Data processing chain for high spatial resolution magnetic survey: Application on the Neolithic site of le Pontet (Charente‐maritime, France)

Abstract

This article focusses on an original magnetic survey protocol and its data processing chain. The use of a high sensitive magnetometer combined with a motorized total station permits production of a magnetic map with a high spatial resolution and a high precision of magnetic and positional measurements. The data processing enhances the final magnetic map by the removal and restoring the outlier values, measurement errors and magnetic disturbances. The magnetic survey was undertaken on the Neolithic site of Le Pontet at Saint-Nazaire-sur-Charente (French Atlantic coast, between Bordeaux and La Rochelle). The site was chosen for geographical and archaeological characteristics such as its settlement period, the topography and the presence of different magnetic disturbances (proximity to roads and residential area). The data processing is mainly made on ungridded data and the gridding is the second to last step. The last step is to correct the relative height variation of the sensors between the prospecting lines (tracks) induced by the topography and operators. The use of a motorized total station allows accurate height determination of the sensors and an upward continuation is performed to correct the slight magnetic intensity variation induced by the relative height variation of the sensors. The results of the magnetic map bring out several causewayed enclosures, pits and postholes. This map will serve as a working basis for interdisciplinary studies and future archaeological studies of this site.

Magnetic field modelling and analysis of uncertainty in archaeological geophysics

“…Clearly, more sophisticated hardware would allow a reduction of acquisition errors. For example, a moving cart equipped with a sensor array will reduce significantly the positioning errors, while a position control hardware could even diminish them to a negligible value (Bruniaux et al, ). Regarding the uncertainty related to external field variations, a synchronized base station can be used to measure the diurnal changes.…”

“…Figure shows the three independent components of the maximum positioning error in the case of regular mapped surveys (Figure (a)), not assisted by specific position control hardware (e.g. Bruniaux, Mathé, Lévêque, Camus, & Ard, ). The component ε x results from a curved shape of the rope (e.g.…”

Magnetic modelling and error assessment in archaeological geophysics: The case study of Urbs Salvia, central Italy

“…ε(x,y)=ξG(x,y)+ε 0 The total uncertainty can be reduced considerably using specific survey strategies and additional hardware. For example, the positioning errors can be reduced to a negligible value using a motorized total station that follows the sensor [Bruniaux et al, 2018], while a base station for the acquisition of diurnal drift variations can be used to eliminate the component of background uncertainty associated with the variance of the estimated diurnal drift curve. However, some survey strategies are not necessarily appropriate and can introduce systematic errors in the resulting magnetic anomaly field.…”

“…To evaluate the maximum displacement of the sensor around the theoretical position we need to consider the three components of sensor displacement around the recorded position during survey line walks ( Figure 2), which depend on the specific terrain and environmental conditions as well as on the available equipment. In absence of a positioning control system [e.g., Bruniaux et al, 2018] the transversal component, e x , is generally of the order of 10 cm and is associated with a curved shape of the rope (e.g., during windy days) and/or small oscillations of the sensor about the rope. The longitudinal, e y , component results from various causes: a) magnetometer readings at fiducial marker points are not perfectly synchronized with pressing of marker commands; b) operator parallax displacements when pressing the fiducial command button, which can be systematically in advance or late; c) variations of velocity during the operator walk along a survey line, as well as from longitudinal oscillations of the sensor.…”

Porsuk - Zeyve Höyük : Rapport préliminaire des campagnes 2018 et 2019

“…Les données magnétiques acquises sont traitées selon un protocole établi par nos soins (Bruniaux et al, 2017). Les données sont ensuite interpolées selon une maille de 0,1 x 0,1 m par la méthode du krigeage.…”