TDEM Waveform and Time Windows

Use the TDEM Waveform and Time Windows tool to specify the time gates to be modelled along with the noise levels for each.

TDEM Waveform and Time Windows dialog options

Waveform definition

The waveform specification can contain either the name of the source database or a comma separated list of defined parameters.
To update the waveform data for inversion, click on the Specify button to open the Specify TDEM Waveform dialog. Modify the waveform parameters and upon closing the dialog, this field will reflect your new waveform definition. The graphical displays (the TDEM waveform and time windows) will be updated as well.

  • See the Application Notes below on how to view and quickly access your waveform specification from VOXI Manager.
    		•

    Offset of time windows from start of waveform

    		 The offset of the designated shutoff time from the beginning of the waveform. The first EM gate containing data may be further offset from the shutoff time.
  • The parameter is updated when the waveform definition is modified.

    • [Zoom]

    		 Press the button to enable a 20x zoom toggle that magnifies the time scale around the first receiver time window. Press again the button to go back to full view.

    Import Time Windows from File

    		 The time gates can also be read from a file – see Application Notes below.

    Application Notes

    You can easily view and modify your current waveform definition in VOXI Manager:

    • In the Data tree, hover the cursor over Windows and Waveform and a tooltip will display your waveform specification if defined.
    • Right click Windows and Waveform, then click on Modify: 
      • If no waveform has been defined yet, the Specify TDEM Waveform dialog will open first in order to define your waveform data for the inversion.
      • If the waveform has been defined, TDEM Waveform and Time Windows will open: further modifications to the waveform specification can be made along with time windows selection.

    The standard waveform database consists of the fields:

    1. Fiducial: Unique fiducial number

    2. Time: Time the current was recorded relative to the start of a cycle in milliseconds

    3. Tx_Current: Current at transmitter at the designated time in Amps

    4. Rx_Receiver: Voltage at receiver, equivalent to dI/dt

    This database has as many lines as flights in the project. The waveforms may vary somewhat from flight to flight. This tool averages them and sends half of the averaged waveform to the inversion engine.

    The time gates of the EM decay array are generally supplied in a separate file and should be loaded onto the EM decay channel prior to invoking VOXI. The zero time of the time gates start at the designated shutoff time. The fields of the decay sampling are:

    1. Start: Start time of the gate

    2. End: End time of the gate

    3. Middle: Middle time of the gate

    4. Width: Width of the gate

    VTEM Decay Sampling scheme  
    Window Start End Middle

    Width

    		  
      Milliseconds

    Noise Floor (pV/Am4)
    4 0.018 0.023 0.021 0.005 0.0792
    5 0.023 0.029 0.026 0.005 0.064
    6 0.029 0.034 0.031 0.005 0.046
    7 0.034 0.039 0.036 0.005 0.034
    8 0.039 0.045 0.042 0.006 0.026
    9 0.045 0.051 0.048 0.007 0.0208
    10 0.051 0.059 0.055 0.008 0.0156
    11 0.059 0.068 0.063 0.009 0.014
    12 0.068 0.078 0.073 0.010 0.01216
    13 0.078 0.090 0.083 0.012 0.01052
    14 0.090 0.103 0.096 0.013 0.00864
    15 0.103 0.118 0.110 0.015 0.00712
    16 0.118 0.136 0.126 0.018 0.0056
    17 0.136 0.156 0.145 0.020 0.0044
    18 0.156 0.179 0.167 0.023 0.0036
    19 0.179 0.206 0.192 0.027 0.0028
    20 0.206 0.236 0.220 0.030 0.0022
    21 0.236 0.271 0.253 0.035 0.002
    22 0.271 0.312 0.290 0.040 0.0016
    23 0.312 0.358 0.333 0.046 0.00132
    24 0.358 0.411 0.383 0.053 0.0011
    25 0.411 0.472 0.440 0.061 0.00096
    26 0.472 0.543 0.505 0.070 0.00084
    27 0.543 0.623 0.580 0.081 0.00073
    28 0.623 0.716 0.667 0.093 0.00065
    29 0.716 0.823 0.766 0.107 0.00059
    30 0.823 0.945 0.880 0.122 0.00056
    31 0.945 1.086 1.010 0.141 0.00053
    32 1.086 1.247 1.161 0.161 0.00049
    33 1.247 1.432 1.333 0.185 0.00046
    34 1.432 1.646 1.531 0.214 0.00042
    35 1.646 1.891 1.760 0.245 0.0004
    36 1.891 2.172 2.021 0.281 0.00038
    37 2.172 2.495 2.323 0.323 0.00036
    38 2.495 2.865 2.667 0.370 0.00034
    39 2.865 3.292 3.063 0.427 0.00032
    40 3.292 3.781 3.521 0.490 0.00031
    41 3.781 4.341 4.042 0.560 0.0003
    42 4.341 4.987 4.641 0.646 0.00029
    43 4.987 5.729 5.333 0.742 0.00027
    44 5.729 6.581 6.125 0.852 0.00026
    45 6.581 7.560 7.036 0.979 0.00024
    46 7.560 8.685 8.083 1.125 0.00023
    47 8.685 9.977 9.286 1.292 0.00022
    48 9.977 11.458 10.667 1.482 0.00021
    49 11.458 13.161 12.250 1.703 0.00021

    The above two tables are shown on the graph of the waveform dialog (see illustration below). In the illustration, the shutoff time is set at 3.4506 ms. The EM decay array start time 0.018 ms is relative to the shutoff. Thus, it occurs at the absolute time 3.4186 ms. Furthermore, the first few gates are normally not used.

    Prior to the inversion, you can further manipulate the settings, for example you could determine which gates are passed to the inversion by checking on/off the Use boxes in the dialog. You can also manually edit the error values.

    In the above table, the first 6 fields are always present, but the next fields are subject to the fit error chosen. They are described below for each Fit error selection:

    Fit error -> Fraction of standard deviation

    1. Std..Deviation: the standard deviation of each gate is calculated and displayed

    2. Fraction: a numeric value <1.0 that the St. deviation is multiplied by to produce the error for that gate

    3. Minimum value: The error for each gate is set to either this threshold value or St. Deviation x Fraction, whichever is larger.

    Fit error -> read from channel: if the standard deviation of the EM array is pre-calculated and loaded on the EM array channel, simply use it.

    1. Multiplier: prior to inversion, the preloaded values for each gate can be scaled by this multiplier to generate the error values.

    Fit error -> Relative error

    1. Relative error fraction: For each observation, the error for each gate is set to a fraction of that gate’s value

    2. Minimum value: The error for each gate of each observation is set to either this threshold value or the calculated relative error, whichever is larger.

    Fit error -> Error channel: No additional fields, the errors are read from the specified array channel.

    The Zoom button allows you to zoom in to the start of the Receiver time.