/s) structure LV (Fig. 12a) located exactly in the same position with the reported natural cavity (Figs. 12b and 12c) and 3) a medium-velocity structure observed below LV, which is also in a very good agreement with the reported zone of compacted tuff with ...

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... provided by Alexiadou, 2001) In the zone of interest C, we can distinguish: 1) a high-velocity structure next to the top of the shaft, which is in a very good correlation with the reported zone of tuff with stones (Fig. 12c); 2) a low-velocity (<400 m/s) structure LV (Fig. 12a) located exactly in the same position with the reported natural cavity (Figs. 12b and 12c) and 3) a medium-velocity structure observed below LV, which is also in a very good agreement with the reported zone of compacted tuff with ...

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... reconstructed velocity field of the imaged area (Fig. 13) is quite complex. Two low- velocity (LV1, LV2) and two high-velocity (HV1, HV2) structures dominate a medium velocity (500-800 m/s) environment. The geologic report (Alexiadou, et al., 2001) for shaft 57 notes the dominance of tephra with volcanic stones intercalations. The zone between depths 1.1 to 3.4 meters in shaft 57 is reported ...

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... zone between depths 1.1 to 3.4 meters in shaft 57 is reported of consisting of more compact material with lower rippability. Figure 13 Cross-hole velocity field between shafts 56 and 57. ...

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... Figure 13 it is definite that the high-velocity structure HV1 is very good correlated with this zone. According to the calibration test discussed in shaft 22, the low-velocity zones LV1 and LV2 are more likely to imply the existence of air-filled or semi-filled cavities situated close to the walls of the shafts. ...

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... image of the reconstructed velocity field is shown in Figure 14. Low (<400 m/s) and high (>1000 m/s) velocity structures dominate in a medium of velocities varying from 500 to 1000 m/s. ...

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... 1000 m/s. The geologic report (Alexiadou, et al., 2001) for shaft 55 reports a low rippability zone, consisting of consolidated pebbles and lava-balls between 1.1 and 3.2m depth, an underlying zone of higher rippability consisting of loose tephra between 3.2 and 4.8m depth, which is in a very good correlation with the low-velocity structure LV1 (Fig. 14), and a deeper zone of very compact volcanic material, consisting of sizable stones and lava-balls. A medium rippability zone of consolidated stones and boulders, between 2.4 and 6m depth, is also reported for shaft 54, which is in a very good agreement with the elongated oblique zone observed in the upper part of the imaged area. A ...

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... medium rippability zone of consolidated stones and boulders, between 2.4 and 6m depth, is also reported for shaft 54, which is in a very good agreement with the elongated oblique zone observed in the upper part of the imaged area. A cavity, extending from the head of the shaft up to 1.8m depth (shaded area A in Figure 14), unveiled during the excavation of shaft 54, prevented the location of geophones up to this depth. The low-velocity zone LV2 (Fig. 14) is likely considered to imply the continuation of the existing cavity toward the imaged area. ...

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... for shaft 54, which is in a very good agreement with the elongated oblique zone observed in the upper part of the imaged area. A cavity, extending from the head of the shaft up to 1.8m depth (shaded area A in Figure 14), unveiled during the excavation of shaft 54, prevented the location of geophones up to this depth. The low-velocity zone LV2 (Fig. 14) is likely considered to imply the continuation of the existing cavity toward the imaged ...

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... reconstructed velocity field of the imaged area is shown in Figure 15. Low to medium seismic velocities (500-1000 m/s), indicative of the pyroclastic formation basement, dominate the imaged area. ...

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... basement, dominate the imaged area. The geologic report (Alexiadou, et al., 2001) for shaft 53 notes the existence of a low rippability zone, observed at 4m depth, consisting of sizable boulders and lava-balls in an older in age volcanic material. This zone is in a very good correlation with the medium velocity (700-100 m/s) structure MV (Fig. 15) observed in the lower part of shaft 53. Figure 15. Cross-hole velocity field between shafts 54 and ...

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... zone is in a very good correlation with the medium velocity (700-100 m/s) structure MV (Fig. 15) observed in the lower part of shaft 53. Figure 15. Cross-hole velocity field between shafts 54 and 53. ...

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... zone of consolidated stones and boulders, between depths 1.5 and 2.8 meters, is also reported for shaft 54, which gives a good explanation for the high-velocity (>1200 m/s) isolated structure HV2 in Figure 15. The same explanation is likely given for the high- velocity structure HV1. ...

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... image of the reconstructed velocity field (Fig. 16) is characterised by low to intermediate velocity values (600-1000 m/s). An impressive low-velocity structure, LV is observed next to the middle part of shaft 49. The geologic report (Alexiadou, et al., 2001) for shaft reports a low rippability zone of compacted volcanic tuff up to 2.4m depth, a medium rippability zone of volcanic tuff ...

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... report (Alexiadou, et al., 2001) for shaft reports a low rippability zone of compacted volcanic tuff up to 2.4m depth, a medium rippability zone of volcanic tuff with altered lava-balls between 2.4 and 4.7m depth and a zone of less compacted volcanic tuff below 4.7m depth. The high-velocity structure observed next to the head of shaft 49 ( Fig. 16) seems to be in a good correlation with the highly compacted zone of volcanic tuff. However since the ray- path coverage in this part of the imaged area is very poor, the high-velocity structure is considered as an artifact and is not taken into consideration. The medium rippability zone reported between 2.4 and 4.7m depth can not ...

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... image of the reconstructed velocity field (Fig. 17) is quite complex. Medium seismic velocities (600-1000 m/s), indicative of the pyroclastic formation basement, dominate the imaged area. Prominent low-velocity LV1, LV2 and high-velocity HV1, HV2, HV3 structures are observed in the imaged area. The geologic report (Alexiadou, et al., 2001) for shaft 43A reports a cavity between the ...

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... HV2, HV3 structures are observed in the imaged area. The geologic report (Alexiadou, et al., 2001) for shaft 43A reports a cavity between the depths 1 and 3.2m, unveiled during the shaft excavation. A zone of older in age ruins is also reported between depths 3.5 and 4.5m, which give a very good explanation for the high-velocity structure HV2 (Fig. 17) imaged exactly in the same position. Structure HV1 is easily explained by the existence of a screen barrier of older in age ruins reported to build the back wall of the cavity. The low-velocity structure LV2 resembles structure LV1 both in shape and seismic velocity. We believe that LV2 is attributed to a hidden cavity situated close ...

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... believe that LV2 is attributed to a hidden cavity situated close to the wall of the shaft. Figure 17. Cross-hole velocity field between shafts 43 and 43A. ...

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... Seismic Tomogram for shaft 22 revealed the existence of three layers of interest. The top zone A which is the remainder of the tephra layer that was removed during the systematic excavations by Prof. Marinatos, the zone B (fig 12a) of older in age ruins verified during the excavations of the foundation shafts and zone C attributed to the pyroclastic formation basement. In general the up-hole experiments were able to image the whole sequence of layers starting from the bottom of the foundation shafts up to the ground surface. ...

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... corresponding up-hole images in the majority of them show the same layer sequence and are interesting from archaeological point of view. Sketch diagram in figure 18 is an E-W cross-section throughout shaft 22 and corresponds to a possible interpretation based mainly on geophysical data. ...