ATSS

atss is a stream ats file (the successor of the old data format of metronix since 1998, “advanced time series”).

An .atss file contains a stream of IEEE 754 double numbers. That is the default format on most computers and is also the default format of C/C++, Python NumPy and Matlab. A double is always 64 bit (4 byte).

The doubles representing units as given in the header, e.g. mV/km for electric field and mV for the magnetic. The system scales the data to the units given in the header.

JSON

RFC 8259 and ECMA-404 European Computer Manufacturers Association specified JSON format is an UTF-8 encoded text file, used for describing the contents of the stream.

Directory Tree

The directory tree HDF5 is the same / similar as in the MTH5 survey.
The purpose of the directory tree is to organize the data in a way that it can be easily found and processed, as well to make it easy into the MTH5 format.

Filename

The run number is obtained from the parent directory.

The filename (and run number) are split by underscores “_”.

nnnSystem_SystemName_ChannelNo_TypeOfData_SamplingRate .atss or .json

As a limitation you can NOT provide names with underscores or spaces like “ADU _ 08e”.

• run_001/084_ADU-08e_C02_THx_2s.atss … the binary data stream

• run_001/084_ADU-08e_C02_THx_2s.json … the JSON header

• run_002/084_ADU-08e_C02_THx_1024Hz.atss … the binary data stream

• run_002/084_ADU-08e_C02_THx_1024Hz.json … the JSON header

additional there may appear atmm mask files:

• run_001/084_ADU-08e_C02_THx_2s.atmm … the mask file

• run_002/084_ADU-08e_C02_THx_1024Hz.atmm … the mask file

File Entries

• serial number > 0 of the logger

• System Name (without underscore, without spaces: protect shell scripts from splitting the filename)

• C NN channel number >= 0

• T channel Type

• Sample Rate with Unit (can be a double); Units: Hz and s

JSON Entries

• datetime: ISO 8601, UTC time the stream started, can contain fractions of seconds like 13:22:01.5

• latitude: ISO 6709, North latitude is positive, decimal fractions

• longitude: ISO 6709, East longitude is positive, decimal fractions

• elevation: in meter, e.g. WGS84 referenced

• azimuth: North to East of the measured direction; 0 = North, 90 = East

• tilt: positive downwards; 0 = horizontal, 90 positive down

• resistance: in Ohm, e.g. the contact resistance of the electrodes or internal resistance of the sensor

• units: text string, e.g. mV/km or mV; the systems scales E to mV/km automatically

• filter: text string, e.g. like ADB-LF,LF-RF-4 -> filter used during recording

• source: text string, e.g. empty or CSAMT

The default values for MT are[1]:

• x = North, angle = 0°, tilt = 0° (horizontal)

• y = East, angle = 90°, tilt = 0° (horizontal)

• z = Down, angle = 0°[2] , tilt 90° (vertical, positive down)

in the sensor_calibration section:

• sensor: name of sensor like EFP-06, MFS-06e and so on

• serial: positive number

• chopper: 1 chopper is on, 0 chopper is off or nothing

• units_frequency: Hz in Hertz, do not use other units

• units_amplitude: mV, should be mV since the timeseries are in mV by default

• units_phase: degrees

• datetime: date of calibration, use “1970-01-01T00:00:00” when unknown

• Operator: person or institute carried out the calibration

• f: []: frequency data

• a: []: amplitude data

• p: []: phase data

… and where are stop date & samples ???

A stream persists as long as the logger records OR a receiver collects.
The later one is undefined, the (network) receiver may stop at any time.
The number of samples can be calculated from the file size and the present stream end with the sample rate. AND the system acts as a streamer; the json header is written once at the beginning of the stream.
A synchronization client (syncthing, rsync) appends the stream to the file; this process may be terminated either by the sender or the receiver. In case the receiver terminates, you would have an inconsistent header - avoided by not using a stop time info.[3]

\(samples == file \ size \ [bytes] \big/ 8\)

\[stop date == \frac{samples}{sample \ rate} + datetime == \frac{file \ size \ [bytes]}{8 \cdot sample \ rate} + datetime\]

It is obvious that the system can write (append) data to the stream without touching the JSON header: a stream has not stop date, he exists as long recorded.

When the system splits the recording into (for example) hourly segments you simply concatenate the streams and use the first header for the newly assembled stream. Hence that the system writes scaled data, so mV/km for the electric field, and therefore the even the electric field can be concatenated.

Header

For the electric field without calibration the entries for f, a, p are there but empty.

{
"datetime": "2009-08-20T13:22:01",
"latitude": 39.026196666666664,
"longitude": 29.123953333333333,
"elevation": 1088.31,
"azimuth": 0.0,
"tilt": 0.0,
"resistance": 572.3670043945313,
"units": "mV/km",
"filter": "ADB-LF,LF-RF-4",
"source": "",
"sensor_calibration": {
   "sensor": "EFP-06",
   "serial": 0,
   "chopper": 1,
   "units_frequency": "Hz",
   "units_amplitude": "mV",
   "units_phase": "degrees",
   "datetime": "1970-01-01T00:00:00",
   "Operator": "",
   "f": [],
   "a": [],
   "p": []
}
}

For a magnetic file with calibration:

{
"datetime": "2009-08-20T13:22:01",
"latitude": 39.02,
"longitude": 29.12,
"elevation": 1088.3,
"azimuth": 0.0,
"tilt": 0.0,
"resistance": 684052.0,
"units": "mV",
"filter": "ADB-LF,LF-RF-4",
"source": "",
"sensor_calibration": {
   "sensor": "MFS-06",
   "serial": 26,
   "chopper": 1,
   "units_frequency": "Hz",
   "units_amplitude": "mV/nT",
   "units_phase": "degrees",
   "datetime": "2006-12-01T11:23:02",
   "Operator": "",
   "f": [
      0.1,
      0.12589,
      0.15849,
      10000.0
   ],
   "a": [
      20.027,
      25.170446600000002,
      31.751886599999995,
      517.07
   ],
   "p": [
      88.566,
      88.187,
      87.711,
      -51.019
   ]
}
}

Dipole Length

The dipole length is not given in the header. (see also [3])
The systems calculate the electric field in mV/km. The position of the electrodes is given in a meta file (JSON). This allows you to calculate the “real” mV input to the logger.
WHY? 99.5% of all users use a North - East setup. If done so, you simply read the data from the disk.
You can concatenate the data from different runs and systems without further transformation.
Likely that other data formats force an entry: use +/- 1000 m for the dipole length.

ATMM

Files ending with .atmm contains pairs of start and stop indices of the stream. The indices are 0-based and the stop index is exclusive (0, 1024 means the first 1024 samples).

You use .atmm files for excluding data from processing; this is the case when a cable was chopped. You also can use it to mask spikes.

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[1]

when archiving the data you shall rotate the data to the North-East system.
Even though all programmers claim that they respect the rotation, they do may do not. Your are always on the safe side when you rotate the data to the North-East system a.s.a.p.
Additionally: when using time series viewers (not specialized for MT), you may not see what you expect.

[2]

for the z component the N-E rotation is meaningless;

[3] (1,2)

For remote observatory networks you may stream hourly segments. On the receiving server you concatenate the hourly segments using the “>” or “>>” operator.
This always works, even if the station has been setup newly with a different dipole length.
Additionally for the magnetic field (assuming the same sensor type) you can concatenate the data without any transformation in case the sampling rate is <= 512 Hz.