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  1. 1. Chapter 16 Applying Magnetotellurics by Arnie Ostrander
  2. 2. Arnie Ostrander Arnie Ostrander is an oil and gas exploration consultant specializing in the integration of magnetotelluric methods and surface geochemistry in frontier basin exploration and in underdeveloped stratigraphic plays in producing basins. He earned his B.A. in geology in 1974 from the University of Montana. He began his professional career with Zonge Engineering and Research Organization from 1975 to 1985, was with Phoenix Geoscience, Inc. from 1988 to 1991, and has been an independent consultant since 1991.
  3. 3. Overview Introduction This chapter discusses the nature and uses of magnetotellurics (MT), a method of surveying the subsurface from the surface. Although MT cannot provide the resolution of seismic surveys, it is less expensive and, more importantly, can be used in places where seismic is impractical or gives poor results. In this chapter This chapter contains the following topics. Topic Page What is Magnetotellurics (MT)? 16–4 What Does an MT Survey Measure? 16–5 How Are MT Data Acquired? 16–6 Case History: Frontier Basin Analysis (Amazon Basin, Colombia) 16–8 Case History: Rugged Carbonate Terrain (Highlands of Papua New Guinea) 16–9 Case History: Precambrian Overthrust (Northwestern Colorado) 16–10 Case History: Volcanic Terrain (Columbia River Plateau) 16–11 References 16–12 Overview • 16-3
  4. 4. What is Magnetotellurics (MT)? Definition Magnetotellurics is an electrical geophysical technique that measures the resistivity of the subsurface. This is the same physical parameter that is measured in a borehole resistivity log. How MT differs from electric logs MT differs from an inductive electric log in three major ways: Magnetotellurics Measurements Electric Log Measurements Made from the surface Made subsurface from inside a borehole Depth of investigation is a function of both frequency at which the measurement is taken and the average resistivity of the subsurface Depth of investigation is the depth of the borehole measuring device below the surface Respond only to changes in average bulk resistivity Respond to individual rock layers along the wall of the borehole The figure below shows the simplified relationship between a lithologic log, an electric log, an MT sounding, and an inversion run using the MT sounding data. Figure 16–1. We can also take electric log data and run a forward MT model to produce an MT sounding curve. Subsurface layers resolved Subsurface layers are resolved by inverse modeling of MT data acquired across a spectrum of frequencies, as illustrated in Figure 16–1. MT resolution The rule-of-thumb for MT resolution for depth of burial vs. layer thickness is 10:1. For example, to “see” a layer at a depth of 1,500 m (5,000 ft), the thickness of the layer needs to be approximately 150 m (500 ft) or more. Low-resistivity layers are more easily delineated than high-resistivity layers. It is difficult for MT to resolve more than three or four subsurface layers. 16-4 • Applying Magnetotellurics
  5. 5. What Does an MT Survey Measure? What is measured? Two basic alternating current (AC) measurements are taken in an MT survey: a horizontal magnetic field (H-field) measurement and an electrical field (E-field) measurement. The E-field is always measured perpendicular to the H-field data. The H-field The H-field is the “source” signal, or the primary field. It propagates across the surface of the earth. Because it does not travel in the subsurface, the H-field data do not provide information about the subsurface geology. Very limited information about the subsurface geology can be interpreted from the vertical H-field if this component is measured. The vertical H-field is called the tipper. The horizontal H-field is measured with a horizontally oriented magnetic coil. The tipper is measured with a vertically oriented coil. Be careful not to confuse an MT survey with a magnetic survey. An MT survey does not measure the magnetic properties of the subsurface rocks, as does a magnetic survey. The E-field The E-field is the secondary field, generated by the H-field propagating across the surface. Each time the primary H-field (an AC signal) switches polarity, a secondary E-field (current flow) is generated in the subsurface. Thus, the horizontal E-field data provides information about the subsurface geology. This is the same physical principle as the alternator in a car. An alternating or spinning magnetic field (H-field) sets up current flow in the wire windings in the alternator, which in turn charges the battery. In the case of an MT survey, the “wire” is the earth. The E-field is measured with a grounded dipole typically 50–200 m long. All subsurface geology information is contained in the E-field data. However, without the H-field data, we cannot calculate resistivity. The figure at right shows the relationship between the E- and H-fields. Resistivity calculation Figure 16–2. The resistivity calculation is a simple ratio of the primary source signal (H-field) and the secondary current flow in the earth (E-field), with a modifier for the frequency at which the data were acquired: 2 1  E  ×  H 5f Apparent Resistivity =  where: E = magnitude of the E-field H = magnitude of the H-field f = frequency Applying Magnetotellurics • 16-5
  6. 6. How Are MT Data Acquired? Acquisition instrumentation The data are collected using a microprocessor-controlled voltmeter. The voltmeter is in fact a system of complex hardware/software devices that includes amplification, filtering, A/D conversion, stacking and averaging, and various data-enhancement algorithms. Types of surveys There are two types of MT surveys: natural source (Vozoff, 1972) and controlled source (Goldstein and Strangway, 1975). The equipment and the operational procedures for these two types differ considerably. Natural-source surveys The natural-source data-acquisition system typically measures four components: Ex, Ey, Hx, and Hy. The Ex component is oriented perpendicular to the Ey component. This is also true for the H-field components. The predominant low-frequency (< 1.0 Hz) signal source for natural-source data is sunspot activity. The dominant high-frequency (> 1.0 Hz) source is equatorial thunderstorm activity. Although H-field data do not provide information on the subsurface geology (when only Hx and Hy components are measured), the vertical H-field component—if measured—provides information on the surface geology. The figure below shows a typical MT setup for a natural-source survey. Figure 16–3. Controlledsource surveys 16-6 The controlled-source system uses a high-power transmitter and motor/generator set to transmit a discrete AC waveform. This signal is transmitted into a grounded dipole typically 600–1,200 m (2,000–4,000 ft) long. The transmitter is normally located 3–6 km (2–4 mi) from the survey line. • Applying Magnetotellurics
  7. 7. How are MT Data Acquired? continued Controlledsource surveys (continued) Normally, only the Ex (parallel to the transmitter dipole) and Hy components are measured. The figure below shows a typical MT setup for a controlled-source survey. Figure 16–4. Which method is better? The choice of MT method depends on the survey objectives. Natural-source data are best suited for regional surveys where the stations are widely spaced (e.g., frontier basin analysis). Controlled-source data are best suited for mapping structural detail where the stations lie along a continuous profile at 100–200-m (300–600-ft) spacings. The maximum depth of exploration for the controlled-source method is 3,000–4,500 m (10,000–15,000 ft) in a typical volcanic, carbonate, or granite overthrust terrain. Natural-source data have considerably deeper penetration but poorer resolution at shallower depths. Where to use MT MT can be valuable in areas that yield poor-quality seismic data and where acquiring seismic data is very expensive. The following table indicates where to use MT and the reasons for using it. Locations Reasons for Using MT Carbonate terrains Poor-quality seismic data Volcanic terrains Poor-quality seismic data Granite overthrusts Poor-quality seismic data Regional surveys Less expensive than seismic; generates prospects to detail with seismic Remote areas Less expensive than seismic Rugged terrains Less expensive than seismic Fracture zones Excellent tool for mapping Applying Magnetotellurics • 16-7
  8. 8. Case History: Frontier Basin Analysis (Amazon Basin, Colombia) Introduction A regional exploration program to study a large unexplored area in the Colombian Amazon basin was conducted by Amoco Production Company in 1987 and 1988 (Burgett et al., 1992). This study area was very large [approximately 300,000 km2 (115,000 mi2)] and remote with dense jungle cover, rugged terrain, and limited road access. The first phase of the program consisted of 31,700 km (19,700 mi) of airborne gravity and magnetics. The large-scale structures delineated in these surveys were then further investigated by MT. The MT survey was feasible with a light helicopter because the crew was small and equipment was light and compact. Data were collected from 43 sites, with a typical spacing of 10–20 km (6–12 mi). Survey results The MT data clearly delineated a thick sedimentary section with internal units that could be correlated from site to site. Three resistivity “packages” were observed: • 40–100 ohm-m (sedimentary) • 150–250 ohm-m (sedimentary) • >1000 ohm-m (crystalline basement) The figure below shows a simulated cross section in the Amazon basin based on MT data. Figure 16–5. Drafted from data in Burgett et al., 1992. Post-MT program Encouraged by the evidence from the MT survey, Amoco decided to shoot a small seismic program and drill a shallow stratigraphic test. This program was positioned on the edge of a subbasin defined in the MT data. There generally was good agreement between the MT data, the seismic data, and the borehole geology. The airborne gravity and magnetic data, followed by the surface MT survey, provided a very cost-effective means of regional basin definition and led directly to a well-positioned seismic survey and well site. 16-8 • Applying Magnetotellurics
  9. 9. Case History: Rugged Carbonate Terrain (Highlands of Papua New Guinea) Introduction The Papuan thrust belt is both an expensive and difficult area in which to acquire seismic data. The area is typified by rugged mountainous terrain, dense equatorial jungle, and thick, heavily karstified limestone. The karstified limestone in some areas is also overlain by heterogeneous volcanics. The few coherent seismic reflectors are lacking in character and continuity, and the data in general are extremely noisy. The sedimentary section in this area, however, is an excellent MT target (Billings and Thomas, 1990). This sequence observed in MT data is a simple three-layer package. The upper layer is the high-resistivity Darai Limestone, the middle layer is low-resistivity Leru Formation clastics, and the third layer is high-resistivity basement rocks. Therefore, the MT data provide a subsurface map of the base of the Darai and the top of the basement. The addition of an upper high-resistivity volcanic layer in some areas usually does not complicate this interpretation, except that it may not be possible to differentiate the base of the volcanics from the top of the Darai. Survey results More than 2,500 MT sites have been acquired in Papua New Guinea by numerous companies involved in exploration in the region (Mills, personal communication, 1994). BP Exploration (Hoversten, 1992) acquired MT data over both the Angore anticline and the Hides anticline. The interpreted models from these two data sets provide depth estimates of the base of the Darai Limestone to within 10% of the measured depth in the Angore 1 well. In both cases, the seismic data aided the interpretation. The figure below shows the 2-D MT model beneath the Angore-1 well and the base of the Darai Limestone as observed in the well. Figure 16–6. Drafted from data in Hoversten, 1992. Applying Magnetotellurics • 16-9
  10. 10. Case History: Precambrian Overthrust (Northwestern Colorado) Introduction MT can be used in an overthrust environment to delineate conductive sediments beneath a resistive thrust plate. It is often difficult to acquire good-quality seismic data in an overthrust area where high-velocity (high-resistivity) rocks overlie low-velocity (low-resistivity) sediments. The Precambrian overthrust in the Bear Springs area of northwestern Colorado is an example (Mills, 1994). Survey results The MT station near the drill hole (see diagram below) shows a thin, near-surface conductor on top of the resistive Precambrian thrust sheet. This is a wedge of Quaternary and Tertiary sediments. Beneath the thrust, a thick conductive section of Cretaceous sediments is observed. The figure below is an 11-station MT profile across the thrust. Figure 16–7. Drafied from data from Mills, 1994. Structural details These data provide the following structural details: • Thickness of Quaternary and Tertiary cover • Thickness of Precambrian thrust sheet • Thinning of Cretaceous sediments to the south • Depth to top of Paleozoic sediments • No differentiation between Paleozoic and basement A very detailed subsurface structural map could be obtained in this area using a 3-D grid, controlled-source MT survey depicting the Precambrian/Cretaceous thrust contact and the top of the Paleozoic section. 16-10 • Applying Magnetotellurics
  11. 11. Case History: Volcanic Terrain (Columbia River Plateau) Introduction Seismic methods do not work well in areas covered by volcanics because of the dispersive nature of the volcanics and because of the decrease in acoustic velocity at the base of the volcanics. Volcanic terrain, however, is an ideal environment for MT because it is a simple, threelayer stratigraphic package: resistive basalts over conductive sediments, which in turn overlie resistive metamorphic or granitic basement rocks. Survey results The cross section below is a 13-station MT natural source survey profile. This east–west section begins near the Idaho–Washington border and extends approximately 75 mi (120 km) to the west (Mills, personal communication, 1994). Figure 16–8. Drafted from data from Mills, 1994. Structural details These data provided the following structural details: • Considerable variation on the thickness of the volcanics • Considerable variation in the depth to top of basement • Basalts thin to the east • Sediments thin to east and eventually disappear • Basement resistivities are an order-of-magnitude higher on the east end of the profile Controlled-source MT data could provide 3-D imaging of individual prospects. Applying Magnetotellurics • 16-11
  12. 12. References Billings, A.J., and J.H. Thomas, 1990, The use and limitations of non-seismic geophysics in the Papuan thrust belt, in C.J. Carman and Z. Carman, eds., Proceedings of the First PNG Petroleum Convention: Port Moresby, New Guinea, p. 51–62. Burgett, W.A., A. Orange, and R.F. Sigal, 1992, Integration of MT, seismic, gravity, and magnetic data for reconnaissance of the Colombian Amazon: 54th meeting, European Association of Exploration Geophysicists, Expanded Abstracts, p. 428–499. Goldstein, M.A., and D.W. Strangway, 1975, Audio-frequency magnetotellurics with a grounded electrical dipole source: Geophysics, vol. 40, p. 669–683. Hoversten, G.M., 1996, Papua New Guinea MT: looking where seismic is blind: Geophysical Prospecting, vol. 44, p. 935–961. Mills, A., 1994, Zephyr Geophysical Services, personal communication. Vozoff, K., 1972, The magnetotelluric method in the exploration of sedimentary basins: Geophysics, vol. 37, no. 1, p. 98–141. 16-12 • Applying Magnetotellurics