Environmental and Exploration Geophysics I

1. Environmental and Exploration Geophysics I Terrain Conductivity tom.h.wilson tom.wilson@geo.wvu.edu Phone - 293-6431 Department of Geology and Geography West Virginia University Morgantown, WV Tom Wilson, Department of Geology and Geography

2. Computer Accounts Log on Geol454-## (i.e. ## =01, 02, 12, 13, …) Password is just geol454 Check out contents of the H: (common) drive And the G: Drive (your personal drive on the network) Your G drive and the common drive are accessible on any machine hooked into our network. Copy the Burger files from the H: drive to your G: drive Store your classwork and models on the G:Drive. That way if you move to another machine those files will still be accessible to you. This also avoide the possibility that someone might inadvertently delete your files from the local C:\Drive. Make Sure your Account Works Tom Wilson, Department of Geology and Geography

3. Electromagnetic Induction In this picture an ammeter is connected in the circuit of a conducting loop.  When the bar magnet is moved closer to, or farther from, the loop, an electromotive force (emf) is induced in the loop.  The ammeter indicates currents in different directions depending on the relative motion of magnet and loop.  Notice that, when the magnet stops moving, the current returns to zero as indicated by the ammeter. http://ww2.slcc.edu/schools/hum_sci/physics/tutor/2220/em_induction/index.html Tom Wilson, Department of Geology and Geography

4. Electromagnetic Induction What would happen if you cut the ring? What would happen if you put a can of coke inside the coil? http://ww2.slcc.edu/schools/hum_sci/physics/tutor/2220/em_induction/experiments.html Tom Wilson, Department of Geology and Geography

5. Bring up tables 8.1 and 8.2 “Dynamic” Tables 8.1 and 8.2  < 30 m (about 100 feet)  ~15 km (about 9 miles)  ~1.5 m (about 5 feet) Tom Wilson, Department of Geology and Geography

6. Tom Wilson, Department of Geology and Geography

7. Colloid concentration Clay particles are a source of loosely held cations Tom Wilson, Department of Geology and Geography

8. Ion clouds in narrow pore spaces can interfere with current flow Cation clouds provide a source of electrolytes, they can also form a partial barrier to current flow through small pores. In this case their effect is similar to that of a capacitor. Tom Wilson, Department of Geology and Geography

9. Empirical conductivity porosity relationships Archie’s Law The general form of Archie’s law is F is the formation factor, and porosity is related to F as follows Note also that b is the conductivity of the mixture (bulk conductivity) and lthe conductivity of the liquid which we assume is water. Tom Wilson, Department of Geology and Geography

10. Terrain Conductivity Survey EM31 EM34 Geonics Limited has specially designed these terrain conductivity meters to take advantage of simple relationships between secondary and primary magnetic fields. The instrument was designed to operate in areas where the induction number is low. Tom Wilson, Department of Geology and Geography

11. Tom Wilson, Department of Geology and Geography

12. VSP Source Point Tracer and soil gas monitors CO2 injection well EM Survey tiltmeters Tom Wilson, Department of Geology and Geography

13. Marshall Co. WV, coal sequestration pilot Tom Wilson, Department of Geology and Geography

14. Hunting for Abandoned Wells Tom Wilson, Department of Geology and Geography

15. Hunting for abandoned Wells Tom Wilson, Department of Geology and Geography

16. The induction number? B induction number s intercoil spacing  skin depth depth at which amplitude of the em field drops to 1/e of the source or primary amplitude e natural base - equals 2.71828 .. 1/e ~0.37 In general for a plane wave, the peak amplitude (Ar) of an oscillating em field at a distance r from the source will drop off as - Tom Wilson, Department of Geology and Geography

17.  is an attenuation coefficient. r =1/ is the skin depth . The distance r=  is referred to as the skin depth Tom Wilson, Department of Geology and Geography

18. The attenuation factor  varies in proportion to the frequency of the electromagnetic wave. Higher frequencies are attenuated more than lower frequencies over the same distance. Hence if you want to have greater depth of penetration/investigation, lower frequencies are needed. As a rough estimate,  (the skin depth) can be approximated by the following relationship We can think of the skin depth as a “depth of penetration” Tom Wilson, Department of Geology and Geography

19. Low Induction Number When that assumption is met, there is a simple linear relationship between the primary and secondary fields when subsurface conductivity and the operating frequency of the terrain conductivity meter are confined to certain limits. Under low induction number conditions the ratio of the secondary to the primary magnetic field is linearly proportional to the terrain-conductivity. Since the secondary and primary fields are measured directly, their ratio is known. Hence, the net ground conductivity is also known. Tom Wilson, Department of Geology and Geography

20. Receiver Transmitter s Hs Hp Surface Contamination Plume  - the net ground conductivity is what we are after Tom Wilson, Department of Geology and Geography

21. Hs Hp Surface Contamination Plume HS secondary magnetic field at receiver coil HP primary magnetic field  = 2f – angular frequency f = frequency o = magnetic permeability of free space  = ground conductivity s = intercoil spacing (m) i = imaginary number Tom Wilson, Department of Geology and Geography

22. s Hs Hp Surface Contamination Plume f refers to the frequency of the alternating current in the transmitter coil The operating frequency is adjusted depending on the intercoil spacing EM31 Together, the EM31 and EM34 provide 4 different intercoil spacings and two different coil orientations. The coils can be oriented to produce either the vertical or horizontal dipole field. EM34 Tom Wilson, Department of Geology and Geography

23. Remember We could also write this as  is the skin depth f is the frequency of the em wave  is the conductivity (in mhos/meter)  is the resistivity The operating frequencies for the different intercoil spacings are EM31 EM34 Tom Wilson, Department of Geology and Geography

24. Since - As the frequency and conductivity increase, the depth of penetration decreases These instruments are designed to work when the induction number is relatively low In the following table we examine the effect of operating frequency, intercoil spacing and ground conductivity on the induction number. Tom Wilson, Department of Geology and Geography

25. In general, for the EM31, operation under the assumption of low induction number is valid for ground conductivity of about 100 mmhos/meter and less. Tom Wilson, Department of Geology and Geography

26. The text isn’t very specific, but a little calculation suggests that induction numbers of 0.2 or less are considered to be “low” induction numbers for the EM31. Perhaps as much as 0.5 or less for the EM34. Generally high ground conductivity is considered 100mmhos/m or greater. Fortunately, ground conductivity in general tends to be much less than 100 mmhos/meter Tom Wilson, Department of Geology and Geography

27. For example, on the Greer site, terrain conductivities in the darker areas are 22 mmhos/meter and greater. The terrain conductivities in the lighter areas are less than 6 mmhos/meter. Fahringer (1999) Tom Wilson, Department of Geology and Geography

28. Vertical Dipole Horizontal Dipole Changing the dipole orientation changes the depth of penetration and thus the instrument response will provide information about apparent ground conductivity at different depths. McNeill refers to these “depths of investigation” as exploration depths. The orientation of the dipole is easily controlled by changing the orientation of the coil. As suggested by the drawing, the vertical dipole will have a greater depth of penetration than the horizontal dipole. Tom Wilson, Department of Geology and Geography

29. Vertical dipole mode of operation Exploration Depths “Rule of Thumb” Tom Wilson, Department of Geology and Geography

30. Those are easy to remember and useful general relationships. However, the apparent conductivity measured at the surface is a composite response - a superposition of responses or contributions from the entire subsurface medium. The contribution from arbitrary depths is defined by the relative response function  (z), where z is the depth divided by the intercoil spacing. Tom Wilson, Department of Geology and Geography

31. We’ve jumped ahead into some of the technical issues associated with terrain conductivity methods. Next Tuesday we will back up a bit and review some more fundamentals. • Continue reading Chapter 8 – pages 499 to 510 (top). • Look over the problem I handed out today and ask yourself how you would solve the problem using methods described in pages 514 – 519. Questions Tom Wilson, Department of Geology and Geography