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6.5.1 Physical principles

Ground penetrating radar (GPR) is based on similar principles to those used in aviation radar (RAdio Detection And Ranging). Rather than propagating an electromagnetic waveform into the air and bouncing it off objects to detect them, it propagates a waveform into the ground and measures (usually) the two-way travel time of pulses of the signal as they are returned from buried objects and interfaces.

An antenna is used, usually with a central frequency somewhere between 100MHz and 1.5GHz, to send very rapid pulses of electromagnetic energy, which are

transmitted as waves into the ground. These are reflected back to a receiving antenna at the surface when there are significant changes in the relative dielectric permittivity (RDP) or magnetic permittivity of the subsurface, often as positive and negative amplitude wavelets. The time each pulse takes to be reflected back is used to make assumptions about the depth of the reflecting material. A composite is generated from all the wavelets created over the many changes in the soil over depth at a given location is called a reflection profile.

In practice, the pulses of the antenna happen too fast to be digitally recorded, so a series of samples is used to build up a reflection profile (usually 512 or more). To collect surveys, a transmitting antenna is dragged along a transect, followed at a fixed distance (usually) by a receiving antenna that measures the returned wavelets and the two-way travel time, building up stacks of thousands of reflection profiles along a traverse. When digitally recorded and recombined, these can be imaged as a radar profile; a two dimensional slice through the ground along the survey transect, showing the reflecting layers and objects. Reflections are not straight forward to interpret because the energy leaves the antenna in a cone, so anomalies ahead of the antenna will produce a response before the antenna is directly over them, and vice versa. Over strongly reflecting targets, this produces parabola and the shape of the parabolas in a survey can be used to estimate the radar velocity in the sediments.

These 2-dimensional radargrams can be further combined (if properly georeferenced) into a three dimensional data set to produce plan view images of anomalies and

changes in the amplitudes of the response- an indicator in the ‘strength’ of the reflecting anomaly (Clark 1996, 118-9; Gaffney & Gater 2003, 74-6; Conyers 2004, 23-6).

There are a lot of factors that influence the choice of antenna and acquisition

parameters, such as the expected spatial extent of the anomalies (in three dimensions) vs. the transect spacing and wavelength (which has a complex relationship with the antenna frequency and the RDP of the material), the known or suspected

soil/sediment properties, and the depth of burial of the targets. In practice, this means GPR is often perceived as a technique that requires a lot of experience or technical knowledge to properly employ. This, combined with perceived ‘underperformance’

given the complex factors affecting how the radar signal will behave in the ground, have meant the technique is less often employed in archaeology, but the situation seems to gradually be changing. In the 1990s Clark (1996, 118) stated that because of problems caused by the wetness of soils, GPR had yet to see many applications in British archaeology. In 2003 Gaffney & Gater (48) stated that it was increasingly used on urban sites in the UK, not because it worked especially well, but because it worked better than the alternatives. They also noted that it was increasingly being used on greenfield sites, but the soil composition on UK sites in general remained a problem.

By 2008 GPR was considered one of the more ‘routine’ techniques employed here (English Heritage 2008); perceptions of the limits on operating environments for this technique have changed as the complexities of RDP vs. radar velocity and attenuation have been worked out.

As discussed above, GPR has a relatively long history of use in peatland

environments outside archaeology, mainly to map and quantify peat deposits and for engineering assessments. It has also had some limited successes in locating

archaeological remains in lowland peat.

6.5.2 Detection capabilities and limits

Despite having a much wider range of applicable environments than was the

perception when Clark was writing there are still limitations to what can be detected with GPR.

Firstly, the target must have sufficiently contrasting RDP and a sharp enough

interface to be detected: gradual changes in RDP or sudden but slight changes will not be detected. RDP is defined as ‘the ability of a substance to store and allow the passage of electromagnetic energy when a field is applied’ (Gaffney & Gater 2003, 50).

Secondly, the signal must be able to propagate to and return from the depth of the target; in an environment where the signal rapidly attenuates it may not be possible to get a return from the required depth, despite using a low frequency antenna.

Linked to this problem is that of the size of the radar footprint; a complex interaction between the antenna frequency, the depth of the target and the RDP of the matrix change the size of the ‘footprint’ of the EM pulse; the area it is actively looking at in the ground. Any anomalous material needs to make up a significant percentage of this footprint to be detected, so the minimum size of object that can be detected and the optimum transect spacing are dependant not only on the antenna, but also on the RDP of the soils/sediments and the depth of burial. The RDP of the sediments and the depth of burial can sometimes be known or estimated before the survey, but often cannot, meaning some trial-and-error is necessary to determine optimal survey strategies.

In practice, this means that, generally speaking, with 250-500MHz antennae, archaeological anomalies 0.5m across are about the smallest that can be detected in routine survey, unless they provide a very strong contrast with the surrounding matrix.

It also means that in area surveys, the maximum transect spacing ought to be 0.5m, to ensure no anomalies of this size are missed between transects. It also means that even with lower frequency antennae, the maximum depth of meaningful archaeological investigation is about 4m. Much greater depths (in the order of several km) have been achieved through ice, but this was to map landforms, not archaeological-scale

anomalies.

6.5.3 Known conflicts and issues

There are some known issues with radar survey, and some common misconceptions.

Firstly, wet environments are not necessarily an obstacle to survey, and neither are

clay soils; it just depends on the physical and chemical properties of the water and or clay.

Water itself is a good propagator of the EM wave and surveys of lakes through the bottom of a boat and using the water column to conduct the signal to the sediments at the base have been very successful. There have also been successful surveys

waterlogged sediments (Clarke et al. 1999a; Utsi 2004). The problem comes when there is any salt present in the water, as saline water is a very effective conductor which rapidly attenuates the electrical component of the signal, causing the wave propagation to fail and the signal to be lost. This can happen with saline intrusions in waterlogged sediments, but it can also be a problem in seemingly dry (and held to be

‘ideal’) conditions over sandy soils if there are salts present in the interstitial water.

Other minerals can cause similar problems; if a material is a relatively good conductor, or has become so due to the chemical makeup of the pore water (for example, in reducing conditions, which tend to be acid), then attenuation is more likely to be a problem. The same thing can happen with the magnetic component of the waveform;

highly magnetically permeable soils (e.g. those with high magnetite content) can also be high attenuation environments (Conyers 2004).

Clays present a problem if the clay is a swelling clay, which can hold water in its matrix, making it a conductor, and this likely to attenuate the signal. Some two-layered clays do not have this property, but making adequate distinctions between the two types in the field is not practicable.

As can be seen, sometimes it is not possible to know in advance if GPR will ‘work’

on a site without advanced knowledge about the geology and expected archaeology.

Even with such foreknowledge, it can take time and trial and error to get the right antenna, travel time window, and estimated velocity to produce a good survey.

GPR also operates over a much used part of the EM spectrum, for radio, television and communications transmissions. As such interference can be a problem, particularly in environments where there are a lot of radio transmissions in the frequency band being used for survey (as has been the author’s experience surveying with 500MHz and 800 MHz antennae on Salisbury Plain, a military training area).

It is essential to maintain good ground coupling (keeping the antenna in constant contact with the ground or at a constant offset) which can be a significant problem over rough terrain or in areas with rapid changes in vegetation cover.

6.5.4 Instrumentation employed

This project employed a MALA Geosciences RAMAC X3M system, utilising Mala shielded 250 and 500 MHz antennae. These antennae are fixed position so cannot be used for the wide angle ranging and reflecting (WARR) or common mid-point (CMP) methods of radar velocity estimation. A survey wheel was used for continuous

distance measurement during survey, rather than the stepped or fiducal markers method. The exact settings, survey strategy and acquisition parameters were adjusted to suit each individual case study site, and are detailed in each Chapter reporting on them.

Section Three: Data processing and ground-truthing methodologies

This section contains two chapters that deal with the post-field handling of the geophysical data, and the principles that guided the ground-truthing work done on some of the sites. Chapter 7 deals with the computerised data processing and looks at the two dimensional surveys, which were processed in GEOPLOT3 (Geoscan

Research 2006) and then the pseudo-three dimensional data, the GPR surveys and the multiplexed resistivity work, which were dealt with in specialist programmes, GPR-SLICE (Goodman 2008) and Res2DInv (Loke 2005) respectively.

Chapter 8 is an overview of the principles and strategy behind the ground-truthing investigations; the specific aims and approaches on each site are discussed in the relevant chapters in Section 4.

Chapter 7: Data processing principles

This Chapter deals with the theory and principles of geophysical data processing.

Proprietary software has been used in the piece of research, and alternatives exist so this Chapter will not deal with the specifics of which tools and settings were

employed (though these are included Appendix A, associated with each case study site), but rather with the broader principles and implications of these processes on the resulting data plots.

7.1 Introduction

The processing of geophysical data can be a contentious issue. The ‘raw’ data gathered in the field is already an abstraction from the ‘real’ characteristics of the sediments. Any further manipulation of the data takes the geophysicist further away from the absolute measurement of the physical properties of the ground. On the other hand, processing techniques can significantly enhance the interpretability of

geophysical data sets, correcting for operator error or unavoidable alterations to the data caused by the fieldwork conditions or environments. They can also be used to enhance, not just correct, the collected data, allowing the archaeologist to emphasise certain parts of the image and present the data in innovative ways that allow better insights into the characteristics of the buried features.

Any manipulation of the data can, however, result in the distortion, loss or

introduction of anomalies and patterns in the resulting images. It is therefore vital that these operations are carried out with an understanding of exactly how the data are being changed by the selected process, rather than being operated as a list of ‘standard processing steps’ with little adjustment for the peculiarities of the individual site and survey being taken into account. With geophysical data acquisition and processing becoming a routine part of commercial archaeology in the UK there is a real danger of such a ‘black box’ approach being adopted by less experienced surveyors, with a resulting problem in the quality of the interpretation and usefulness of the surveys.

The information in this Chapter is synthesised from a number of sources (Scollar et al.

1990; Clark 1996; Loke 2000; Wheatley & Gillings 2002; Gaffney & Gater 2003;

Lock 2003; Conyers 2004; 2006a; Geoscan Research 2006; Goodman 2008).