Friday, July 4, 2008

Geophysical training programme

Participated in a geophysical training programme since last Monday, courtesy of Danida. We had a visiting professor from Aarhus Universitet of Denmark to share Denmark's technology and knowledge in multi-electrode profilling (resistivity survey). We managed to carry out 4 resistivity profiles, with an accumulated distance of 1.7km over 2 locations. The first location is within a prominent golf course in Kuala Lumpur territory, and the second, on a vacant land somewhere in Lembah Beringin (exact location is not disclosed, don't want to get sued).


This week's training module is a follow-up training that we first had in Denmark, back in March this year. Being exposed to 2D resistivity in the past, this training module is more like an affirmation for me, because I've already have had some basics back in uni days and extensive experience using this technique for groundwater exploration in Perak . But this training module is an added knowledge to me because a relatively new array, i.e. gradient array is being adopted for the acquisition of field resisitivity data. Apparently this array is a rojak between dipole-dipole, Wenner, Schlumberger etc. arrays, and widely used in Denmark for subsurface imaging. But I've always pondered if this widely used array in Denmark can be successfully applied here, on our terrain. The reason being is, in Denmark, the land is generally flat and underlain predominantly by sediments. And here on our terrain the geology is a complex one, we have all sorts of rock and formations - igneous, sedimentary and metamorphic rocks, you name it. And there is also limited papers describing the usage of the gradient array on tropical land like Malaysia, where we experience more vigorous weathering.

I was wrong eh... the signal over noise ratio for gradient array was so impressive that a profile conducted profile over electrical cables doesn't seemed to be affected that much and there weren't many negative readings as compared to other array that I've used before, e.g. pole-dipole (forward or reverse) and dipole-dipole. And looking at the pseudosection derived from the gradient arrangement, I think this array is the best to apply when you're carrying out a resistivity survey. But the downside is, over a profile of 400m long, the depth of penetration that you can expect is between 40-60m below ground surface, which is kind of shallow for 'our' standard. A quick look at the pseudosection that I've generated using the data acquired in Perak last year indicate that penetration depths were generally greater, whereby for pole-dipole and dipole-dipole arrays, the penetration depths are generally 90-100m and 50-70m, respectively.
But anyway, it's an eye opener for everybody because this is one rare opportunity where we actually have had a visiting professor from Denmark to teach us and share his knowledge on resistivity survey.

2 comments:

Unknown said...

Thank for sharing the topic ... I was surfing and find it by accident

I am Geophycisist , and looking fot geoelectric training programme ...

could you please provide me any information where I could find any training of geophyscial resistivity training ?

Best Wishs

Khalid

ahchong said...

Hi Khalid,

You can try contacting 'The Hydrogeophysics Group' at hgg@geo.au.dk. Their url: http://www.hgg.au.dk/

They have affiliation with some lecturers from University of Aarhus, Denmark. Perhaps they could advise you on the training programme, if any.