A magnetotelluric transect of the Jacobsville Sandstone in northern Michigan
CHARLES T. YOUNG I Department of Geology and Geological Engineering, Michigan Technological University, Houghton, Michigan 49931 TED R. REPASKY*
ABSTRACT at Sturgeon River Falls about 30 km south of the MT survey line. Com- Eight magnetotelluric soundings were conducted across a basin plex structure within the Jacobsville Sandstone is indicated by highly of the Jacobsville Sandstone along a northwest-southeast line, begin- contorted gravity contours in some areas, and by folded, faulted, and tilted ning north of Toivola, Michigan, northwest of the Keweenaw fault beds at Limestone Mountain (the letter "A" in Fig. IB) (Bacon, 1966; and ending near the village of Keweenaw Bay in Michigan's Upper Kalliokoski, 1982). Peninsula. The soundings have provided an estimate of the thickness Meshref and Hinze (1970) interpreted cross sections of the western and resistivity of structures in the Jacobsville Sandstone as well as the Upper Peninsula of Michigan on the basis of aeromagnetic data. One resistivity, stratigraphy, and structure of the underlying strata. One- profile lies a few kilometres southwest of the MT profiles and is approxi- and two-dimensional models indicate a northwestward thickening of mately parallel to it. They estimate a thickness of about 2 km for the the Jacobsville Sandstone, a faulted basement uplift, the Keweenaw Jacobsville Sandstone near the Keweenaw fault and interpret a reversely fault, and a deep conductor at the southeast end of the profile. faulted anticline in the underlying basalt basement southeast of the Ke- weenaw fault. This fault and anticline intersect the magnetotelluric (MT) GEOLOGICAL AND GEOPHYSICAL SETTING profile as shown in Figure IB, and we used their model as a reference for our interpretation. The study area is a sedimentary basin southeast of the Keweenaw PRINCIPLES OF THE MAGNETOTELLURIC METHOD fault in northern Michigan, as shown in Figures 1A and IB. It is one of several basins flanking the Midcontinent (Keweenaw) Rift System. Explo- Here we present a qualitative description of the MT method to illus- ration for petroleum has been conducted in some of these basins in recent trate the significance of the survey data. The mathematical and physical years. The Jacobsville Sandstone was deposited in Proterozoic time on a formulation of MT has been given by Kaufmann and Keller (1981). basement of unknown nature and later faulted against the Portage Lake Summaries of MT principles and examples of surveys to evaluate a sedi- Volcanics. The Portage Lake Lavas (PLL), Copper Harbor Conglomerate mentary basin are given by Stanley and others (1985) and Vozoff (1972). (CHC), and Freda Sandstone (FSS) lie to the northwest of the fault; the The method utilizes natural transient electric and magnetic fields to Jacobsville Sandstone (JSS) and the Michigamme Formation (MF) lie to determine the surface electromagnetic impedance of the Earth. These fields the southeast. Early studies summarized in Heiland (1968, p. 661 and 662) are due to the flow of charged particles in the ionosphere and to distant reported resistivities of 120 to 44,000 ohm-m for the lavas and 35 to 120 lightning storms. The electromagnetic impedance expresses an assumed ohm-m for the Jacobsville Sandstone (originally termed "Eastern Sand- linear relationship between the applied magnetic field and the resulting stone"); thus it is likely that resistivity measurements can detect contacts electric field. The observation depth of a given measurement is dependent between these dominant rock types. upon the frequency of the detected signal and upon the subsurface resistiv- Various geophysical methods such as gravity (Bacon, 1966), aero- ity. Low-frequency electromagnetic waves penetrate more deeply than do magnetics (Meshref and Hinze, 1970), seismic-reflection (Aho, 1969), and high-frequency waves. Waves of a given frequency penetrate deeper into seismic surface-wave studies (Van Horn, 1980) have been used to estimate resistive rocks than into conductive rocks. The impedance is expressed as the thickness of the sandstone. Kalliokoski (1982) compiled geophysical apparent resistivity and plotted versus frequency (Fig. 2, for example). and borehole measurements which indicate that the sandstone may be as These plots resemble a highly smoothed electrical log with an axis of thick as 3,000 m near the Keweenaw fault and that it tapers to a thin edge frequency rather than depth. The structures giving rise to these curves are to the southeast. determined by comparison of the data with synthetic data computed from Gravity and magnetic data, as well as a few outcrops, indicate com- models. plex structures within and beneath the Jacobsville Sandstone. Gravity data MT equipment normally records two horizontal components of elec- suggest (1) major faulting generally parallel to the Keweenaw fault. Minor tric field and three orthogonal components of magnetic field. The cross-faulting (Fig. IB) and magnetic data suggest extensive buried basalt horizontal electric and magnetic fields are used to determine the imped- flows in a band parallel to the Portage Lake Lavas about 30 km southeast ance tensor, which is usually mathematically rotated to the principal axis. of the Keweenaw fault. This interpretation is confirmed by basalt outcrops The direction of principal axis (impedance maximum) and the ratios of the maximum to minimum impedance express the electrical fabric at the 'Present address: Mobil Exploration and Production Service, Inc., P.O. Box measurement site. 900, Dallas, Texas 75221. The vertical component of the magnetic field is used to define the
Geological Society of America Bulletin, v. 97, p. 711 - 716,6 figs., June 1986.
711
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-r 90 88
Figure 1A. Regional geologic map from Kalliokoski (1982). The location of the detailed map below is indicated by the heavy rectangle.
PALEOZOIC
UPPER PROTEROZOIC Jacobsville Sandstone
MIDDLE AND UPPER PROT. [Wq Lavas; sediments £ ^ I* ' • J P. Powder Mill Gp. LOWER PROTEROZOIC | 1 Metased metavolc. I I rocks ARCHEAN —I X X Granite, gneiss, r x it X greenstone o 50 Km
88 45 88 30 88 15 88 00 "tipper," which is also used to reveal the electrical fabric of the area. If the Earth consisted only of flat layers, there would be only uniform cunents, but if there is more complicated structure, electric current will concentrate in conductors. The magnetic field of concentrated current curls around the current, as described by the right-hand rule, and thus a horizontal corrent concentration will have a vertical magnetic field component on either side of it. The ratio of the vertical magnetic field to horizontal magnetic field is known as the "tipper magnitude," and the tipper direction is the direction of the horizontal projection of the total magnetic field. The tipper is directed at right angles to the current concentration causing it, and it points away from the concentration. Tipper vectors are presented in the next section to supplement the impedance orientation data.
EXPERIMENTAL PROCEDURE AND SURVEY RESULTS
The MT profile transected the Jacobsville Sandstone at -90° to the Keweenaw fault (Fig. 1) and was located where the structure would be as nearly two dimensional as possible, several kilometres northeast of the complex magnetic and gravity anomalies associated with the faulting and the buried basalts. Detailed site selection depended on land access and on minimizing powerline and pipeline interference. The data quality lor the survey was generally good, except for occasional problems caused when high winds vibrated the ground in which the magnetometers were buried. Figure IB. Detailed geologic setting and location of MT stations. A sample plot of apparent resistivity versus frequency is shewn in The interpreted faults and anticline are from Meshref and Hinze Figure 2. The depth scale given by the diagonal lines is strictly valid only (1970) and Bacon (1966). Limestone Mountain is indicated by the bold for a two-layer earth. Apparent resistivities over a perfectly conducting face A near the liottom of the map. Key to rock types: FSS, Freda layer buried at a depth, h, will have asymptotic values that plot on these "h Sandstone; CHC, Copper Harbor Conglomerate; PLL, Portage Lake lines" at very low frequencies (Berdachevsky and Dmitriev, 1976, p. 174). Lavas; JSS, Jacobsville Sandstone; MF, Michigamme Formation. For preliminary interpretation, one assumes that the depth to a given
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SITE 3 datum is equal to the h line upon which it falls. The lowest apparent resistivities left of center therefore represent depths of 500 to 1,000 m. The Rho xy = c Rho yx = ] h lines also indicate proximity of the MT station to significant resistivity > 10,000 > contrasts (edge effects). The splitting of pxy and pyx that begins at about .3 Hz thus is due to structures about 3,000 m from the station and is probably due to the Keweenaw fault. These approximate distances and depths are iÖS 1000 helpful in formulating a starting model to be tested by more accurate CO • computations. LUS OCX \ The fact that pxy and pyx are very nearly identical at high frequencies hO 100 indicates that the Earth is nearly isotropic in the upper 2 to 5 km and may S3 S \ s t 3 ' be modeled locally with flat layers. The divergence of the curves at lower S* \<'i.c frequencies indicates the need for a two- or three-dimensional model there. o
Tipper Scale
1.0
Figure 3. a. Apparent resistivity curves from all sites. All data are rotated so that the x-axis (for pxy) is 45° east of North and the y-axis (for Pyx) is 135° east of North, b. Tipper magnitude and direction for 0.047 Hz. Note north arrow, distance, and tipper scales, c. Rotated impedance polar diagrams for 0.047 Hz. The radius of the ellipse is proportional to the impedance in the direction of the radius vector.
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2. Sites 2, 3, and 4 diverge at low frequencies. The frequency at which this divergence begins is lower for stations farther from the Kewee- naw fault, suggesting that the resistivity contrast at the fault causes the divergence. 3. The apparent resistivities are low for low frequencies at sites 6 and 7, indicating a deep conductive body. 4. Sites 1 and 8, at the west end of the profile, have a different character than the other curves have, and they are located on relatively resistive rocks. In addition to the apparent resistivities of all stations, we also present tipper and impedance orientations in Figures 3b and 3c for the frequency .047 Hz. Data at this frequency behave in a reasonably consistent manner across the survey, and the penetration is appropriate to show regional electrical fabric. The most striking feature of the tipper arrows (Fig. 3b) is the general agreement between sites 2 to 6. These arrows point approxi- mately northwest and indicate a concentration of current in the central and southeast portions of the profile. This may be due to a deep, very conduc- tive body at the southeast of the profile, interpreted as the buried slates of the Michigamme Formation. Currents in this conductor and in the con- ductive Jacobsville Sandstone could give rise to the observed tippers. Site 7 has a small tipper and may be almost directly above the axis of the current, where the vertical magnetic field due to the concentration would be rnininrijil To the northwest, site 1 is directly over the Keweenaw fault, and site 8 is about 10 km northwest of the fault. Tippers at these locations are probably influenced by local current concentrations which depart from two-dimensional geometry. Rotated impedance diagrams are shown in Figure 3c. The radius of
the ellipse or peanut-shaped diagrams represents Zxy, which is the magni- tude of the electric field in the direction of a given radius generated by a unit orthogonal horizontal magnetic field. Each diagram is normalized by
its maximum value of Zxy. The orientations are generally consistent with the regional structure. For example, site 2 is highly anisotropic (the ellipse is narrow) near the Keweenaw fault (between sites 1 and 2) and sites 3 and 4 are more isotropic near the center of the survey, where the edge effect of the fault is weaker. The impedance anisotropy change at sites S and 6 may be due to a nearby fiult, as expressed in our model, discussed later. Site 7 is nearly isotropic. c MODELING AND INTERPRETATION
MT data sets ( or the results of model computations) are often sum- marized as a pseudo-depth section (Fig. 4A, for example). In this display, the sites are indicated at the top of the cross section. Data at each station are tabulated by frequency: the highest frequency is at the top, representing the shallowest penetration; the lowest frequency is at the bottom, repre- senting the deepest penetration at that site. The section thus compactly displays data for the entire profile, but the vertical dimension is distorted along the profile du e to the variation of penetration depth with resistivity along the section. Our Pxy (p-parallet) data are presented as; a pseudo depth section (Fig. 4A). They show: (1) generally high apparent resistivities at great depth (low frequencies), representing volcanic rocks; (2) an abrupt transi- tion between sites 1 and 2, marking the known location of the Keweenaw Figure 4. Comparison of calculated and observed results pre-
fault; (3) relatively thin sediments at site 4, indicated by the 40-ohm-m sented as pseudo sections: A. pxy data, B. p^ computations, C. tipper contour which is close to the surface; and (4) low resistivities at great depth data, and D. tipper computations. The units of the resistivity com ours beneath sites 6 and 7. are ohm-m, and the tipper contours are unitless.
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4, 2 10 60 6 7 V V JLI A. Figure 5. Final two-dimensional resistivity model 40 40 1400 cross section of transect for frequencies of 0.094, 0.047, and 0.0117 Hz. Resistivities are given in ohm-m. °r—i—? 1400 / n L
One-dimensional models found to fit p^ apparent resistivity and The main similarities are high tipper magnitudes at frequencies below .3 phase have been given by Repasky (1984). The phase response was useful Hz and high values at intermediate frequencies at site 1. The main differ- in indicating that at least two conductive layers of different resistivity were ences are the high tipper magnitudes in the data at 9, 36, and 96 Hz for needed to model the conductive sandstone. Synthetic data generated from sites 4, 5, and 6. These large tipper values must represent a three- these models match the above data very well, but there are obvious incon- dimensional structure near sites 5 and 6. The inhomogeneity that gives rise sistencies near the Keweenaw fault and at depth, which indicates the need to the high tippers does not appear on the magnetic anomaly maps of for two-dimensional modeling. Further modeling is also needed to investi- Meshref and Heinz (1970) or on Bacon's (1966) gravity map; thus it gate effects of the dip of the Keweenaw fault. represents a contrast in resistivity properties only, not in magnetic proper- A finite-element, two-dimensional modeling program (Stodt, 1978) ties. It may be a fault or fracture, associated with basement uplifting, which
was used to generate synthetic pxy and pyx values for comparison with is locally filled with fluid. data. The program computes synthetic data over the top edge of a grid A geological cross section interpreted from the geoelectrical model is representing a cross section of the Earth. The rectangular grid consists of shown in Figure 6, obtained by smoothing the rectangular boundaries in 20 vertical elements and 90 horizontal elements. The user compares the the geoelectric model and assigning rock types according to available results of a computation to the field data, makes changes in the model in surface exposures. The unit indicated as Keweenaw Volcanics includes an attempt to improve the fit, and reruns the program. both Portage Lake Lavas and Powder Mill Volcanics, which are indistin- Model responses were computed for frequencies of 96, 36, 9, 4.5, guishable on the basis of resistivity. 1.12, 0.374, 0.094, 0.047, and 0.0117 Hz. The model cross section for Figure 6 indicates that the Jacobsville Sandstone is about 2 km thick frequencies 0.094,0.047, and 0.0117 Hz is shown in Figure 5. The models near the Keweenaw fault. The base of the sandstone is warped into an for the other frequencies have only minor changes and are not shown. Due asymmetrical anticline that is parallel to the Keweenaw fault, with gentler to program limitations, the model for the highest frequencies does not dips to the northwest. Along the crest of this feature, the Jacobsville extend under site 8 at the left, and thus the synthetic data pseudo section is Sandstone is about 1 km thick. Southeast of this anticline, the underlying, blank. The quality of fit is demonstrated by the comparison of field data more resistive unit seems to be faulted down, and the Jacobsville Sand- and computed results for the p-parallel (p^,) resistivities shown in Figures stone thickens to about 2 km. 4A and 4B. p-perpendicular (pyx) data and the model results indicate a The high resistivities of the basement northwest of this fault could be similar quality of fit and are not shown. All of the features in the data either the Portage Lake or the Powder Mill volcanics extending beneath enumerated above are present in the synthetic data of Figure 4B. the sandstone. Southeast of the anticlinal crest, a thin resistive zone, proba- The p-parallel computation also yields tipper magnitudes, which are bly part of the Powder Mill group of mafic volcanics, separates the Ja- compared to data in Figures 4C and 4D. The model was not adjusted to cobsville Sandstone from much more conductive strata below. The produce a best fit to the tipper data, and the pseudo sections are presented conductive material is interpreted to be slates of the Michigamme Forma- primarily to indicate differences between the computations and the data. tion, which crops out at the south end of Keweenaw Bay. The model
ry] Freda ITTI Keweenaw | I JacobsvUe HX] Michigamme hw resistivity I'-' I Sandstone I A| Volcanics I I Sandstone IMJ Formation regions
Figure 6. Geologic section interpreted from final two-dimensional model.
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resistivities are consistent with Heiland's (1968) values for graphitic slates The two-dimensional modeling procedure has allowed identification (3.5 ohm-m and lower). The data also indicate a. vertical conductive zone of certain features in the data which are probably due to local structures, between sites 1 and 2 which may be brecciated material in the Keweenaw and has allowed an estimate of the dip of the Keweenaw fault. fault. This is interpreted as a fault zone with less than 1 km offset. Meshref and Hinze (1970) also show a fault near here in their interpretation of ACKNOWLEDGMENTS aeromagnetic data. Small 2-ohm-m zones under sites 3, 4, and 6 were needed to force a The field work and modeling in this paper was carried out by match to the high-frequency data at these sites. Such local conductive T. Repasky. C. Young condensed the thesis into this manuscript and ¡tdded zones in the Jacobsville Sandstone are expected from ground-water hy- new explanations and figures. The writers sincerely appreciate the assist- drology studies, which indicate local regions of highly mineralized ground ance of J. C. Rogers and students at Michigan Technological University water (Allan Johnson, MTU Institute of Mineral Research, 1985, personal who helped with the field work, J. Kalliokoski for advice regarding the commun.) geological significance of our findings, and an anonymous review sr for thoughtful suggestions which helped us improve the paper. Significant CONCLUSIONS support was received from the Board of Control of Michigan Technologi- cal University for the purchase of the magnetotellurics equipment. The This study shows that the Jacobsville Sandstone consists of three research was supported by a Creativity Grant from Michigan Technologi- layers of differing resistivities. The top unit, about 450 m thick, is the most cal University. resistive with a resistivity of about 60 ohm-m. This layer is underlain by
about 300 m of more conductive material of about 10 ohm-m. The lower REFERENCES CITED
unit of the Jacobsville Sandstone has a resisti'/ity of about 40 ohm-m. Abo, G. D., 1969, A reflection seismic investigation of thickness and structure of the Jacobsville Sandstone, K eweenaw These three resistivities may represent different water content and porosity, Peninsula, Michigan [M.S. thesis]: Houghton, Michigan, Michigan Technological University. Bacon, L O., 1966, Geologic structure east and south of the Keweenaw fault on the basis of geophysical evidence, in which may imply significant changes in the depositional history of the Steinhart, J. S., and Smith, T. J., eds., The Earth beneath the continents: American Geophysical Union Monograph 10, p. 3-27. sandstone. There may also be small, highly conductive units within the Berdachevsky, M. N., and Dmitriev, V. I., 1976, Basic principles of interpretation of magnetotelluric sounding curves, in Jacobsville Sandstone, which may contain saline water indicated as small Adam, A., ed., Geoelectric and geothermal studies: KAPG Geophysical Monograph: Budapest, Hungary, Aka- demiai Kiado, p. 165-221. near-surface inclusions in Figure 6. Overall, the thickness of Jacobsville Heiland, C. A., 1968, Geophysical exploration: New York, Hafner Publishing Co., 1013 p. Kalliokoski, J., 1982, Jacobsville Sandstone, in Wold, R. J., and Hinze, W. J., eds., Geology and tectonics o:' the Lake Sandstone ranges lietween 1 and 2 km. Superior basin: Geological Society of America Memoir 156, p. 147-156. Kaulmann, A. A., and Keller, G. V., 1981, The magnetotelluric sounding method: New York, Elsevier Scientific By varying the model, it was found that the Keweenaw fault has a dip Publishing Co., 595 p. of about 65°, with a resolution of ±20°. The data indicate high resistivity Meshref, W. M., and Hinze, W. J., 1970, Geologic interpretation of aeromagnetic data in western Upper Peninsula of Michigan: Michigan Geological Survey Report of Investigations 12. northwest of the Keweenaw fault, representing the Portage Lake Lavas. Repasky, T. R., 1984, Magnetotelluric profile of the Jacobsville Sandstone [M.S. thesis]: Houghton, Michigan, Michigan Technological University. Rocks underlying the Jacobsville Sandstone also show high resistivities Stanley, W. D., Saad, A. R., and Ohofugi, W., 1985, Regional magnetotelturic surveys in hydrocarbon exploration, and consequently are interpreted as basalt. This electrical basement can be Parana basin, Brazil: American Association of Petroleum Geologists Bulletin, v. 69, p. 346-360. Stodt, J. A., 1978, Documentation of a finite element program for solution of geophysical problems govencd by the traced southwestward from the Keweenaw fault across a faulted anticline. inhomogeneous 2-D scalar Helmholtz equation: University of Utah Technical Report, National Scienc: Founda- tion Grant AER 76-11155. From correlation with outcrops along the southeast edge of the Jacobsville Van Horn, J. E., 1980, Seismotectonic study of the southwestern Lake Superior region utilizing the upper Michigan- Sandstone, the high-resistivity basement is interpreted as Powder Mill northern Wisconsin seismic network [M.S. thesis]: Houghton, Michigan, Michigan Technological University. Vozott, K., 1972, The magnetotelluric method in the exploration of sedimentary basins: Geophysics, v. 37, p. <>8-141. Lavas. If this is so, the lower Keweenawan Powder Mills stratum probably Wanna maker, P. E., Hohmann, G. W. and Ward, S. H., 1984, Magnetotelluric responses of three-dimensional bodies in extends up to the Keweenaw fault and quite possibly may exist on the layered earths: Geophysics, v. 49, p. 1517-1533. north side of the fault as well, below the Portage Lake Lavas. This MT transect has determined resistivity values and structure for a deep part of a Proterozoic basin. The structural interpretation conforms with previous interpretation, especially with that of Meshref and Hinze MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 12,1985 REVISED MANUSCRIPT RECEIVED JANUARY 6,1986 (1970), made essentially on the basis of aeromagnetics. MANUSCRIPT ACCEPTED JANUARY 13,1986
Printed in U.SA
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