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A Magnetotelluric Transect of the Jacobsville Sandstone in Northern Michigan

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A Magnetotelluric Transect of the Jacobsville Sandstone in Northern Michigan 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/6/711/3445109/i0016-7606-97-6-711.pdf by guest on 26 September 2021 712 YOUNG AND REPASKY -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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/6/711/3445109/i0016-7606-97-6-711.pdf by guest on 26 September 2021 MAGNETOTELLURIC TRANSECT OF JACOBSVILLE SANDSTONE, MICHIGAN 713 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<c 1 \ \ 1 10 c The gravity and magnetic data indicate an absence of significant three- a. * t it J\ Ì \ <Q. \ dimensional structure, suggesting that two-dimensional modeling is proba- \ bly adequate for our data set.
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