Field Setup

Location

The location of the MT station is the most important part of the field setup. The location should be chosen carefully to avoid noise sources. The following list gives an overview of the most important points to consider:

Avoid power lines, power stations, transformers, and other electrical installations
Avoid railways, highways; especially DC railways destroy the signal completely

The distance to the noise sources depend on the signal type and the surface resistivity.

A harmonic signal is less harmful compared to a transient signal or rectangular signal
In conductive areas the signal is attenuated faster than in resistive areas

So, simply spoken: a 50 Hz power line in a conductive, sedimentary area has a reasonable amplitude and does not destroy the spectrum.

A small village with a lot of power lines and transformers in a resistive area can destroy the spectrum completely over a distance of several kilometers (or water pumps in farming areas operating at night time … and so on).

Over all there is no other way but making a test measurement and check the spectrum. As a rule of thumb: after 15 min of recoding with a 512 Hz sampling rate you should get short but reasonable resistivity curve[1]. The amplitude in the H channels should be less than 4 V.

Orientation

Ideal is true North and East. You always want to align your data to the geographic North and East and with the geology.
Hence that the EDI format may support different orientations, the MT software is designed to work with N-S and E-W orientation only.
So, if you rotate, and sometimes not … you leave an open door for errors.

Warning

Layouts differing from N-S and E-W are trouble makers

E-Field

Warning

The electric field is represents the most difficult part of the MT measurement!
Most of the noise intrudes by the electric field.

Check your electrodes.

Always orientate your setup N-S and E-W. This is the only way to compare your data with other data sets.
Additionally most of the MT software is designed to work with N-S and E-W orientation only.

Electrode Spacing

The electrode spacing depends on the frequency range you want to measure.

100 m (2 x 50 m) or more for LMT (long period MT), must use non-polarizable electrodes
25 m for AMT, steel rods are fine
5 - 10 m for frequencies above 25 kHz, must use buffered electrodes
50 - 25 m for MT, 8 hours recording time, steel rods can be fine, periods down to 1 or 10s
50 - 100 m for MT, more than 8 hours recording time, non-polarizable electrodes

Hence that we talk here about the total cable length including the cable drum. In case your use MFS-07e and make a broad and recording including some AMT, 50 m on each reel is fine.

In older versions you read that a greater spacings improves the data quality. So using 200 m spacing is better than 100 m spacing. This is still a valid assumption for low frequencies; however the pre-amplifier of the system is so good that you do not need to spend efforts in laying out a long E line.
In case you can only use 60 m spacing in total for LMT, use 60 m spacing.

Electrode Depth

As deeper as better for LMT; if you reach the depth of natural moisture you are the winner.

Drill a hole!

In dry areas use an auger to dig a hole.
Understand: no good contact, no data!

Bentonite - NO SALT

Use bentonite to fill the hole and water it.
Never ever use salt!

Connect the cable to the cable drum:

Turn upside down, that gives a tiny protection against water intrusion.

Why can’t you spend 300 €?

… and get data …

Until now I don’t understand why people spend a lot of money for undertaking a MT survey - but are not able to purchase an auger (or motor driller).
To improve the data quality, drill deeper:

Common Problems

the electrode is not buried deep enough
contact resistivity is above 4 k\(\Omega\)
electrode falls dry
cable is chopped by animals

Noise and Trash

Electrodes have their life time. Noise will increase, e.g. below 10 Hz. That is not easy to see in the data.
Follow the link here for EFP-06. Their lifetime is about 2 years (regardless of the usage time).
You can test the electrodes by measuring the self-potential.

H-Field

Use the full cable length to the sensor. The sensors will “see you” when changing the battery or when connecting the cable to your laptop.
20 m is the default cable length and you can use it for AMT and LMT.

Keep Distance

Use the full cable length to the sensor.

Make sure that the coils is not directly placed beside the E line - keep 2 m distance.

Placement

20 - 40 cm deep for coils is fine. For a fluxgate, which has some temperature dependencies, go as deep as you can.
For the North coils the cable end is at South; for the East coil the cable end is at West.

dig a trench
level the bottom
place the coil
use the level again
use a compass to orientate the coil (either N or E)

Direction

The coils are magnetic! Stand upright and orientate the coil to North or East, don’t lay the compass on the coil!.

Remote Reference Station

The basic idea of a remote reference station is to measure at the same time but not at the same place.
The distance between the stations should be estimated that way, that the noise sources are not the same, but the inducing magnetic fields are the same.
The electric field strongly depends on the local geology, and is mostly not used for RR; advanced processing techniques are needed.
In case you use RR for long periods, the distance should be 10, 20 km or more.

Local Reference Station

For AMT and LMT you can use a local reference station.
Especially for AMT in the dead band by simply using \(H * H_r\) (H against H reference of the next local station) you can improve the data quality; reason: the dominating problem here is often not external noise - but internal noise of the sensor. And exactly this is used here.

Combination

In case you don’t have many systems, try a combination from the above.

Laptop

WiFi

The ADU acts as a WiFi hotspot.
You will see system and serial like “ADU-10e 22”, and connect.
You need to authenticate with user and password when accessing the system.
In most cases this is the easiest way.

WiFi - not visible

The WiFi switches off after 10 minutes of no activity.
Shortly press the power button to reactivate.

WiFi - no Internet

Your device may report “poor” or “limited” Internet access.
That is because the ADU does not connect you trough.

Tip

Auto Connect
Some devices may disconnect and search for alternatives.
Auto connect while using the ADU may solves that problem.

Cable

Cat6 and above works for 50 m and more (20 m and above is needed to operate the ADU without any interference from you (your laptop / mobile phone))[2].

You must set your IP address manually.

Table 1 IP Addresses
system	IP	netmask
ADU	192.168.40.10
Laptop	192.168.40.2	255.255.255.0
ADU	192.168.20.10
Laptop	192.168.20.2	255.255.255.0

As you can see, in case you have several generations of ADU (07e,08e,10e,11e,12e), you may configure multiple static addresses.
It is possible to configure the ADU IP address also.

Warning

Same IP Address
If you have set your IP address identical to the ADU’s IP address you get no connection!