Geophysical Survey in Archaeological Field ... - English Heritage

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Electrical imaging has been employed with some success to characterise archaeological anomalies and three-dimensional surveys can be constructed by measuring a sequence of parallel sections and stacking the results (Collier et al 2003). However, the technique is slow compared to area survey methods, as a large number of electrodes need to be prepositioned for each section. Electrical sections are therefore usually employed to improve the characterisation of anomalies rather than for their initial discovery. For this reason they have been little used in UK archaeological evaluation surveys and should only be considered when there is an agreed need to further characterise potential archaeological anomalies after initial discovery by area survey techniques. Nevertheless, they are increasingly employed in geomorphological studies to provide details of buried landscapes associated with archaeological activity. In this application, large geological-scale sections are measured at strategically targeted locations, typically using more widely separated electrodes than for direct analysis of archaeological-scale anomalies (Bates and Bates 2000; Bates et al 2007). Where electrical sections are employed, an inter-electrode spacing suited to the scale and depth of the expected anomalies should be chosen.This might be as narrow as 0.5–1m when imaging archaeological features, but may be much wider (2m, 5m or more) for geomorphological studies. Different electrode configurations (Wenner, dipole-dipole, etc) have different response characteristics (Loke 2004), so the configuration used and the reasons for its selection should be noted in the survey report. Care should also be taken to minimise the contact resistances of each electrode in the array (typically to
A wide range of site surfaces may be considered for GPR survey, including concrete, tarmac and even fresh water, although the technique is limited by the attenuation of the signal in conductive media. In practice, this will largely be determined by the concentration of clay and the moisture content of the soil at the site. Highly conductive media, such as metal objects or salt water will prove largely opaque to the GPR signal. Strong reflectors in the nearsurface will also reduce the energy transmitted to immediately underlying targets and this may include the local water table (or other nearsurface interface). Ferrous reinforcement bars in concrete are also readily imaged by GPR but their presence will not, necessarily, preclude the identification of underlying reflectors. For normal ground-coupled antenna, good physical contact with the site surface is necessary to ensure adequate coupling of the radar energy with the soil. As far as possible, vegetation and any other surface obstructions should be removed from the site prior to the survey. High-frequency, air-launched horn antennas are designed to be operated from above the ground surface for civil engineering applications (eg road deck investigations), but do not have sufficient depth penetration for archaeological surveys. Air-launched antenna may prove useful for surveying delicate architectural features (eg plaster mouldings, wall paintings or mosaic pavements) when it is desirable to have no physical contact between the instrument and the surface under investigation. Many site-specific variables must be considered when using GPR, but in general it will respond to a wide range of archaeological features (Table 7), and is often successful over sites where earth resistance survey has proved fruitful (eg presence of masonry walls, void spaces, etc). GPR is sensitive to the interface between differing materials and some target features produce highly distinctive GPR anomalies (eg hyperbolic responses from point reflectors). However, the identification of complex material properties, for example distinguishing either human or animal bone from the surrounding substrate, is considered to be beyond the capabilities of the technique under typical field conditions. While the use of GPR for detailed large area surveys (>1ha) has increased it is often applied as a complementary technique, following the acquisition of magnetic or earth resistance data, to target specific archaeological anomalies identified over a more limited area of the site. Care must be taken to ensure that GPR survey is appropriate to a site, particularly if it is the only technique to be applied.The proximity to sources of radio-frequency (RF) interference that may affect the data quality – such as mobile telephone transmitter base stations or the radio modem of an on-site differential GPS system – should be considered. 1.4.2 Instrumentation GPR systems utilise an electromagnetic source, generated by a transmitter antenna on the ground surface, and record the amplitude and time delay of any secondary reflections from buried structures.These secondary reflections are produced when the GPR pulse is incident upon any media with contrasting conductivity (σ) or (dielectric) permittivity (ε), or both, to the medium above.The magnetic permeability (μ) of the sub-surface will also influence the propagation of a radar wave, but for most practical considerations it may be ignored. In general, the GPR response will be largely determined by the local variation of water content in the sub-surface.The maximum depth of penetration for a GPR is governed by a combination of signal scattering and attenuation within the subsurface, through the dissipation of radio-frequency energy as eddy currents within conductive media. The majority of archaeological materials and soils are semi-transparent to the GPR signal and this is able to penetrate to some depth, creating a series of secondary reflections from buried objects distinguished by an increasing time delay.The resulting time-amplitude data is displayed as a two-dimensional profile with the X-axis indicating the horizontal location of the antenna on the ground surface and the Y-axis representing the increasing time delay (depth) from the initial impulse. While radar waves propagate more slowly in the ground than in the air, velocities are still extremely high and the receiver electronics must be capable of recording events separated by less than a nanosecond (10 -9 s).The recorded delay represents the total time required for an incident pulse to travel from the transmitter to the target and then for the reflection to return to the receiver. This dual pathway is known as a two-way travel time and can be converted to provide the approximate depth of buried targets where an accurate estimate of the sub-surface velocity can be made. GPR systems consist of an antenna unit housing the transmitter and receiver, an electronic control unit, a data console and a power supply. Different configurations of these components are offered by the major manufacturers and each may have advantages in particular survey conditions (Table 8; Fig 13). Antenna units The GPR impulse covers a comparatively broad band of frequencies, usually defined by a nominal ‘centre frequency’. Precise depth estimation from GPR surveys is often difficult to achieve, yet is a critical process for the successful presentation of results. Unprocessed GPR data, expressed in terms of the time delay of returned reflections, can always be recalibrated in the light of additional information (eg trial excavation results) to present a more accurate physical depth estimate for other unexcavated targets. Fig 13 (above) Annotated photograph of a Sensors and Software Pulse Ekko 1000 GPR system.The sledge accommodates either a 900 MHz, 450 MHz or 225MHz centre frequency antenna and maintains good coupling with the ground surface through its flexible plastic skid plate. 29
Page 1 and 2: 2008 Geophysical Survey in Archaeol
Page 3 and 4: Contents Preface to the Second Edit
Page 5 and 6: description linked to lucid and obj
Page 7 and 8: with other features, sites or the w
Page 9 and 10: Further information: anything furth
Page 11 and 12: Most HERs include information about
Page 13 and 14: Part III Guide to Choice of Methods
Page 15 and 16: deterrent to successful geophysical
Page 17 and 18: anomalies within peat, and there ar
Page 19 and 20: Part IV Practitioner’s Guide to G
Page 21 and 22: It can identify thermoremanently ma
Page 23 and 24: is achieved when the traverse separ
Page 25 and 26: where soil and geological condition
Page 27: factors into account and, preferabl
Page 31 and 32: unit, consisting of a separate tran
Page 33 and 34: a) b) c) d) (within ±0.5m) the ele
Page 35 and 36: that they utilise. While such a bro
Page 37 and 38: measurements have been used to succ
Page 39 and 40: This system is based on a cart-moun
Page 41 and 42: the subject of methodological resea
Page 43 and 44: positive peak of such small dipolar
Page 45 and 46: a) b) c) possible to model the geom
Page 47 and 48: that outlines the important archaeo
Page 49 and 50: A different type of three-dimension
Page 51 and 52: Conyers, L B 2004 Ground Penetratin
Page 53 and 54: Powlesland, D J, Lyall, J and Donog
Page 55 and 56: magnetometer scanning the informal
Page 57 and 58: Appendix II Contacts Advice on geop
Page 59 and 60: Appendix IV List of consultees Orga

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