I am wondering about the difference between ERT and TEM for shallow depth investigation. It seems that TEM is a very fast and easy way to find a conductor if a mobile array is applied. It may be slow if we use ERT to cover a large area.
Hi Rujun, I think you have the wrong topics caught because with TEM you mean certainly not Transmission Electron Microscopy, as indicated above?! ;-)
Hi Volker Klemm. I am sorry I make a mistake. The meaning of TEM in my question is transient electromagnetic method. The meaning of ERT is electrical resistivity tomography. Theses methods are geophysical methods. I proposed this question this morning using my phone to access internet. Because of communication problem, I can not change topic of this question and give up. I have fixed this problem.
Hi, Rujun. The difference between ERT and TEM for near surface (0.1-50 m depth) is very simple: ERT works, TEM does not work there. for the next level 50-200 ERT works much better, than TEM. then, deeper than 200 m in more or less conductive media TEM works better. However, I used to find fresh water at more than 200 m depth with ERT.
Hi, Yuriy. I have a different idea. What''s kind of TEM system used by you? The turn-off time of a TEM system targeted for near-surface application can be less than 2.5 us for a 40m x 40m loop. It has high resolution as 1 m for near-surface application. TEM is used to find UXO as small as 23 mm in diameter. It's impossible for ERT to find such small object. TEM is very powerful in finding conductor (small or large).
Rujun,
I think even with a turnoff of 2.5 microsec, getting very shallow information is difficult. The first few meters are missed. If you believe you are getting resolution in the first meter, then either the ground is very conductive or else the turnoff is faster than 2.5 microsec. With ERT, you have the option to put electrodes close together and collect information on the upper few meters. Further, TEM responds well to good conductors, whereas ERT is better suited for mid-range to high resistivities.
Rujun, to find metal UXOs it's better to use mine sweepers. Yes, you can get answer from metal object at 2, and even at 1 us, but you will never invert this into resistivity. Because in 40x40 loop the current cut-off time is more or less 1 us. What are you measuring in 2.5 us?
Hi Rujun, If we consider 2D view of both results, TEM for slicing horizontal and ERT for vertically. For me, it is good if we can combine both methods. But I totally agree TEM is very fast!
Thank you, Mr. Hohd Hariri Arfin. There are some conditions that we can not do ERT. For example, it's very difficult to carry out ERT in area as rocky, desert, high way, glacier, railway. If the resistivity of surface is very high, I think TEM may be a good choice.
Dear Yuri, thank you very much. I think it's possible to get correct apparent resistivity to shalow depth as 1 m or less using TEM. I study the principle of current TEM instruments and tested some TEM instruments. I find that no TEM instrument can satisfy the requirement of shallow depth exploration. The problems exist in transmitter, receiver, and sensor. According to my experience of geophysical instrumentation, I think it's possilbe to get much better result if we design a TEM instrument targeted for near surface investigation. There is a gap between current TEM instrument and GPR for shallow depth investigation.
Dear Les P. Beard, thank you very much. The turn-off time may be less than 1 microsec for 10 m x 10 m loop or smaller loop. I am wondering the turn-off time of 3 m x 3 m loop. The depth of exploration may be great than 15 meters for 3 m x 3 m loop. It's better for TEM to find good conductor in high reisitive background.
Dear Rujun, IMHO there is no need to trick the nature trying to make TEM at 1 m. The gap you've mentioned is already filled. You can use FDEM. It is done. See Geonics EM-31, GSSI EMP-40, and our NEMFIS (www.nemfis.ru)
- Badges
- Science topic
- Similar topics
- Filing