FAQ MT

10,000s with coils

Can I get 10,000s data with coils?

Yes, but …

First of all, that is around a 3hrs period, and we are talking of depths of 20-100km. Including spectral smoothing we need 4 hrs for a stack. If you would have 20 perfect stacks that would be 80hrs only.

This is completely unrealistic - or rare.

../../_images/Rhoden_10kHz-10000s.png

The output voltage at 10,000s is small and the magnetic activity should be good or even high in order to compensate that.

As a rule of thumb for the metronix MFS-06e coil :
Until 1000s there is no need for a fluxgate.
Recordings should be between 1 day (excellent place) to 3 days.
For 10,000s you should a) record for 6-10 days and may consider using a fluxgate.

10s with fluxgate

No.

Even though many fluxgates are specified up to 1 kHz, they do not work for MT in that frequency range.
100s is a good starting point … sometimes 30s.

How long do I have to record?

That is the overall question in MT.

The property you should know: TIME EQUALS DEPTH

As in the example above: For 1000s to be evaluated you need 1 hour of recording (including the smoothing in the spectral domain, which adds frequencies at the beginning and end of the spectra). That means continuous and error free in order to mark this stack as valid. If you say you want 20 stacks, you need 1 day.
If you have 30 to 50% loss you end up in realistic 2 days.
If someone tells you “1000s a day” it may one of these commercial guys, making you believe that MT is super cheap and fast (or he is simply stupid … or both). We have seen in almost all recent publications in geothermal that the data quality is poor (here they want to be very fast because of many stations to measure).
… and please do not believe that “robust” is the big iron, making your curves smooth. It works to a certain extend. However, a minimum of valid stacks is always needed.

How deep?

This is a question which can not been answered directly. Learn that please.
Question: How far do I get when I drive for one hour on a motorway?
Answer: depends on our speed (aka conductivity) and traffic jam (aka noise level).

coil prefered

duration

application type

environment

periods

depth

MFS-06e

1-2 days

general studies / geothermal

sediments

1000s

3-6 km

MFS-06e

3-6 days

general studies

sediments

5000s

8-12 km

MFS-06e

1-2 days

general studies

rocks

1000s

8-15 km

MFS-06e

3-6 days

general studies

rocks

5000s

20-100 km

MFS-07e

1-2 days

general studies / geothermal

sediments

1000s

3-6 km

MFS-07e

1-2 days

general studies / geothermal

rocks

1000s

8-15 km

MFS-07e

8 hours

general studies / geothermal

sediments

100s

2-3 km

MFS-07e

8 hours

general studies / geothermal

rocks

100s

4-6 km

SHFT-02e

15 mins

environmental

normal soils

1 kHz

30-100 m

These are basic estimates. If you would calculate the skin deepth you get deeper; but skin depth is much deeper compared to the result getting from MT processing which includes spectral smoothing.

For geothermal the important two questions are: how thick is the clay cap and how deep is the reservoir? If you commercially estimate 3 km and planning to make 300 stations with 1 day recording time, you end up with the MFS-07e. This caclulation can be completely destroyed in case you have small vibrations due to micro seismic activity of a volcano nearby - as we often see in Indonesia.

How shallow?

coil prefered

duration

application type

environment

periods

depth

MFS-06e

15 min

general studies

sediments

4096 Hz

80-100 m

MFS-06e

15 min

general studies

rocks

4096 Hz

120-300 m

MFS-06e

5 min

general studies

sediments

10,000 Hz

30-100 m

MFS-06e

5 min

general studies

rocks

10,000 Hz

70-120 m

MFS-06e

is not a dedicated AMT coil

prefer MFS-07e

MFS-07e

15 min

general studies

sediments

4096 Hz

80-100 m

MFS-07e

15 min

general studies

rocks

4096 Hz

120-300 m

MFS-07e

5 min

environmental studies

sediments

50,000 Hz

10-40 m

MFS-07e

5 min

environmental studies

rocks

50,000 Hz

30-100 m

SHFT-02e

15 sec

environmental studies

sediments

132 kHz

5-10 m

SHFT-02e

15 sec

environmental studies

rocks

524/132 kHz

5-10 m

For frequencies above 20 kHz: buffered cables with length 25 m are needed; for 132 kHz and above: 5 m buffered cables are needed. Use a steel rod.

In case you use the MFS-07e and catch AMT signals for general studies, you do not need to change your electrode cables and electrodes. With some luck and not too high contact resistivity the result will be fine.

If you make dedicated environmental studies (shallow water contermination for example) where you need results for each station you need buffered cables.

For all measurments we always recommend to use the full frequency range of the sensor. For the MFS-06e you should schedule a 64 or 32 kHz run, for the MFS-07e a 64 or 132 kHz run. In case AMT data was successfully catched, maybe one of the phases is close to 45°. If this is the case, the chance of correcting static shift is high. Additionally your inversion results will be better - because they always have to guess the surface layer.

If size does not matter

As explained here chopper: If your disk size does not matter (and additionally you do timeseries pre-processing), you divide into chopper on and chopper off job.
Typical jobs (as also scheduled by the mobile app) with MFS-06:

f sample [Hz]

duration

chopper

MB for 5 channels

32 kHz

3:25 min

off

128

4 kHz

15:25 min

off

72

256 Hz

1-10 day(s)

on

420-4200

The last job may be longer and depends on your target depth.
If you want to get more AMT data for the MFS_06e make the fist job longer

f sample [Hz]

duration

chopper

MB for 5 channels

32 kHz

10:25 min

off

390

4 kHz

15:25 min

off

72

256 Hz

1-10 day(s)

on

420-4200

For the MFS-07e you may be interested to catch frequencies above 10 kHz

f sample [Hz]

duration

chopper

MB for 5 channels

64 kHz

3:25 min

off

256

4 kHz

15:25 min

off

72

256 Hz

1-10 day(s)

on

420-4200

or with more AMT focus

f sample [Hz]

duration

chopper

MB for 5 channels

256 kHz

3:25 min

off

1025

64 kHz

10:25 min

off

781

4 kHz

15:25 min

off

72

256 Hz

1-10 day(s)

on

420-4200

With parallelized software (like ProcMT) processing is no problem.

Hence that on high resistive ground the 524kHz job may require buffered cables”.

Why do I need to stack timeseries?

Imagine you see a coin at the bottom of a swimming pool.

../../_images/coinsInPool.jpg

You will see that the face is changing; that is not the true image of the coin because the light is changing. If you wait for a while and make, let’s say 20 pictures, you will be able to combine these pictures (by statistics) and come close to the “true image”.