Hi Sergio - I have done a fair bit of frequency-domain modelling in Maxwell, and I have always used the inphase and quadrature data. By my reading of the documentation, the Maxwell FEM file format does not support apparent resistivity for plate modelling.
Greg has given a good summary of the two main algorithms used to calculate apparent resistivity [pseudolayer (inphase-quadrature) and amplitude-altitude]. There is also a good article by Nick Valleau in Exploration Geophysics (2000), v31 pp 584-594. You will need the altimeter data and the pseudolayer depth (if the former algorithm was used) to recover the inphase and quadrature from the apparent resistivity. If the pseudolayer method was used and you don't have altimeter data, you might be able to assume a nominal altitude and see what you get. I hope I am not going to provoke a storm of controversy here, but I think for plate modelling the anomaly shape is more important than the absolute amplitude. If your anomalies are local, and the altimeter variations can be assumed to be minor over the width of the anomaly, then you might be able to get something useable. In terms of highly-conductive regolith, it is rare to come across an airborne EM survey in Australia where the maximum conductivity is not ~1 S/m or higher. I am currently working on some data from Western Australia where the unconsolidated sediments have conductivity ~ 10 S/m... the maximum water salinities are on the order of 150,000 ppm. These high conductivities have greatly limited the application of airborne FEM for mineral exploration in Australia, as the minimum frequency available is quite high (typically ~400 Hz). Of course there are many places in Australia where the cover is so thick and conductive that time-domain AEM doesn't cut it either, even with the very low base frequencies now available (12.5 Hz or 6.25 Hz). Cheers, James -----Original Message----- From: SEGMIN <[email protected]> On Behalf Of Bill Peters via SEGMIN Sent: Monday, 23 November 2020 8:48 AM To: [email protected] Cc: Bill Peters <[email protected]> Subject: Re: [SEGMIN] SEGMIN Digest, Vol 29, Issue 26 This message originated outside Mira Geoscience. Absolutely no need for an apology Greg. It was said in good humour and that how we take it. You are correct in my opinion that we have long favoured TDEM over FDEM here mainly because of our highly conductive regolith. In Canada you don't have that problem in general. Cheers Bill Peters 39 Richardson Street, West Perth WA 6005 SGC CHAIRMAN T +61 (8) 6254 5000 M +61 419 962 274 CONSULTANT GEOPHYSICIST E [email protected] S billpeters Crosmin Pty Ltd SOUTHERN GEOSCIENCE CONSULTANTS www.sgc.com.au -----Original Message----- From: SEGMIN On Behalf Of [email protected] Sent: 23 November, 2020 1:00 AM To: [email protected] Subject: SEGMIN Digest, Vol 29, Issue 26 Today's Topics: 1. Apology (Greg Hodges) ---------------------------------------------------------------------- Message: 1 Date: Sun, 22 Nov 2020 16:55:14 +0000 (UTC) From: Greg Hodges <[email protected]> To: SEGMIN <[email protected]> Subject: [SEGMIN] Apology Message-ID: <[email protected]> Content-Type: text/plain; charset="utf-8" It has been suggested to me that my attempted tease at my Australian friends about FDEM was in poor taste.? I agree, and I apologize for it. I have tremendous respect for the EM science developed in Australia, and have gotten incalculable value from the EM modelling software developed there.? Greg Hodges -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://lists.geosoft.com/mailman/private/segmin/attachments/20201122/721298bb/attachment-0001.htm> ------------------------------ Subject: Digest Footer ----------------------- SEGMIN community mailing list service ([email protected]). Change your personal options here: %(user_optionsurl)s Colleagues can join here: https://lists.geosoft.com/mailman/listinfo/segmin Archives: https://lists.geosoft.com/mailman/private/segmin/ NOTE that <Reply> will reply to all members of the list. ------------------------------ End of SEGMIN Digest, Vol 29, Issue 26 ************************************** ----------------------- SEGMIN community mailing list service ([email protected]). Change your personal options here: https://lists.geosoft.com/mailman/options/segmin/jamesr%40mirageoscience.com Colleagues can join here: https://lists.geosoft.com/mailman/listinfo/segmin Archives: https://lists.geosoft.com/mailman/private/segmin/ NOTE that <Reply> will reply to all members of the list. ----------------------- SEGMIN community mailing list service ([email protected]). Change your personal options here: https://lists.geosoft.com/mailman/options/segmin/archive%40mail-archive.com Colleagues can join here: https://lists.geosoft.com/mailman/listinfo/segmin Archives: https://lists.geosoft.com/mailman/private/segmin/ NOTE that <Reply> will reply to all members of the list.
