Application of Passive Seismic Methodologies to the Determination of Overburden ThicknessWednesday, November 21, 2018 - 16:30 to 19:00 Multiplex Gym (DND)
Diamond mining is central to the economic development of the Canadian North. Innovative methods are needed to identify new prospective targets, as many of them are hidden beneath a thick overburden of glacial sediments.
Passive seismics is an emerging method used to map the thickness of near-surface geological layers. Vibrations from distant earthquakes are used as a source of signal and data is processed to estimate the depth of the interface between the overburden and the underlying bedrock.
In July 2018, four Tromino seismographs were taken to a study site located approximately ten minutes driving north of Yellowknife. A total of 146 Tromino measurements and associated GPS elevation measurements were taken at 6 m intervals along a dirt road. Elevation measurements were averaged over the course of four days and the survey line was approximately 740 m long. Results indicated that depth to bedrock decreases near outcrops and increases in valleys however; another geophysical dataset is needed to validate the passive seismic data.
In addition, 36 repeatability measurements were taken at a borehole location to validate the Tromino’s functionality and stability. Measurements were taken with 4 Trominos at the same location for 9 consecutive days to determine if data remains consistent even if the source signal changes each day. Each measurement yields a peak resonant frequency in (Hz). Key results from this study were that the average standard deviation of the peak resonant frequency associated with using the same instrument at the same location on different days was 0.07 Hz. This indicated that even if the source signal is different each day the standard deviation is very stable. Also, the average standard deviation of the peak resonant frequency associated with using different instruments at the same location on the same day was 0.05 Hz, which indicated that all instruments were performing well in terms of instrumental error.
A borehole study was conducted using two borehole locations. One was located at the repeatability study site and the other was approximately 65 m off the road, near a lake. To estimate the depth of the overburden-bedrock interface (Z) requires knowledge of the peak resonant frequency (F0) and shear wave velocity (Vs) in the near subsurface: Z = Vs/4*F0. Depth to bedrock was calculated using varying Vs values ranging from 200-500m/s in steps of 25m/s. The RMS error was calculated between borehole depths and calculated depths. The Vs value corresponding to the minimum RMS error value was 300m/s, which was used to calculate depth to bedrock.