Abstract
Electrical resistance geophysical surveys are known to produce variable results at different times of the year. This is a problem which can often lead to a misinterpretation of an archaeological feature under investigation. The dynamic relationship between a natural soil matrix and an archaeological feature is a complex one which greatly affects the success of its detection through electrical resistance. For electrical surveys, the change in contrast is mainly a function of moisture and ion concentration within the soil, the geology and the environment in which the archaeological deposits are situated. This paper will present the initial stages of a study monitoring the gradual variation of moisture by electrical methods over a selection of test areas. Monthly earth resistance surveys have been conducted over the last six months over known ditch features on four sites.

Introduction
The success of an electrical resistance (ER) survey is dependent on many pedological, ecological, geological and archaeological variables which exist within the natural landscape. The prospection method of resistance survey is greatly affected by soil moisture content, which changes, both diurnally and seasonally throughout the year. Past studies have not looked at archaeological detection in traditional ‘problem’ areas such as those on clay geologies or areas of low contrast. Past research has thus concentrated on large ditch features in areas of high contrast and not fully investigated why the detection problems exist.

Benefitting from a multi-disciplinary framework, as part of The DART Project, the research includes monthly geophysical data collection from four sites, and will incorporate data from parallel investigations into archaeological detection using hyper-spectral and spectro-radiometry survey, soil analysis, and time domain reflectometry (TDR) and weather measurements. This will allow a full environmental and geophysical case history for each site to be built, and enable the targeting of ER surveys at the ‘best detectable’ times, based on telemetry from the TDR and weather data.

At the four test areas, a Twin-Probe multiplexed earth resistance survey has been conducted every month since June 2011 and will continue until October 2012. By using a multiplexer to increase the separation of the mobile probes at each data station, deeper volumes of earth can be measured. Each resistance survey yields a dataset from 4 sequential depth investigations.

Preliminary results
The key of detection is ‘contrast’ between the feature under investigation and the surrounding soils so initial data analysis from the surveys is looking at the data collected over the ditch features and comparing these readings to a selected ‘background’ response as well as the complete dataset. By calculating the percentage difference between these populations, we are able to assess how the contrast between the ditch and background is changing and becoming more (or less) detectable.

The results will be compared to the local weather in the area to assess its impact on the changing geophysical contrast.