Rayleigh‐wave Resonance Analysis: a Methodological Test on a Viking Age Pit House

Self Cite

Structures in Iceland were traditionally built of turf, earth and, to a lesser extent, stone. As turf was the primary construction material, the contrast in geophysical parameters between building ruins and surrounding soil is expected to be low. To investigate the extent to which the remains of turf buildings can be detected by geophysical measurements, we applied several geophysical techniques to a known turf ruin in southwestern Iceland. The methods used were magnetics, ground-penetrating radar (GPR), electromagnetic induction (EMI), electrical resistivity tomography (ERT) and seismic Rayleigh-wave resonance mapping (RRM). Magnetics identified an accumulation of stones inside and beside the ruin. The in-phase component of the EMI measurements, which can be related to magnetic susceptibility, showed the same pattern. A very precise image of the stones lining the inside of the former turf walls was generated by GPR. In contrast, EMI conductivity and ERT imaged the actual turf in the walls. Turf walls have lower electrical conductivity compared with the surrounding soil, probably as a result of different porosities. The mapping of Rayleigh wave resonance clearly revealed the outline of the ruin, as indicated by weaker amplitudes compared with the surrounding soil. Overall the results indicate that geophysical methods can be used for subsurface mapping of Icelandic turf structures and that the combined application of the methods maximizes this potential.

Section: Discussionmentioning

confidence: 99%

“…A detailed description can be found in Wilken et al . (). The basic concept is to process single vertical component recordings in order to determine Rayleigh wave resonance frequencies.…”

Section: Methodsmentioning

confidence: 97%

“…The processing follows the procedure described in Wilken et al . (). The data were first checked for unsuitable traces, which when found were muted.…”

Section: Methodsmentioning

confidence: 97%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

On the Ability of Geophysical Methods to Image Medieval Turf Buildings in Iceland

Wunderlich

Wilken

Andersen

et al. 2015

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The Passive Seismic Technique ‘HVSR’ as a Reconnaissance Tool for Mapping Paleo‐soils: The Case of the Pilastri Archaeological Site, Northern Italy

Zeid

Corradini

Bignardi

et al. 2017

Horizontal-to-vertical spectral ratio (HVSR) is a widely used geophysical technique in seismic microzonation studies. It is based on a specific analysis of seismic ambient noise. The method allows to obtain the frequency and amplitude of the resonance peaks of a layered earth with increasing acoustic impedance contrasts. The peaks can be interpreted to obtain an estimation of depth(s) of the impedance contrast horizon(s). Based on the assumption that long-term human trampling results in sediment's stiffening, which increased both density and velocity of seismic shear waves, the HVSR method was applied to investigate the shallow subsurface of an important, Middle Bronze Age, archaeological site called 'Pilastri Terramara' discovered at the end of the last century. Following recent excavations, archaeologists supposed that the settlement could extend outside the initially hypothesized borders, and decided to involve geophysicists to verify the truthiness of this new hypothesis and consequently to map the possible spatial extent of the paleo-surfaces frequented by ancient occupants. The purpose of the geophysical investigation was then to detect and possibly to map one or more anthropogenic paleo-surfaces over a relatively large area (about 12000 m 2 ). Unfortunately, direct evidences showed that the paleo-surfaces were embedded in clayey sediments and laying at depths ranging between 50 and 170 cm below ground level. Furthermore, the area to be investigated is occupied by a farm with glasshouses and other buildings. These obstacles constituted a real challenge that hindered the utilization of the most commonly used geophysical methods in archaeology, i.e. ground penetration radar (GPR), magnetometry and electrical resistivity tomography. For these reasons, we decided to use the HVSR method as a reconnaissance exploration tool, to confirm or rule out the presence of such paleo-surfaces. Spectral peaks related to acoustic horizons provided evidences about their presence and allowed to estimate their depths as was later confirmed by a new excavation.

What over 100 drillings tell us: a new method for determining the Koenigsberger ratio of soils from magnetic mapping and susceptibility logging

Pickartz

Rabbel

Rassmann

et al. 2020

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We investigate the relative fractions of remanent and induced magnetization of the fillings of neolithic long pits in order to develop remanent magnetization as an additional parameter for the archaeological interpretation of magnetic maps. We determine the Koenigsberger ratiothe ratio between induced and remanent magnetization intensitiesfor key targets by combining magnetic mapping with downhole measurements of susceptibility, numerical modelling, and inversion computations. The susceptibility data were acquired in drill holes along profiles crossing the targets identified by magnetic mapping. The targets of this exemplary study are house-accompanying pits at the Linearbandkeramik site Vráble 'Farské'. For this purpose, we conducted auger drillings with a point distance of 25 cm and measured the susceptibility with a downhole susceptometer. The resulting two-dimensional susceptibility distributions were used to calculate synthetic magnetic anomalies corresponding to the case of solely induced magnetization. The comparison to the observed magnetic data showed a considerable discrepancy that can only be explained with remanent magnetization. To determine the Koenigsberger ratio we developed a new interpretation approach, using parts of the measured susceptibility distribution as a basis function. The free parameters of this numerical problem are determined by non-linear inversion. We applied the novel approach to six exemplary profiles and found Koenigsberger ratios between 1.6 and 10.5 with the majority of the values being smaller than 4. These values apply to soil volumes with susceptibility values larger than 27-160 × 10-5 SI. Laboratory measurements on soil samples were used to examine the possible causes of the observed magnetization. The analyses suggest that the increase in susceptibility and remanent magnetization in the pits is caused by an increase of the population of magnetotactic bacteria and deposition of magnetized material, followed by the alignment of the ferrimagnetic iron compounds in the waterlogged environment of the pits and accumulation of viscous remanent magnetization.