Detection of resistive features using towed slingram electromagnetic induction instruments.
код для вставкиСкачатьArchaeological Prospection Archaeol. Prospect. 16, 103–109 (2009) Published online 24 March 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/arp.350 Detection of Resistive Features using Towed Slingram Electromagnetic Induction Instruments JULIEN THIESSON1*, MICHEL DABAS2 AND SE¤BASTIEN FLAGEUL1 1 2 ABSTRACT UMR 7619 Sisyphe, UPMC, Case 105 4 place Jusieu 75252 Paris cedex 05, France Geocarta, 16 rue du sentier 75002 Paris, France Slingram frequency domain electromagnetic (FDEM) instruments allow simultaneous measurement of both magnetic susceptibility and electrical conductivity, which should justify their widespread use in archaeological surveying. However, this is not the case and their application remains quite limited due to: (i) a lack of knowledge about the role of coil orientation and spacing in terms of the detection abilities for archaeological features (especially for resistive bodies); and (ii) a lack of instrumentation specifically designed for shallow targets.We present here a test of a new version of the CS60 instrument (VCP coil configuration and 0.6 m intercoil spacing) for shallow depth resistive feature detection. This experiment was undertaken on the Roman site of Vieil-Evreux where a complete series of control resistivity and radar data was obtained. Detection of buried Roman walls was successful, in accordance with what can be expected from three-dimensional modelling.This confirms that the application of this type of instrument in archaeological surveys merits to be extended significantly. Copyright # 2009 John Wiley & Sons, Ltd. Key words: Slingramelectromagneticinstrument; resistivitymapping; walldetection; susceptibility mapping Introduction Among the different solutions that can be adopted to measure and to map electrical resistivity using electromagnetic induction (EMI) methods, Slingram instruments consist of a transmitter coil and a receiver coil separated by a distance, L, the depth of investigation being of the same order as L. In archaeological surveying, as in other areas of exploration geophysics, EMI was considered in order to bypass the limitation of requiring sufficiently good galvanic contacts for the d.c. resistivity method. The first field experiments * Correspondence to: J. Thiesson, UMR 7619 Sisyphe, UPMC, Case 105 4 place Jusieu 75252 Paris cedex 05, France. E-mail: julien.thiesson@upmc.fr Copyright # 2009 John Wiley & Sons, Ltd. showed that the observed responses depend mainly on magnetic susceptibility (Musson, 1968). A series of field trials coupled with an improvement of the theoretical approach (Tite and Mullins, 1969, 1970) allowed the recognition that both electrical conductivity (s) and magnetic susceptibility (kph) could be measured simultaneously using frequency domain instruments. As s corresponds to a 908 out-of-phase response and kph to an in-phase response, it has been possible to propose instrument characteristics in accordance with archaeological surveying requirements (Tabbagh, 1974). As this type of instrument is also able to detect metallic objects (Tabbagh and Verron, 1983), they constitute very flexible tools for geophysical surveying in archaeology (Tabbagh, 1986a). However, in spite of the existence of both prototypes and commerReceived 4 February 2009 Accepted 12 February 2009 104 cial instruments, the most widespread being the EM38 (Geonics Ltd), their use has remained limited in archaeology because of: (i) misunderstanding of the spatial response of such instruments and consequently erroneous data interpretation; (ii) uncertainties about the knowledge of the depth of investigation (Dabas and Tabbagh, 2003), together with feature detection abilities; (iii) difficulties to compensate for the thermal drift of in-phase responses and mechanical instabilities. To overcome these issues, it is useful to first recall the physical constraints and assumptions that govern the interpretation of the Slingram results. To measure s using EMI instruments (or its inverse electrical resistivity, r, both representing at the macroscopic scale electric charge displacement) without mixing with the dielectric permittivity e (that represents at the macroscopic scale the electrical polarizability of a material), one must respect first the condition s >> ev (where v ¼ 2pf is the angular frequency, f being the frequency). Given the range of values of soil conductivity and permittivity, this implies that f < 300 kHz. As the intercoil spacing, L, must stay in the range of both the depth of investigation and the lateral resolution, it cannot be very different from 1 m in the majority of archaeological applications. Therefore, the low induction number (LIN) condition, L2smv << 1 (where m is the magnetic permeability), is always respected in archaeological prospection. Under this condition the s response, the secondary field generated by eddy currents, is proportional to the induction number and 908 out of phase with the primary field. Consequently this response does not overshadow the in-phase component of the secondary field generated by magnetic susceptibility, as is the case for deeper target investigations with larger intercoil spacing, for example in mining geophysics. The simultaneous measurement of both properties by taking into account the phase of the responses is thus possible (and far better than the measurement of the modulus of the secondary field, which Copyright # 2009 John Wiley & Sons, Ltd. J. Thiesson et al. would be difficult to interpret due to the cumulative effect of the two independent properties). Another consequence of the LIN condition is that the depth of investigation and the lateral resolution depend both on L and on coil orientation but not on frequency (Frischknecht et al., 1991). The choice of the coil orientation must be based on the objectives of archaeological surveying, which is first to characterize archaeological features that are three-dimensional and may correspond to both conductivity and/or susceptibility contrasts. According to these objectives, the definition of the best coil orientation has been studied by forward numerical modelling (Tabbagh, 1986b). This study concluded that the best orientation is the perpendicular one, PERP, followed by the vertical coplanar (horizontal dipoles) one, VCP. No later field trial refuted this conclusion. These results contradict the driving choice of most instrument manufacturers that tend to favour depth of investigation for soil conductivity layering, thus the HCP (horizontal coplanar, vertical dipoles) configuration (Geonics Ltd, 1996). In the present paper, to contribute to a better understanding of Slingram instrument abilities we present a field trial of resistive targets detection. We chose the walls of a Roman building, buried in silt on a plateau, a conductive context. This case corresponds to an unfavourable test situation because EMI methods are more sensitive to conductive than resistive targets. Test site, control data and apparatus The Roman city of Gisacum is located on a plateau in the sedimentary Paris Basin at Vieil– Evreux, 7 km east from the modern city of Evreux (Eure, France). It is formed by a large polygonal worship place of more than 100 ha, which surrounds a series of public religious buildings including temples, thermae and fana (Dabas et al., 2005). It has been surveyed using electrical and magnetic methods (Aubry, 2003). Now, as it constitutes an archaeological reserve, tests of new instruments and surveying techniques can be Archaeol. Prospect. 16, 103–109 (2009) DOI: 10.1002/arp Detection of Resistive Features Using Towed Slingram easily performed over grassland plots where tillage has stopped. The test area, 40 by 40 m2, covers the central part of the west fanum that comprises in its last state (second century AD) a 13-m-sided square stony wall cella (square central room with sacred statue) surrounded by a rectangular 72 m T 36 m outer wall. The resistivity mapping was achieved 105 using the ARP1 multidepth instrument. It has a ‘vol de canards’ array configuration with three different receiver dipoles corresponding to three different depths of investigation of about 0.5 m, 1 m and 2 m (Panissod et al., 1997). Figure 1a–c, shows the apparent resistivity maps corresponding to these depths of investigation. The cella wall generates a marked anomaly that can be Figure 1. ARP1 (seewww.geocarta.net) apparent resistivitymapsonthe cellaofthewestern fanumat Vieil-Evreux (Eure,France): (a) 0.5 m dipole spacing, (b) 1m dipole spacing and (c) 2 m dipole spacing. Copyright # 2009 John Wiley & Sons, Ltd. Archaeol. Prospect. 16, 103–109 (2009) DOI: 10.1002/arp J. Thiesson et al. 106 Figure 2. Ground-penetrating radar time-slice of the reflected amplitude at 4 ns (one-way travel time) over the fanum west cella (Radar Pulse Ekko1000 with a 225 MHz central frequency antenna and a 0.2 m horizontal distance step). Figure 3. The CS60 andits cart in use.This figure is available in colour online at www.interscience.wiley.com/journal/arp must take care to pull with a constant speed along a profile. interpreted by the presence of a wall and its foundation, having its top just below the topsoil and a significant extent in depth until about 0.8 m. A central smaller resistive feature can be observed inside the cella, another shallow small resistive feature appears 20 m to the east, and part of the outer wall can be observed in the northern part of the test area. The cella internal structure was also surveyed by GPR using a 225 MHz antenna, the 0.3 m depth amplitude slice is presented on Figure 2; it confirms the existence of the thin internal central feature. The CS60 instrument has a 27 960 Hz frequency, a 0.6 m intercoil spacing and is normally used in VCP (horizontal dipoles) coil configuration; when necessary it can be used in HCP (vertical dipoles) coil configuration to increase the investigation ability for deep layers. Its characteristics, coil orientation and spacing, were originally designed for irrigated salted soil studies (Job et al., 1995) and not for archaeology (for which PERP configuration would be better). Measurements can be achieved either point by point or by continuously pulling the apparatus on a cart (Figure 3). In the first case, the coil axes are 0.05 m above the ground surface, and 0.07 m in the second case. The acquired data are transferred via a microcontroller to a PC at a constant time step that corresponds to a distance step of approximately 0.02 m. Thus, the operator Copyright # 2009 John Wiley & Sons, Ltd. Modelling control A wall corresponding to those remains existing in the west fanum can be modelled by a twodimensional feature of 300 V.m resistivity, 10 105 (SI) susceptibility, 0.6 m thickness and 0.45 m width embedded in a 25 V.m and 20 105 (SI) layer and covered by a 50 V.m, 40 105 (SI) and 0.2 m overburden topsoil. We compared the different responses of three coil configurations (PERP, HCP and VCP) for different intercoil spacings from 0.4 m to 1 m (Figure 4). Generally speaking, it can be observed that on the one hand, the wall is clearly detected for any spacing and coil configuration, on the other hand, that shorter spacings produce stronger anomalies. This can by explained by the stronger relative contribution of the wall response to the total response, compared with the surrounding medium. Then, discussing the shapes of the anomalies, only the VCP configuration gives a symmetric and simple shape. In fact, the PERP configuration is clearly asymmetric whereas the HCP tends to suggest two resistive bodies when there is only one. According to these results, the VCP configurations have to be preferred for towed survey. For all coil configurations, the susceptibility contrast is too weak to create any observable anomaly in the apparent susceptibility. Archaeol. Prospect. 16, 103–109 (2009) DOI: 10.1002/arp Detection of Resistive Features Using Towed Slingram 107 Figure 4. Calculated in-line apparent resistivity variation above a two-dimensional wall measured with a Slingram: (a) PERP, (b) HCP an (c) VCP configured instrument for different values of the intercoil spacing from 0.4 m to1m. Copyright # 2009 John Wiley & Sons, Ltd. Archaeol. Prospect. 16, 103–109 (2009) DOI: 10.1002/arp J. Thiesson et al. 108 appears, this is also the case for the thin resistive internal feature. The small feature to the east and the outer wall of the fanum are less marked than on d.c. ra maps but recognizable. The noise that can be observed results partly from the errors in restored positions. In fact, a true distance measurement by coding wheel or by Doppler radar would be better than a regular time step. The presence of many small metallic (mainly iron resulting from World War II bombing) pieces in the topsoil generates outlier values that exist on both the ra and kpha maps and have been observed previously in the magnetic field map (Aubry, 2003). Figure 5. CS60 apparent magnetic susceptibility map of the test area. Field Results The data have been acquired by pulling the instrument on N–S profiles (1 m apart) following a zigzag route. The kpha map, Figure 5, shows that the cella corresponds to a slightly less magnetic area, suggesting the existence of a floor. By comparison the zone just north of the cella appears more magnetic. On the ra map (Figure 6) the range of values is more limited than in the d.c. ra prospecting but the wall of the cella clearly Figure 6. CS60 apparent electrical resistivity map of the test area. Copyright # 2009 John Wiley & Sons, Ltd. Conclusions In archaeological surveying, frequency domain Slingram EMI instruments are recognized as light and flexible tools able to map both kph and s, but commonly not considered for resistive feature mapping. The test presented here indicates that, without considering the abilities of multidepth electrical prospecting, they can succeed in resistive feature detection if suitable coil orientation and spacing, in accordance with the expected depth of the features, are respected. The advantages and limitations of these instruments that can (and must) be assessed using three-dimensional modelling are different, and better for archaeological applications, from those expected from using only one-dimensional modelling. The present example is one of the first published results where a Slingram instrument is used for resistive archaeological feature mapping and other experiments must be undertaken to improve these conclusions. However, commercially available Slingram instruments exist, some of them can be used in PERP or VCP coil configurations. They are light, of low power consumption and can be transported easily everywhere. The fact that, in addition to their ability to simultaneously measure magnetic susceptibility and electrical conductivity, resistive feature detection is possible, leads us to consider a future significant increase of their application in archaeological surveying. Archaeol. Prospect. 16, 103–109 (2009) DOI: 10.1002/arp Detection of Resistive Features Using Towed Slingram Acknowledgements This experiment was performed within the framework of the Agence Nationale de la Recherche (ANR), number ANR-06-BLAN-0049 Celtecophys contract that aims to improve the capabilities of Slingram instruments in order to design, build and test a new combined magnetic/ electromagnetic multisensor instrument for extensive in-field surveys. References Aubry L. 2003. Acquisition traitement et restitution des données d’une reconnaissance archéologique: la ville gallo-romaine du Viel-Evreux. Thèse Université Pierre et Marie Curie, Paris, 259 pp. [in French] Dabas M, Tabbagh A. 2003. A comparison of EMI and DC methods used in soil mapping – theoretical considerations for precision agriculture. In Precision Agriculture, Stafford J, Werner A (eds). Academic Publisher: Wageningen; 12–129. Dabas M, Guyart L, Lepert T. 2005. Gisacum revisité. 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