V is for more versatile.
There are many conventional EM's.
With the Aerial:
There are multi-frquencies systems that are more sensitive and discriminatory between conductors, and therefore promise a higher rate of rejection and positivity with a trade off of poorer depth penetration compared to TDEM systems. The TD part stands for time domain. The MF systems mix the primary signal on several frequencies with the secondary signal coming from the ground and try to make sense of it. They look at phase shift, polarity flip (out of phase signal), rise time (shape) of response wave form, and amplitude for clues about conductors. Anomalies to mines in the Timmins camp is about 250 to one surprisingly. With Texas Gulf the ratio was in one year, 64 anomalies to one mine. This older more powerful system had magnetic anomaly discrimination so that you did not drill BIF (banded iron formation), unlike TDEM or input systems from questor and geoterrex. It also does not generate the same number of conductors as TDEM. With TDEM or Input, you get about 37,000 conductors in Timmins. With AM from the Canadian Aero system about 400. Discrimination of these with a few parameters/criteria and mag/geology results in about 100 drill targets. At least 4 of them are mines. Which would you rather fly?
There are time domain systems which pulse current and wait for a reply with respect to differing microsecond windows after the pulse.. to get reflected and induced mag field from current. Variously called, pulse, time domain or TDEM, the VTEM system is one of these. The VTEM systems uses a large coil, and smaller receiver in the same plane. older TDEM systems used a separate sending bird and receiving aerial. Time domain tries to measure mag at the same time, but turned off the mag when they were sending pulses, in order not to confuse it. The later and stronger the response, the better and supposedly deeper the conductor. Anomalies to mines is perhaps 100,000 to one. There is very low discrimination.
The VTEM people claim you can site a drill hole purely from their GPS located data and aire photos, which are flown slow and accurately. This was not possible before.
You can fly AFMAG which uses a higher frequency system around 900 hz and measures electrical field and magnetic field. It proved good for finding faults and its results echo the Questor TDEM system although its principle is different. It is good for finding gold mine faults and Iron formation.
There are methods that utilize large ground loops of wire like Turair and Turam. Turam finds a lot of small sulfide veins but seems to crap out in recon on larger bodies. This is probably because there is poor conductor size discrimination with this method. On the other hand it finds sulphides very well.
IP is a popular ground method that used many pulses of differeing frquencies sometimes one polarity sometimes two and measures in the off times at a received or during on times too, the rate of decay of the "phantom continuing polarization current" between pulses. This will find chargeable zones that while not conducting contain bits of metal that will retain a charge and like a capacitor leak current to ground after the last impress of a charge. IP has found many orebodies.
The best method of them all for finding orebodies and the best method to correct a drill hole to target is down hole geophysics. Blaine Websters (Goldeye) group JVX practices down hole IP, which has promise of even more powerful results. Downhole is the single most mine-finding geophysics ever invented and has a positivity that is very high. Hit rate for mines over anomalies is 1000's of times greater than for aerial or other ground geophysics.
Ground carried dual coil systems are used. These are called Slingram types. Among these are Max Min and Shoot-back. These systems are deceptively simple, but are very low in positivity and badly misused often. In fact experimentation with coil separations and configurations is necessary to find conductive ore with these methods. Their hit rate of anomaly to ore with no other indicators is on par with aerial TDEM. 10,000 to one. A better method is being developed by a Calgary geophysicist, Duckworth which is called co-planar co-incident coil method. It will be no better at figuring conductor size and/value ultimately but much more positive and easy to use. Perhaps one operator instead of two.
If you know what you are doing, the Ronka and Geonics EM-16 and the Ronka EM-14 are among the best geofizz instruments to use, they are simple and very positive. EM-16 has no near field problems that most impressed current or field systems have. That is because it uses very remote sources of field.
As if you were using frequency mode discriminants in Induced Polarization or IP (dominant freq), the EM-14 will distinguish graphite from sulphides, something only self-potential and IP, among all the other geofizz methods, are capable of.
Self potential, which used electrical generation of a conductor on the ground i.e. the current it natively produces, is the most powerful discriminatory, and positive of all the geophysical methods. It also uses the cheapest insurumentation has good depth penetration and should be more widely used. It is basically two porous pots, a cable and a milli-voltmeter. Some orebodies can play a transistor radio, if wired up properly!
Every sulfide ore body in Timmins, or major sulfide body is also an EM-16 (hand held) geonics.com VLF conductor as well as being a deep conductor of other methods. Many large sulfide bodies in Timnmins with valuable mineral do not conduct by geophysics. The Kidd Creek orebody did not conduct by TDEM 6 channel Questor(Ken Darke, chief geologist Texas Gulf, personal communication) when the open pit was 150 feet deep and exposed. Kidd Creek was an AM anomaly by Canadian Aero's system and also by other aerial methods suchs as rotating coil, Teck Corp's Dighem and Inco's MF method of the day (1965).
The oil industry uses AMT and CSAMT which use like Afmag a different system to measure currents of differing frequencies than other geophysical methods. They are much more depth penetrating. AMT used natural earth currents called telluric current. These are created by among other causes lightning strikes at the equator.
en.wikipedia.org
This site clues you in a bit about multi frequency ground dual coil systems. They are similar in system to aerial systems.
heritagegeophysics.com
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