Multi‐offset ground penetrating radar methods to image buried foundations of a medieval town wall, Great Yarmouth, UK

Abstract

In conventional common-offset (CO) usage, ground-penetratingradar (GPR) methodsmay be problematic where a target is hosted within a structurally complex subsurface, or where coherent noise energy obscures the response to that target. This often occurs when imaging archaeology in urban environments, where reflected and diffracted air-wave energy can be pervasive in the presence of cars, walls and other nearby features.To overcome such issues we show a multi-offset (MO) GPR approach to urban surveying. Multi-offset velocity analysis is used as a filter by which ground-propagating signal energy is separated from airborne arrivals, with the latter suppressed on application of normal moveout corrections and stacking. Pre-stack migration is applied and yields considerable improvement to target resolution compared with post-stack routines. The MO-derived model of GPR velocity can also show the subsurface distribution of archaeological layering. Methods are demonstrated for a 25-fold MO GPR profile acquired, using unshielded antennas of 200 MHz centre-frequency, over foundations of the medieval town wall of Great Yarmouth, UK. The buried foundations represent a vertical discontinuity in the ground, which truncates both natural geological and archaeologically prospective layers.In addition to this complexgeometry, whichitself hinders GPRimaging, coherent air-wave noise is problematic because there are numerous above-surface features (e.g. parked cars, abovesurface walls) in the immediate vicinity of the profile. The improvement offered by MO techniques is benchmarked against a conventional CO routine and compared with co-located borehole records for a comprehensive subsurface interpretation.Target foundations appear ca.1.5 m beneath the ground surface, and GPR data support artefactual evidence of the construction of a sixteenth century rampart on their interior side. Despite the increased survey effort, werecommend MOmethods at anylocationwhere CO data are dominatedwith coherent noise energy, andwhere complex subsurface geometries degrade the output from standard migration algorithms.

Influences on the resolution of GPR velocity analyses and a Monte Carlo simulation for establishing velocity precision

“…; Booth et al . ). For improving velocity accuracy, algorithms are available for correcting, e.g., for dipping reflectors (Levin ; Yilmaz and Claerbout ; Bradford et al .…”

“…; Bradford and Harper ; Berard and Maillol ; Booth et al . , ) and requires various systematic corrections before inferring physical quantities (e.g., Schneider ; Sheriff and Geldart ; Yilmaz ; Bradford et al . ; Booth et al .…”

Spectral velocity analysis for the determination of ground‐wave velocities and their uncertainties in multi‐offset GPR data

“…2007; Turesson ; Booth et al . ) and direct ground‐wave (DGW) velocity surveying (e.g., Huisman et al . ; Huisman et al .…”

“…In addition to finding an optimum ground-wave velocity from multi-offset data gathers, our method can be used to estimate the associated uncertainties. After introducing the methodological background, we use simple synthetic data to illustrate the principles of our method and investigate uncertainties in ground-wave velocity estimates, including their impact on derived values of soil water content for different surveying and Knoll 2005), multi-offset reflection profiling (e.g., Greaves et al 1996;Brosten et al 2007;Turesson 2007;Booth et al 2010) and direct ground-wave (DGW) velocity surveying (e.g., Huisman et al 2001;Huisman et al 2003b;Galagedara et al 2005a) can be used. DGW velocity surveying employing either constant or multi-offset acquisition techniques focus on integrative velocity estimation within the upper half metre of soil.…”

Ground‐penetrating radar assessment of the medieval arch bridge of San Antón, Galicia, Spain

“…The GPR method is quite commonly applied to historical structures (Ramírez‐Blanco et al ., 2008; Booth et al ., 2010; Udphuay et al ., 2010). However, although it is known that GPR is effective for bridge evaluation, there are, to date, few published studies on the testing of historical structures, especially the evaluation of masonry arch bridges, with some notable exceptions (Colla et al ., 1997; Flint et al ., 1999; Pérez‐Gracia, 2001; Fernandes, 2006; Arias et al ., 2007; Diamanti et al ., 2008; Orbán and Gutermann, 2009; Lubowiecka et al ., 2009; Solla et al ., 2010).…”