Construct a survey with a fixed 100x100m loop with 20 stations (20m between stations starting at centre of loop). Set the Earth in EMVision to a layered half-space (layer over a half-space).
Do these exercises and where it seems appropriate provide diagrams/images in your Lab report.
1. Model the in-loop (look at the decay of the first station) response with an upper layer (of thickness of 50 metres) resistivity of 1,10 and 100 ohm-m and with the lower half space set at 1000 ohm-m. What are your observations about the response (eg. amplitude, shape and effect of upper vs lower layer).
2. Now do the same as in exercise 1, but keep the upper layer at 100 ohm-m and change the basement to 1 and 10 ohm-m.
3. Set the upper layer to 300 ohm-m and lower layer to 50 ohm-m. Note on the various stations the time at which the decay curve changes in polarity. Construct a plot (in Excel) of the time of this change verses distance that the station is from the centre of the loop.
4. What is the approximate lateral speed of the current vortex based upon the curve from exercise 3.
In the first graph, the orange part of speed changes more steeply than the blue part. That means the lateral speed increase sharply in the first 3 channels and the lateral speed increase gradually in the following 3 channels.
In the second graph, the blue graph shows the general tendency of lateral speed.
Therefore, the approximate lateral speed of the first 3 channels is 1E+06 m/s and the approximate lateral speed of the following 3 channels is 454054 m/s.
5. With the earth settings the same as in exercise 3 comment on the ability of an in-loop system to discriminate the effect of the water table at 50m depth vs 30 m depth. Note 50 ohm-m is about right for a saturated sand with fresh water and 300 ohm metres is about right for a sandy clay that is mostly unsaturated.
In this graph, the orange part of lateral speed is quite steep and the blue part of lateral speed is smoother than the orange one. It is easily observed through gradient of this tow linear equations. The gradient of the orange one is 1E+06, and the gradient of blue one is 45405.