G FIZZ

[trowelhead]

Has any of you guys ever been presented with a flux gradiometer survey that indicated no ditches or other positive archaeological features other than ridge and furrow yet on excavation found the site littered with archaeology that had not been caputered by the G fizz. Thank gawd I went for the just in case evaluation trenchs!!!

The guy was an experienced bloke who i had used loads to great effect. The method was suitable for the geology type and everthing. Very Strange.

Any similar experiences.

[Sparky]

Yep. It happens.
I guess it comes down to getting what you pay for and a bit of bad luck. I get pissed off with Gfizz reports coming back with all the images played with already and no raw-data. It would certainly help to mess with the images and data myself to be sure or to account later why we didn't see stuff.
Anyway, how deep was the cover over the archaeology and what was it comprised of?

[Hugh]

I've not had any problems with Gradiometer surveys but I have seen a magnotometer miss a Roman pottery kiln lying just below the surface. My experience is that it can be a bit pot luck with G Fizz, but the positive results are well worth the occassional poor one.

When I look at the smiles on all the children's faces, I just know they're about to jab me with something.

[Sith]

I have experienced this both ways in the past; sites with features not detected by geophysics and geophysical anomalies not identified during excavation. Is it possible that the ridge and furrow responses were strong enough to swamp the 'real' archaeological ones?

D. Vader
Senior Consultant

Vader Maull & Palpatine
Archaeological Consultants

Our geophysicists have completed their scan of the area and found nothing

[Hugh]

quote:Originally posted by Sith

I have experienced this both ways in the past; sites with features not detected by geophysics and geophysical anomalies not identified during excavation. Is it possible that the ridge and furrow responses were strong enough to swamp the 'real' archaeological ones?

Ah yes I forgot false positives for some reason It's fairly common to see R&F on the surveys here, even when there are no archaeological features asscoiated with it. Presumably it's a sort of ghost signal of long ploughed out R&F. I'm not sure how much it swamps out underlying features but it does seem to.

When I look at the smiles on all the children's faces, I just know they're about to jab me with something.

[Sparky]

I would imagine that the signal is stronger because the soils infilling the furrows is full of burnt material and other matter that is distinct from the local and present day magnetic resonance but in sufficient quantity to mask any underlying material of different distinct magnetic resonance.

[geodan]

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 ?

[Sparky]

Its not up to me who my unit employees to undertake the geophysics. Its up to my managers and their decision comes down to price and who they know. It bothers me greatly and any questioning is usually met with contempt. I might tell you who they are but not now.

[EarlySlav]

As said above it does come down to contrast of material with the soil. How deep was the archaeology and what type of soil layers above it?

[Hugh]

quote:Originally posted by geodan
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?

I'm afraid that I cannot comment fully as it was in another authority than the one I work for now. The survey was done in the noughties, I think around 2001, in a county that is notorious for poor geophysics results. I don't blame the contractors/specialists as it seems to be a genuine problem with the geology/soils of the area.

When I look at the smiles on all the children's faces, I just know they're about to jab me with something.