I worked at field with EM38-MK2 instrument. Results of raw readings vizualised at picture below. I don't understand why readings for 0.5 m base are predominantly negative. Calibration? But I calibrate instrument before survey at 1.5 m. Conditions around were uniform.
What are you showing is Q or conductivity? You write Q, which is the mutual coupling rate (Hs/Hp) and is dimensionless, but write units in mS/m which is conductivity.
Q can be negative, especially in the presence of heterogeneities, e.g., metallic objects, wires, faults, fractures, etc.. This negative values of Q leads, erroneously, to negative values when using the low induction number approximation equation.
Another possibility is the fact that manufacturers doesn't always use the LIN approximation equation to evaluate the conductivity but a calibration curve that sometimes tends to show this negative behavior when the conductivity is very low. I had the same problem using GF Instruments CMD-Explorer and CMD-MiniExplorer and in my case the problem was mostly due to their calibration curves.
Thanks. Are these Q values apparent conductivity or not?
May be you dealt with calibration curves for EM38-MK2?
I want to invert my data for modelling vertical distribution of EC.
Q can be easily converted into apparent conductivity by a simple multiplication if the the instrument is working at very low induction numbers B which is defined by the ratio between coil separation (s) and the skin depth, i.e., it is valid when s is very small in relation to the skin depth which is inversily proportional to the conductivity. Of course this formula applies only to homogeneous media or layered earth. when the subsurface has heterogenities it fails, e.g., can lead to negative conductivities...
Unfortunately I did not work with EM38-MK2...
If you want to take a look at my PhD thesis maybe it could help you... it is avaialable here in research gate.
- Badges
- Science topic
- Similar topics
- Mathematics
- Algebra
- Matrix