Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Terrain Conductivity Phone

Tom Wilson, Department of Geology and Geography 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.

Tom Wilson, Department of Geology and Geography 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.

Tom Wilson, Department of Geology and Geography What would happen if you cut the ring? What would happen if you put a can of coke inside the coil?

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

Tom Wilson, Department of Geology and Geography

Clay particles are a source of loosely held cations

Tom Wilson, Department of Geology and Geography 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. Ion clouds in narrow pore spaces can interfere with current flow

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

Tom Wilson, Department of Geology and Geography 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

tiltmeters Tracer and soil gas monitors EM Survey VSP Source Point CO2 injection well

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

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

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

Tom Wilson, Department of Geology and Geography 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 /e ~0.37 In general for a plane wave, the peak amplitude (A r ) of an oscillating em field at a distance r from the source will drop off as -

Tom Wilson, Department of Geology and Geography  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 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 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 HpHp Surface Contamination Plume HsHs TransmitterReceiver s  - the net ground conductivity is what we are after

Tom Wilson, Department of Geology and Geography HpHp Surface Contamination Plume HsHs H S secondary magnetic field at receiver coil H P 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 HpHp Surface Contamination Plume HsHs s f refers to the frequency of the alternating current in the transmitter coil The operating frequency is adjusted depending on the intercoil spacing 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. EM31 EM34

Tom Wilson, Department of Geology and Geography The operating frequencies for the different intercoil spacings are 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 Remember EM31 EM34

Tom Wilson, Department of Geology and Geography In the following table we examine the effect of operating frequency, intercoil spacing and ground conductivity on the induction number. 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

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

Tom Wilson, Department of Geology and Geography 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 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. 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.