Options in Decay chart tabEditing field data for further inversion and resistivity mapping, namely:
- Edit decays (delete spikes, noisy tails, and early-time parts with off-scale ADC)
- Median filtering of several sweeps
- Splice curves measured at different transmitter currents
- Reverse TEM response sign
- Estimate rapidly sounding depth and apparent resistivity
Cutting off noisy tails
Deleting noisy decay segments (tails) is a standard procedure in TEM data preprocessing (Fig. 1). Select an end point at which to cut off the curve and press (Cut off all points after selected). Several curves of the same type can be edited simultaneously for better efficiency.
Median filtering over several sweeps
Soundings are often run in duplicate or triplicate, etc., but only one decay is to be inverted. Median filtering several replica sweeps reduces noise and improves the data quality (Fig. 2). To apply median filtering, load two or more curves and press (Calculate average for active curves). After averaging, delete the original curves. For this first hide the resulting (median) curve: select the curve names in legend and press button (Delete active curves) (fig.3), then display the median curve.
Splicing curves measured at different transmitter currents
Soundings may be run at the lowest or highest possible currents in order to extend the range of the transient process and minimize distortion from too long ramp time. For example, the minimum and maximum currents in the TEM equipment TEM Explorer (T1 transmitter) are 0.7-1.5 and 17-20 A, respectively. The responses at 1 A are the least distorted by the ramp time and bear high-quality information on the near-surface resistivity while those acquired at 13-15 A penetrate to greater depths. So, two such curves are spliced (Fig. 4) at the zone where they overlap at points selected by the user. If necessary, time correction is applied to the 13-15 A sounding to increase the overlap zone to the maximum.
For splicing, load two curves to Decay tab. Then select the splicing point where the two curves are the closest to one another (Fig. 5) and press Splice curves . The resulting decay will consist of the low-current response at early times and high-current response at later times. The original curves are deleted automatically.