High-resolution magnetic gradient and electrical ...

Near Surface Geophysics, 2009, 207-215

High-resolution magnetic gradient and electrical resistivity tomography survey at the Plaka Petrified Forest Park in Lesvos Island, Greece G. Vargemezis1, N. Zouros2, P. Tsourlos1 and I. Fikos1 1 2

Geophysical Laboratory, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Department of Geography, Aegean University, 61100 Mytilini, Greece

Received April 2008, revision accepted April 2009 ABSTRACT Lesvos Island, in the North Aegean area of Greece, exposes large accumulations of fossilized tree trunks. They are collectively known as the Petrified Forest of Lesvos, a designated protected natural monument. The paper describes the results of a geophysical study that has been carried out in the area of Plaka on the western part of Lesvos in order to investigate the near-surface geology and detect buried fossilized tree trunks. In situ and laboratory measurements of magnetic and electrical properties of the trunks have been conducted. Considering the magnetic susceptibility distribution and the contrast between the trunks and the surrounding material, magnetic gradient anomalies of negative signature were expected to reflect the existence of trunks. Electrical resistivity tomography has also been conducted in order to detect buried trunks, based on their resistivity contrast with the pyroclastic surrounding material. Flow paths of the pyroclastic material that can be related to the tectonic features at the time have been detected. In general, petrified trunks have been detected as high resistive bodies. In one case, where the trunk was buried very close to the surface, the resistivity was lower than the surrounding material. 2D and 3D subsurface resistivity models of the surveyed area have been constructed, pointing out probable locations of buried petrified trunks. Excavations that followed the geophysical survey revealed petrified trunks in most of the cases although some of the excavated resistive targets proved to be surface fracture systems filled with resistive oxides. INTRODUCTION The GeoPark of Lesvos’s Petrified Forest comprises a core zone (15 000 hectares of the Petrified Forest protected area) and a broader buffer zone that includes more than 20 000 hectares of the central volcanic terrains. The main accumulations of the fossilized tree trunks appear at the western part of the island. The coastal area of Plaka (Fig. 1) in western Lesvos is one of the main sites of research interest for exploring both terrestrial and marine fossil sites. It has been decided that the area is to be developed as a visiting park due to the abundance of standing and lying petrified tree trunks as the quality of the findings and the special features of the Plaka fossil site compared favourably to neighbouring similar sites in Lesvos. Many fossilized trees are present along the coast on the beach and in the sea. Systematic research has been carried out by the Natural History Museum of the Lesvos Petrified Forest during the last years in order to explore the fossil sites. The Geophysical *

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© 2009 European Association of Geoscientists & Engineers

Laboratory of the University of Thessaloniki contributed to this research carrying out geophysical investigations to detect buried petrified trees (Vargemezis et al. 2001). The results of a geophysical investigation carried out at the area of Plaka are presented in this study. Magnetic gradient mapping and electrical resistivity tomography techniques have been applied aiming to investigate the near-surface geological structure of the park as well as to detect buried trunks. The magnetic gradiometry technique is well-known and it is used widely in near-surface geophysics, particularly in archaeological prospection (Gafney 2008). It relies on the principle that nearly all materials generate a secondary magnetic field, known as induced magnetism, when exposed to a strong primary magnetic field. The strength of this induced magnetism is related to the magnetic susceptibility. The initial idea leading to the present survey was to take advantage of the contrast between the magnetic susceptibility of the trunks and the surrounding materials. Estimates of magnetic susceptibility have been obtained both from in situ and sample laboratory measurements. 207

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FIGURE 1 Location map of the surveyed area.

FIGURE 2 Photo depicting the typical strati graphy at the western part of the Plaka geopark.

The electrical resistivity tomography (ERT) technique (Dahlin 2001) is very popular in a wide variety of near-surface applications ranging from hydrogeological and geotechnical studies to environmental and archaeological prospection (Daily et al. 1992; Papadopoulos et al. 2006). A systematic ERT survey was applied to determine the geoelectrical structure of a limited part of the study area that had already been surveyed by the magnetic gradiometry method. Petrified trunks buried at a small depth (0.5– 1 m) from the surface are expected to have a strong resistivity contrast with the surrounding, more conductive, material. Therefore, the shape of the located resistive anomalies was a criterion for the characterization of the resistive target as a possible petrified trunk. GEOLOGICAL SETTING-FORMATION OF THE PETRIFIED FOREST The geological structure of Lesvos Island consists of an autochthonous unit of Permo-Triassic age (mica schists, quartzites,

metasandstones and phyllites as well as a carbonate sequence) of significant extend on the south-east part of the island and two allochthonous units representing the volcanosedimentary nape and the ophiolitic nape respectively. On top of the above formations rests a Neogene volcanic rocks unit, including a thick pyroclastic sequence that dominates the central and western part of the island (Pe-Piper and Piper 1993, 2002). Kinematical analysis carried out in Lesvos showed that several successive tectonic events took place during Cenozoic. Published results of regional neotectonic studies in the wider North Aegean area as well as local studies suggest that the island of Lesvos suffered at least two major post-volcanic tectonic events since Miocene. The first one produced E-W to ENEWSW trending sinistral strike-slip faults in the Late Miocene. The second during Pliocene, caused NW-SE trending normal faults and NNE-SSW trending sinistral strike-slip faults (Papazachos et al. 1991). The formation of the Petrified Forest is directly related to the

© 2009 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2009, 7, 207-215

High-resolution magnetic gradient survey

intense volcanic activity in Lesvos Island during the Early Miocene times that lasted almost two million years (18.5– 17 Ma). The fossilized tree trunks appear within the thick pyroclastic sequence. Volcanic eruptions triggered pyroclastic flows that caused severe damage to the Miocene vegetation. An explosive blast of hot gases travelled ahead of the debris avalanche, flattening thousands of trees that later were covered by the pyroclastic materials (Pe-Piper and Piper 1993). The rapid covering of tree trunks, branches and leaves led to isolation from atmospheric conditions. Along with the volcanic activity, hot silicon rich solutions penetrated and impregnated the volcanic materials that covered the tree trunks. Thus, the major fossilization process started with a molecule by molecule exchange of the organic plant by inorganic materials. In the case of the Petrified Forest of Lesvos, the fossilization was perfect due to favourable fossilization conditions. In the coastal area of Plaka, the pyroclastic formation consists principally of pumice flows, mud flows, debris flows and stream conglomerates intercalated with air fall pyroclastic deposits (Fig. 2). As seen in other sites of western Lesvos, as loose slag and ash accumulate, plants grow preferentially in the erosion channels that develop on volcanic slopes. When an eruption occurred, lava and pyroclastic flows or glowing avalanches followed existing drainage lines. Thus any vegetation concentrated in these channels was covered and mixed within the debris. In the terrestrial part of the Plaka Park several clusters of petrified trees have been found and identified in 45 different fossil sites. The majority were standing while others were lying. The diameters of petrified tree trunks vary ranging from 0.5–4.5 m. IN SITU AND LABORATORY MEASUREMENTS OF MAGNETIC AND ELECTRICAL PROPERTIES Both magnetic and electrical properties of the petrified tree trunks and of the surrounding material have been studied. The magnetic susceptibility has been measured in situ using a JH-8 susceptibility meter. In addition, representative samples have been collected from the site and then measured in the laboratory. In situ measurements of magnetic susceptibility were collected from petrified trunks buried in the pyroclastic material, as well as from volcanic bombs that are commonly found within the same pyroclastic material. These measurements have been taken at their original places in the pyroclastic formation. A Bartington susceptibility meter has been used for the laboratory

measurements of cylindrical cores constructed from samples collected from the study area. The volume of the cylindrical cores was 10 cm3 and the units of magnetic susceptibility in Table 1 are in the SI system. Electrical properties of cylindrical samples (shown in Table 1) were measured using a standard 4 electrical contacts configuration (van der Pauw 1958) using a DC source and a high impedance voltmeter (KEITHLEY model 2010). Even a cursory examination of the geophysical properties of petrified trunks and the surrounding materials (Table 1) indicates that magnetic and electrical methods are applicable because: 1. The magnetic susceptibility of the petrified trunk is practically zero while the susceptibility of the volcanic bombs and of the surrounding pyroclastic material is quite significant (Table 1). Therefore, it is expected that the buried trunks would demonstrate a dominantly negative magnetic signature in relation to the environment. 2. Volcanic bombs have high susceptibility that is actually much higher than the surrounding pyroclastic sediments and thus are expected to produce mostly positive magnetic anomalies. 3. The resistivity ratio of the petrified wood and of the volcanic bombs compared to the pyroclastic material is found to be more than 3000 in laboratory measurements as shown in Table 1. Thus, it is expected that petrified trunks or sizeable volcanic bombs would appear as highly resistive elongated or rounded anomalies. Actually, these anticipated differences in the shapes of the various highly resistive bodies were subsequently used as a criterion for target characterization. Overall, the geophysical evaluations of potential petrified tree trunks suggest that they must correspond to linear (in case they are buried horizontally) or rounded (in case they are buried standing) anomalies showing both a negative magnetic anomaly as well as having high resistivity. GRADIOMETRY MEASUREMENTS An FM256 gradiometer has been used in order to measure the vertical gradient of the vertical component of the magnetic field. The study area has been divided into grids of 20 m × 20 m and 40 m × 40 m and was measured with a sampling interval of 0.5 m along parallel profiles 0.5 m apart. The GEOPLOT (v.3.0) program (Walker and Somers 2000) has been used to process and present the gradiometry data. Processing

TABLE 1 In situ and laboratory measurements of magnetic and electrical properties of materials Sample Volcanic bomb

Resistivity (Ohm.m)

Pyroclastic material Petrified trunk

>1 × 10

4

55

> 20 × 10

4

Magnetic susceptibility (SI) Laboratory measurements

209

1454 × 10

In situ measurements

-5

1370–1530 × 10-5

707 × 10-5

460–556 × 10-5

–1 × 10

-5


0

210


involved the application of a zero mean traverse, which sets the background mean of each traverse within a grid to zero. This helps to remove striping effects commonly occurring in gradiometer data as well as to eliminate discontinuities between adjacent grids. The interpolated data have been geo-referenced and imported to GIS mapping software in order to be examined in their real geographical location. In Fig. 3 the gradient of the magnetic field is presented in grey scale colours. White areas show positive gradients while black areas show negatives ones. Raw magnetic gradiometer data exhibit a very high variation range (–2300 nT to 2300 nT) due to the presence of the highly magnetized volcanic bombs. This is considered to be the major problem of the magnetic survey since petrified trunks are expected to show small negative anomalies and thus the existence of such strong anomalies often mask their signature. To reduce this

FIGURE 3 Georeferenced map of the vertical gradient of the vertical magnetic component.

FIGURE 4 Linear features (black lines) corresponding to faults derived from the interpretation of the magnetic gradiometry data.

effect caused by the presence of the highly magnetized volcanic bombs, the magnetic gradiometer data are depicted in Fig. 3 applying a cut-off threshold of ±100 nT, which has been chosen by considering the overall data amplitude range. Thus, important magnetic variations in that range can be further examined. Given the large area covered by the gradiometer prospecting (approximately 3 acres) a magnetic map at that scale does not depict details. However, it is very informative about the near-surface geological features. It does depict linear large-scale anomalies that correspond to the flow of the pyroclastic material within the pre-existing drainage system. In the western part of the surveyed area a zone of 20 m in width (parallelogram with dashed line in Fig. 3) can be identified that includes linear magnetic dipoles having a NNW up to a NW orientation.

FIGURE 5 Photograph showing a fracture zone revealed after excavation (location at Fig. 4).

FIGURE 6 Detail of the gradiometry data image where the path is drawn with a white dashed line.



FIGURE 7 Map of the processed gradiometry data.

FIGURE 8 Comparison of initial and reduced to the pole data.

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Anomalies having large amplitude appear to be, in most of the cases, bipolar and linear. Cases of large linear anomalies can be interpreted as being related to faults as they are expected to be filled with clay materials and oxides that typically produce such anomalies due to the higher magnetic susceptibility of iron hydroxides. The interpreted linear magnetic anomalies that are associated to possible fractured zones are depicted in Fig. 4. The cracks that are detected exhibit two main orientations: the first group has a strike of approximately 65o and the second of 120o. This result is in accordance with the general tectonic setting of the area as the former orientation refers to the ENE-WSW sinistral strike slip faulting system of Late Miocene and the latter to the NW-SE normal faulting system. Excavations followed in selected locations of linear anomalies revealed cracks having a width of 0.5 m. A representative photo of such a crack is shown in Fig. 5. Isolated high magnetic anomalies depicted in Fig. 3, having rounded or rectangular shapes, are correlated to the existence of big boulders of volcanic rocks. It is noticed that small rocks that were placed on both sides of a path to guide visitors of the area produced anomalies clear enough to allow the mapping of the path (shown in Fig. 6 (dashed white line in the middle of the path)). Negative magnetic anomalies in Fig. 6, with characteristic geometrical shapes, could be related to buried trunks as they have almost zero magnetic susceptibility. Magnetic data usually display plus-minus anomalies that are due to the dipolar nature of the magnetic sources and the interaction with the Earth’s magnetic field. These anomalies may be converted into positive anomalies by using the Hilbert transform to create the analytic signal, which was applied to the collected gradiometer data using the software developed by Young (2004). The described processing is considered to be equivalent to the reduction to the pole of magnetic data. The results are shown in Fig. 7 where highly magnetic bodies are presented in white colours. Of significant interest are linear anomalies of considerable length and relatively high amplitude, which are probably related to pyroclastic material flow. Since observation of the data distribution in the complete data set is quite difficult to be sufficiently clear, the area of the special interest in the eastern part of west Plaka has been focused on in Fig. 8. On the left figure the data are presented in their original bipolar form while in the right part the result after the application of Young’s algorithm is presented. The linear anomalies with NNW-SSE direction are more clearly shown in both figures. The difference between the two figures is that bipolar magnetic anomalies in the processed data are transformed to monopolar ones in the reduced to the pole data. White areas in the results, after the reduction to pole processing, show highly magnetic bodies that are related mainly to volcanic rocks. ELECTRICAL TOMOGRAPHY INVESTIGATIONS The observations made in magnetic gradient results have led to the decision of the application of the electrical resistivity tech-


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FIGURE 9 a) Petrified trunk in the seawater, b) measured ERT line and c) 2D section of the inverted resistivity data.

FIGURE 10 Location of the ERT grid.

nique. To evaluate the potential of the technique and to calibrate measurement settings a trial electrical tomography line has been conducted at sea level and over a petrified tree trunk that was partly situated at the bottom of the sea at a depth of less than 1 metre. The trunk was horizontal and was partly buried in the sea-bed while a part of it was visible (Fig. 9a). The tomography that was placed perpendicularly to the long axis of the trunk had 0.5 m inter-electrode spacing and 24 electrodes (total length of 11.5 m, Fig. 9b). Data were obtained with the pole-dipole array and results were processed using a 2D smoothness inversion algorithm (Tsourlos 1995) that produced an inverted image having an RMS error of 2.1% after 6 iterations. In the inverted image (Fig. 9c), the signature of the trunk is depicted as a rounded resistive anomaly centred at 7.5 m coinciding with the visible part of the trunk. The upper part of the trunk is not

distinguishable in the resistivity section and this can be due to seawater intrusion into cracks that are visible along the trunk. Considering the above trial electrical tomography section as well as the laboratory resistivity measurements suggests that the resistivity technique could be used to locate trunks. Therefore, a larger scale resistivity survey was conducted at the eastern part of the area of West Plaka. The area has been chosen due to the interesting number of anomalies obtained by the preceding gradiometry survey. A total of 41 independent parallel 2D lines with an inter-line distance of 2 metres (Fig. 10) have been measured using the pole-dipole array. Electrode spacing was set to 0.5 m and in every line 48 electrodes were used suggesting a total line length of 23.5 m. Thus, a resolution of 0.5 m in the direction of eastwest and 2 m in the direction south-north has been achieved.



Overall an area of 80 m (X) × 23.5 m (Y) was surveyed and the maximum investigation depth was estimated to 3 m (Z) given that maximum pole-dipole separation was 8a, 8(2a) and 8(3a), where a = 0.5 m, which is the inter-electrode spacing. The data were fully 3D inverted using a 3D smoothnessconstrained algorithm (Tsourlos and Ogilvy 1999) producing an rms error of 3.5% after five iterations. The inversion results are depicted in Fig. 11 in XY slices for different depths. In Fig. 12 the 3D volumetric image of the high resistivity values is depicted and in Fig. 13 the respective 3D image of only the low resistivity values is shown.

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A very low resistivity zone (white colour anomaly marked as (a) in Fig. 11) is dominating the central-south part of the resistivity image below the depth of 0.7 m having a northwestern orientation. The zone is also clearly depicted in the 3D volume image of Fig. 13. It is considered to be a zone filled with fine grained pyroclastic materials (clay, silt) and it coincides with the low magnetization zone found by the gradiometry method. This is depicted in Fig. 14 where the resistivity slice at the depth of 1 m is compared to the distribution of the magnetic gradient. The depicted linear anomaly is believed to be the signature of the drainage system, related to, at that time, the active tectonic FIGURE 11 XY resistivity depth slices produced by the 3D inversion of the parallel ERT lines.


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FIGURE 12 3D volumetric image of the high resistivity values. FIGURE 14 Petrified trunk discovered after the excavation at location B shown in Fig. 11.

high resistivity values suggested that there is also a possibility of containing petrified trunks. Subsequent excavation that took place in that selected area, due to the geophysical survey, revealed two petrified trunks buried at a depth of 0.7 ms. The photo given in Fig. 14 shows the excavation results. Further, throughout the northern part of the resistivity grid isolated high resistivity anomalies can be seen (some of them are marked as (c) in the 0.15 m slice of Fig. 11) and based solely on their resistivity signature can be correlated to either petrified trunks or volcanic bombs. However, the gradiometer survey map does not exhibit similarly isolated high magnetic anomalies in this area. Therefore, it could be assumed that most of these anomalies probably reflect the existence of buried trunks, a fact that has to be verified by excavation yet.

FIGURE 13 Resistivity slice (right) compared to the distribution of the magnetic gradient (left).

regime, which was then filled when the flow of pyroclastic material occurred. The south-west corner of the surveyed area is dominated by high resistivity values from top to 0.7 m (marked as (b) in the 0.15 m slice of Fig. 11). The anomaly is also clearly depicted in the 3D volume image of Fig. 12. This was initially interpreted as a conglomerate of pyroclastic material strongly cemented but the

CONCLUSIONS AND DISCUSSION The principal aim of this work was to detect petrified trunks at the Plaka Petrified Forest Park in the island of Lesvos. Sample laboratory measurements suggested the presence of significant contrasts between the magnetic and electrical properties of petrified trees and of the surrounding material. As a result the gradiometry and electrical resistivity tomography techniques were employed as the main geophysical survey tools. Processed geophysical survey results revealed much more complicated geophysical property images than expected. This is a reflection of the complicated survey environment consisting of petrified trunks buried in flows of pyroclastic material and mixed with volcanic bombs. Additionally, the localized cementation of the pyroclastic material as well as the existence of oxidization and fractures within the trunks renders interpretation even more difficult. Since, the forests were initially covered by pyroclastic material it was expected that buried trunks would show negative magnetic anomalies and high resistivity anomalies. In this case



the combined use of both geophysical techniques certainly enhanced the ability to produce more reliable interpretations. In several cases the specific geophysical signature of petrified trunks was identified and has been verified by subsequent excavation yet most of the potential buried trunk locations pinpointed by the geophysical survey still need to be verified. In the rest of the cases isolated highly magnetic bodies represented as local anomalies have been recognized as possible volcanic bombs. Both gradiometer and ERT surveys showed the existence of an old drainage system that is believed to have been filled by the pyroclastic material during its flow. This drainage system reflects the faulting system in the area. Application of the magnetic gradient method, in particular, could be widely used in such areas in order to enlighten researchers interested in the paleoenvironment and detect the pre-existed drainage system, especially as the technique is very rapid and accurate. Gradiometry results can be further investigated by the application of electrical tomography, which can give detailed depth information concerning the topographic relief of areas where petrified trees might be buried. Overall, the presented case study suggests that geophysical investigations in a petrified forest environment can provide valuable information not only about the existence of petrified trunks but also about the paleoenvironment. In such environments, there is scope for combined application of more than one geophysical technique aiming to reduce ambiguities in final interpretations. ACKNOWLEDGEMENTS Results presented in this paper were part of a research project that was financed by the Museum of Petrified Forest of Sigri. The authors would like to thank the editors and two anonymous reviewers for their constructive suggestions. We would especially like to thank Prof. O. Valassiadis and Dr S. Spassov for the laboratory measurements. Finally, we would like to thank Dr A. Giannopoulos for his useful suggestions concerning this paper.

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