26th January 2006, 02:35 PM
The problem will vary dependent on the strength of the magnetic anomalies from the ridge and furrow and the need for a contrast between the magnetic susceptibility of the fills of features and that of the surrounding natural matrix.
Basically your site can be littered with archaeology but if there is no magnetic contrast between the fill of features and the surrounding natural then a gradiometer will not pick the archaeology up.
I've seen surveys over extant ridge and furrow where the soil has a strong magnetic susceptibility and consequently show as broad bands of black and white when plotted in greyscale. Although altering greyscale display parameters and some processing methods may help reduce this effect and reveal underlying archaeology, it is more often the case that they will not.
More complex processing could help reveal underlying archaeological features, but is unlikely to be carried out given the low reporting costs and rate per hectare charged by geophys companies and, more importantly, expected by those who sub-contract them
It is more usual to see weak linear parallel anomalies that illustrate the former presence of r and f or more modern ploughing. In this case (remembering the need for that magnetic contrast) anomalies that indicate the presence of archaeological features, unless severely truncated, should be discernible.
In response to a couple of points:
As already pointed out gradiometers pick up contrasts in magnetic susceptibility and not archaeology. The presence of magnetically anomalous areas, e.g. the remains of last years guy fawkes night bonfire, that are not caused by cut and infilled features is possible and probable.
If somebody has missed a kiln with a magnetometer then dont use them again. Was this in the 1970's and if not why were they using a magnetometer ? Did they in fact miss the kiln, or just misinterpret the origin of the magnetic anomaly?
If you receive reports without raw data, then don't use the company concerned again. They haven't followed EH guidelines.
More to the point - if you are so unsure that the geophys report is of sufficient quality and wish to examine and play with raw data yourself (it's no good playing with images as thats what they are - a snapshot of the data captured within certain parameters and after certain processes have been applied), rather than believe the considered interpretion of the specialist, why are you using that specialist ?
Basically your site can be littered with archaeology but if there is no magnetic contrast between the fill of features and the surrounding natural then a gradiometer will not pick the archaeology up.
I've seen surveys over extant ridge and furrow where the soil has a strong magnetic susceptibility and consequently show as broad bands of black and white when plotted in greyscale. Although altering greyscale display parameters and some processing methods may help reduce this effect and reveal underlying archaeology, it is more often the case that they will not.
More complex processing could help reveal underlying archaeological features, but is unlikely to be carried out given the low reporting costs and rate per hectare charged by geophys companies and, more importantly, expected by those who sub-contract them
It is more usual to see weak linear parallel anomalies that illustrate the former presence of r and f or more modern ploughing. In this case (remembering the need for that magnetic contrast) anomalies that indicate the presence of archaeological features, unless severely truncated, should be discernible.
In response to a couple of points:
As already pointed out gradiometers pick up contrasts in magnetic susceptibility and not archaeology. The presence of magnetically anomalous areas, e.g. the remains of last years guy fawkes night bonfire, that are not caused by cut and infilled features is possible and probable.
If somebody has missed a kiln with a magnetometer then dont use them again. Was this in the 1970's and if not why were they using a magnetometer ? Did they in fact miss the kiln, or just misinterpret the origin of the magnetic anomaly?
If you receive reports without raw data, then don't use the company concerned again. They haven't followed EH guidelines.
More to the point - if you are so unsure that the geophys report is of sufficient quality and wish to examine and play with raw data yourself (it's no good playing with images as thats what they are - a snapshot of the data captured within certain parameters and after certain processes have been applied), rather than believe the considered interpretion of the specialist, why are you using that specialist ?