NCAG Geophysics Section
This section of the group was formed in 2009 when the committee decided to investigate the possibilities of purchasing a Resistance Meter.
In June 2009 we purchased a Geoscan RM15 Resistance Meter and our first experience of using this was at a training day given by Earthsound Archaeological Geophysics in August. On this day 17 of our member’s had experience with the machine and saw its potential in the data obtained.
What Is Geophysics?
Geophysics is a non-invasive probing of the earth’s surface and is used to seek details of a site – known or unknown – before excavation, or indeed to substantiate whether or not there is any possible archaeology in a given area.
There are several different types of survey available in Geophysics –
• Resistivity – a small electric current passed between probes through the ground up to a depth of 1.5m. It is used to detect stone, compacted surfaces, buildings, ditches and pits.
• Magnetometry – this is the quickest of all techniques and measures the difference between the magnetic content of the earth and the soil within the survey area. It is used to detect burnt or fired remains including hearths and kilns
• Magnetic susceptibility – this is used to assess areas of soil that are susceptible to changes of magnetic field indicating possible burning, industry and occupation.
• Ground Penetrating Radar – this passes an electromagnetic pulse into the ground which deflects off the underlying layers which will show the depth of an anomaly. It can detect voids such as tombs, caves, masonry and ditches as well as soil variations.
and Metal Detecting are the main ones.
The Resistance Meter
The technique of resistivity derives from the principle that the damper the soil the more easily it will conduct electricity. During a survey an electrical current is passed through the soil between two or more metal probes and readings are made on a meter of the degrees of subsurface resistance.
High Resistivity is where less moisture is retained and could be stone walls, roads, compacted surfaces.
Low Resistivity is where more moisture is retained – possibly silted up ditches, filled in pits, etc.
Once downloaded, the data is subjected to a series of processes to enhance its reading and interpretation and the processed data can then be used to find anomalies within the survey area and targets for further investigation eg. excavation, can then be identified.
Geophysics shows POSSIBLE archaeology – nothing is confirmed until it is investigated further usually by a ‘dig’. It allows a greater area to be archaeologically examined in a shorter time and allows more specific areas to be targeted for greater investigation and is therefore more time and cost effective.
In September 2009 we were given a target for the winter 2009/spring 2010 – a field by the Norton to Baldock Road and our first target was to identify a cursus found when the A1 was cut in 1968.
We held a series of survey days and completed an area of some 2.8.hectares in the given time. We experienced surveys in the cold, wet, fog and even snow in this time but recorded excellent data and managed to locate the cursus as requested. The field area is not finished and we hope to return to it at some point to establish the length of the cursus – a target for future years.
In 2011 we were given a different survey area – to try and identify targets for trenches in Church Field. Again this was over the winter time but in the period January to the end of March we managed 14 survey days (only 2 had to be called off for rain) and completed an area of just under 3 hectares. The data obtained was very interesting and somewhat unexpected as we appear not to have found the deserted medieval village of Norton thought to be there, but prehistoric features together with a possible Roman anomaly and medieval field boundaries. Targets have now been identified for the Easter dig and we hope to prove (or disprove) our theories.
Aims of NCAG Geophysics
We aim to give all interested members the chance to participate in surveys and encourage people to experience working together to find new anomalies. In some instances it will be possible to take this further and join a ‘dig’ to investigate further those anomalies.
To find out more about joining our team please email the membership secretary stating your interest in Geophysics (you must be a member of the group to participate).