EM-31 results at the perpetual slide

Geophysics for the EOS field school

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EM-31 data from the perpetual slide

Since 1997 there has been interest in obtaining geophysical information that could shed light on the kinematics of the land slide that is studied as one of the geol335 engineering exercises. We had intended to bring a ground penetrating radar unit to this year's field school, but there was no instrument available. However, an apparent conductivity instrument (the Geonics EM-31) was brought in order to investigate the shallow (less than 6 metres) electrical conductivity of the site. This should help anticipate the penetration depth that could be expected for a GPR survey since electrical conductivity is the primary physical property controlling attenuation rate of radar signals in the ground.

The two figures shown here are scanned from areal photographs take in 1962 (left) and recently (right). The excavation carried out to lighten the load and prevent further sliding is evident by comparison of these two airphotos.

There were only a few hours available for this investigation, so one 400 m line was surveyed with 2 m station spacing. The instrument was used in the "normal" mode, with coils 1 m off the ground, in the "vertical dipole" position, and oriented along the survey line. The first figure to the right shows a topography of the slide area with the surveyed line overlayed.

The following figure (below) provides results a apparent conductivity versus line position (not corrected for slope). Remember that apparent conductivity is essentially a weighted average of the true conductivity of the soils within 6 m of the instrument (below and around). Basics of operation, and the weighting with respect to depth are outlined on a separate page.

Comments on interpetation of EM-31 results

The instrument begins to see more conductive materials only within 150 m of the toe. The implication is that Whitelake sediments that underlie the sand/gravel surface materials is within 6 metres of the surface at between 125 and 150 metres. This is also where the hummocky, wooded region begins.
There are variations in conductivity that appear to be correlated with microtopography of the hummocks between 30 m and 120 m.

This could reflect the variations in thickness of material overlying more conductive Whitelake sediments.
This is also likely caused by the variation in height of the sensor above the sediments. Since the instrument is 3.66 metres long, when standing in a trough between hummocks, both receiver and transmitter coils were closer to the ground. One way of alleviating this would be to use the instrument with coils oriented perpendicular to the survey line.

The region between 5 m and 40 m has the highest apparent conductivities, and I understand this to be the region where Whitelake sediments predominate. Above that there is gravel overlying the sediments, and below that zone the slope increases significantly, suggesting that we have reached the end of sediments, and underlying granites essentially outcrop.
Given the seriously mixed up nature of materials within 50 m of the toe, it could be estimated that the conductivity of Whitelake sediments is between 80 or 90 mS/m. This is a significant contrast with respect to overlying gravels that apparently average less than 5 mS/m, so unless the interface between these units is not clear, it should be possible to detect a reasonable GPR reflection from the top Whitelake sediments through fairly thick sequences of overlying gravels.
Also, the gravels themselves are resistive enough to leave us optimistic that signals from bedding planes within these materials should be detectible using a GPR survey.

To conclude, this survey provides a strong incentive to conduct GPR work at the perpetual land slide site. There are also several modifications to the survey that would be beneficial, and in fact, several adjacent EM-31 survey lines would help characterize the structures perpendicular to the slide's centre line.

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