geoelectrical data

[lorenzo76]

Hy to all!
For my post i would ask to you any questions.
I'm an italian geophysicist and i have a PASI georesistivymeter, max input current 500 mA and max voltage 300 V.
For a specific geological conditions (outcrop of limestones, rocks very hard) I would like to know the minimal values for the potential measurements for considering a good geoelectrical acquisition.
Have you any experiences in similar situations?
I have imposted 500 mA of input current but sometimes it flows only 4-5 mA and the potential measurements are on the order of 0.01 for the lowest values.
Are they correct values?I am not so sure!
thanks

[Hemasri]

Resistivity survey in Hard rock terrains

We use resistivity surveying, here in Sri Lanka to find Caverns in Metamorphosed Limestone (marble), Groundwater explorations (of course in Hard rock-Pre Cambrian Metamorphic) and in Landslide investigations. We use OYO brand, that has capacity of dlevering fix current (can change from 5mA to 200mA), but potential is automatically change to delever that selected current to the ground. For our surveys, we use mostly 5-20mA range.

If we increase the current, if the instrument can not increase the potential to delever that current to the ground always give a error massage. My instrument can automatically change voltage maximum upto 600V without a booster.

So, I think, less that 20 mA is enough. Is that correct?.

Regards,

[lorenzo76]

Dear Hemasri,
thanks for your interest to my post.
I used 500 mA of input current because of the difficult conditions of the survey.
If I use low input current (such as 20-50 mA) in that geological situations (very tough limestone outcrops) I fear the current doesn't pass through the rocks.
In your experience,which minimum values of electrical potential (in terms of voltage) have you measured?
My instrument gives an error message if the values are too low but maybe it depends on the precision of the resistivimeter.
Do you think that potential measurements on the order of 0.01 for the lowest values are acceptable?
regards, Lorenzo

[Hemasri]

Resistivity survey in Hard rock terrains

Dear Lorenzo

May I know something, What was your elecreode configuration and what was the electrode distances, when you receive 0.01 value, is that mV or V. If it is Schlumberger with 300m /20m electrode difference, Row= aprox. 140 (Resistivity). Is that true?. So, the accuraccy of your results is depends with the electrode difference. If the same potencial value you got with 500mA for 100m/10 elctrode difference you will get Row=62. So, I think we have to consider many thing prior to finalize.

I like to discuss more on this matter. Also if you could give your personal e-mail address I will send you some set of data to examine and compare

REgards,

Hemasri
[email protected]

[lorenzo76]

Hy Hemasri, i don't know if you have read my email I have sent to you 20 days ago.
If not so, maybe there is an error in the email address, so I give to you my personal email and, if you want, you can contact me.
[email protected]

Regards