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Conventions and default values

Conventions

Sign of corrections

All corrections are subtracted from the measured gravity value.

Data

Units are assumed as follows (if not provided and parameters are passed only as numbers). Input data in other units is converted.

  • Gravity: nm s-2
  • Gravity gradient: nm s-2 m-1
  • Heights: m
  • Tilt angle: rad
  • Polar angles: arcsec
  • Coordinates: degree
  • Orientation of sensor: degree from North
  • Timestamps are always UTC.

Terminology

Point

A precisely defined measurement location, usually identified by a point code. For example, the gravimeter lab at the Geodetic Observatory Wettzell has points WET_CA, WET_DA, WET_EA, and WET_FA. The vertical gravity gradient is a property of each point. An Earth tide model can apply to multiple points.

site_name

The AQG control software has an input field for the measurement site name. It is recored in the metadata (.info file) as measurement_site_name. Here, this parameter name is shortened to site_name and its value kept unchanged. Since this field can contain supplementary metadata, such as sensor orientation, it is not necessarily identical to the point code. When reading a dataset, Gravitools attempts to guess the point code and sensor orientation from the site_name by a formatting pattern.

Default values

Signal correction and outlier removal

By default, all corrections provided in the raw AQG output file (columns starting with dg_) will be corrected for. An outlier correction will be applied, using default values, if not excplicity disabled.

Tides

If the tide signal was not included or wrongly estimated (for several potential reasons), it can be included or changed in post production via calling set_dg_earth_tide().

Coriolis

In AQG software >= 2.0, the Coriolis effect is listed as a individual column (dg_coriolis) and NOT included in the systematic bias (dg_syst_effects) anymore (as was the case for all lower AQG gui versions). It is therefore automatically applied. If this is not desired, which is common if the same point was measured with two 180° opposed instrument orientations, this has to be specific in a loaded config file or passed directly as an option to process(). See also the documentation part on setting up a config file.

Outlier correction

Input for the outlier correction function detect_outlier().

Standard values are:

   threshold=3000
   sigma_threshold=3
   neighbors=15
   min_gap="30s"

Statistics calculation

Input for the function get_statistics(), which calculates SEM, SD_h, TAV and instrument sensitivity.

    h=STANDARD_HEIGHT
    adev_threshold=3600
    adev_wn_tau0=10.0
    adev_wn_tau1=100.0
    incl_outliers=False

Defined parameter value defaults

  • AQG sensitivity (as by manufacturer): 500 nm/s²*sqrt(Hz)
  • Standard height to which gravity value is transferred to (using the provided vertical gravity gradient): 0.0 m (=marker on point)
  • AQG measurement interval = 0.54 s