Decay Calculation

Use the Target Inversion > Tools > Decay Calculation option (geosoft.uxo.gxnet.dll(Geosoft.GX.UXO.UxaDecay;Run)*) to calculate the EM61-MK2 apparent time constants. 

Decay Calculation dialog options

Target Database

Name

Select/enter the target database name.

Script Parameter: UXANALYZE.TARGETGDB

Group

Select the target group.

Script Parameter: UXANALYZE.TARGETGROUP

ID channel

Select the target ID channel.

Script Parameter: UXANALYZE.TARGETID

Mask channel

Select the target database mask channel to filter out specific targets. Only the targets with a mask (channel value) of 1 will be processed. If the value is set to 0 or dummy (*), the corresponding target will be ignored.

If the field is left blank, no mask will be applied.

Script Parameter: UXANALYZE.TARGETMASK

Subset Database

Name

Select/enter the subset database name.

Script Parameter: UXANALYZE.SUBSETGDB

Data channel

Select data channel to process. This channel must be an array channel of size 4.
In case of EM61-MK2 Mode D, the gates are b216, b366, b660, and t660.

Script Parameter: UXANALYZE._SENCHAN

Parameters

Gates for Tau calc

Pick the gates used in Tau calculation. The available gates are: 12, 13, 14 [Default], 23, 24, and 34.

Script Parameter: UXANALYZE.TAUGATES

Acceptance threshold

Enter the acceptance threshold, which replace all the readings that are below its value.

If empty, no thresholding is done.

Script Parameter: UXANALYZE.THRESHOLD

Gate 1 threshold

Enter the amplitude threshold. This threshold is used to filter the data before calculating the median of the Tau values.

If empty, no thresholding is done.

Script Parameter: UXANALYZE.AMPTHRESHOLD

Application Notes

The apparent time constant is calculated on the different time gates of the EM61-MK2 instrument in order to normalize the time decay, represent it in a single number, and as a result facilitate the interpretation of the data. This calculation makes the assumption that the targets have a simple geometry with closely equal dimensions. The time constant between each 2 time gate channels is calculated and saved under the channel name Tau_A_B, where Tau is the prefix, A and B are the gates indexes used to calculate the time constant.

The exponential time-decay is represented as:

V(t)= k e -t/t

Where:

V(t) is the output signal as a function of time, V is in mV and t in ms

t is the time constant and

k is a constant

For each combination of time gates, the time constant is calculated as:

 tm-n = (tn-tm)/(ln(Vm/Vn)

Where:

 tm-n Is the time constant between time gates m & n (n>m).

 Vm & Vn  are gate outputs at times tn& tm

The Geonics EM61-MK2 can provide 4 time gates that are geometrically spaced in time. The standard time gates are 216,366,666,1266. This feature enables the discrimination of different target types based on time decay rates. Based on the assumption that the time decay is an exponential, the apparent time constant normalizes the time decay curve into a single number, removes the difference in the magnitude of the response, and facilitates the recognition of targets of a similar nature residing at different depths.

In practice it is recommended to calculate the time constant between time gates 1 & 3 and also the time constant between the time gates 2 & 4, then compare the results. If the two responses of 2 different targets are similar, they most likely are of the same type. Note that in order to calculate only these two time constants you will have to run the process twice, once which each time gate combination. Running the time constant calculation on all four channels you result in more output channels than desired.

The apparent time constant can also be used to discriminate between small target residing at or near the surface and deeper larger targets. Typically small targets have responses of about 100 microseconds and less while larger targets have responses in the order of several thousands of microseconds.

*The GX tool will search in the "gx" folder. The GX.Net tools, however, are embedded in the Geosoft.uxo.gxnet.dll located in the bin folder. If running this GX interactively, bypassing the menu, first change the folder to point to the bin folder, then supply the GX.Net tool in the specified format.

Acknowledgments

The following document has been used in the compilation of this help file and further information can be obtained directly from it:

  • Technical Note TN-33 – Why did Geonics Limited Build the EM61-MK2? Comparison between EM61-MK2 and EM61, M.Bosnar, March 2001