Potential-field geophysicists have been aware of the delete-rious effects of glacial till on magnetic maps as far back as the1940s. However, in the last decade or so, a new generation ofgeologists and geophysicists entering the potential field arenadoesn’t seem to have as solid a footing in geology as the ear-lier generation, and they are often unaware of the problemsthat can arise when performing low-level aeromagnetic sur-veys over areas covered by glacial till. The present paper waswritten to fill this information gap. In 1940, Nettleton inGeophysical Prospecting for Oil referred to “troublesome” prob-lems with magnetics in glacial areas; in 1946, Heiland inGeophysical Prospecting stated: “Magnetic interference mayoccur... by magnetic rocks in glacial drift;” and in 1951, Vacquieret al. (Interpretation of Aeromagnetic Maps) stated (p. 1): “Thereis some benefit [to be] derived from removing the magne-tometer a few hundred feet above glacial till and other near-surface magnetic disturbances ...” Today, however, one neverhears about problems resulting from low-level magnetic fly-ing over areas of glacial deposits, although they commonlyoccur without being recognized. Such problems can be astrongly limiting factor in the accuracy of interpretations fornear-surface magnetic sources and for any attempt to mapsources within the sedimentary section via so-called HRAM(high-resolution aeromagnetic) or “depth slicing” methods inglacial till covered areas.

Brief tutorial on Pleistocene glaciation in North America.Mountain glaciers, with their conspicuous lateral and termi-nal moraines, were observed from the earliest days of geol-ogy in the 1600s and 1700s, but continental glaciation was notrecognized until the early 1800s. The name most often asso-ciated with glaciation is that of Swiss geologist Louis Agassiz(1807-1873), who became aware of its validity in 1836 after dis-cussions with Johan von Charpentier, another Swiss geologist.Agassiz was able to convince many of his reluctant colleagues,both on the continent and in the British Isles, of the reality ofa gigantic ice cap covering all of northern Europe, similar tothe present-day ice cap on Greenland. In 1847, Agassiz leftEurope and accepted a professorship at Harvard, transferringhis knowledge of glaciation across the Atlantic. By the turn ofthe 20th century, glacial studies by geologists on both conti-nents had established the boundaries of glaciation in the north-ern hemisphere. In North America the presence of twoprincipal ice sheets was documented, the “Labrador” on the

east and the “Keewatin” in the west, that covered almost allCanada and extended into the northern United States (Figure1). Where the two sheets coalesced, as they did in the last glacia-tion, a single sheet, the “Laurentide,” resulted. Rough calcu-lations of the thickness of ice in this sheet based on crustalrebound in the Hudson Bay area are approximately 2800 m.However, many geologists believe that figure should be muchhigher. The two present icecaps, Greenland and Antarctica,have, at the present time—not a glacial period but an “inter-glacial”—approximate maximum thicknesses of 4000 m and3000 m, respectively. In the mid-1900s, four accepted glacial

542 THE LEADING EDGE JUNE 2004 JUNE 2004 THE LEADING EDGE 0000

Coordinator’s comments: This article is an excellent synopsis of a very realproblem—that of geologically derived “noise” contaminating a deeper sig-nal. However, many designers of magnetic surveys would not agree withGay’s recommendation to fly higher above the ground to suppress the highfrequency contamination. They would argue that it is better to sample theglacial signal accurately by flying low, and then to do one’s best to elimi-nate the glacial signal with digital filtering, rather than using flying heightas a de facto filter. Furthermore, there may be unexpected value in the shal-low signal. In Canada, such shallow signals have been used in sedimentarybasins to find kimberlites and gravel deposits, as well as to map shallow gasreservoirs and drilling hazards caused by boulder filled channels.

Glacial till: A troublesome source of near-surface magneticanomaliesS. PARKER GAY JR., Applied Geophysics, Salt Lake City, Utah, U.S.

THE METER READERCoordinated by John Peirce

Figure 1. Extent of Pleistocene glacierization of North America. (Fromvon Engeln, 1949, after R. S. Tarr and L. Martin, CollegePhysiography, MacMillan, 1915.)

Figure 2. Photograph of glacial till in an esker at Hopeville, Ontario,Canada. Note the large percentage of pebbles and boulders resulting fromreworking of the till by fluvial processes. (From Saunderson, “Bedforddiagrams and the interpretation of eskers” in Research in GlacialSystems, 1982.)

JUNE 2004 THE LEADING EDGE 543

Table 1. Areas containing documented magnetic anomalies over glacial tillEx. No.1.

2.

3.4.

5.

6.

7.

8.

9.10.

11.12.13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.24.

25.

26-29.

LocationNorthern New York

StateWestern New York

StateSouthern MichiganSouthern MichiganAlbion-Scipio Oil FieldNortheast Wisconsin

U.P. of Michigan

E. Minnesota

E. Minnesota

N. MinnesotaNE Montana

S. MontanaNW Ontario, CanadaN. Ontario, Canada

N. Saskatchewan,CanadaNW Alberta, Canada

W. Alberta, Canada

NE Alberta, Canada

Central Alberta, Canada

N. Central Alberta,Canada

SE Alberta, Canada

NE British Columbia,Canada

SE Alaska

AlaskaAlaska

Offshore U.K. in NorthSea

NE Montana-NW N.Dakota

DescriptionLake Sylvia, a dammed river drainage, is filled with glacialboulders. Documented by AMS*, GMS*, and a drill hole.

Cayuga and Seneca Lakes, two of the Finger Lakes, arealso filled with magnetic glacial materials. AMS

MS Thesis—GMS profiles over eskers.Glacial till anomalies caused problems in interpretation of

“near-surface magnetic sources.” AMSGlacial till anomalies resulted in erroneous depth-to-base-

ment calculations. AMSAMS anomalies over glacial till in the Ontonagon River

valley were misinterpreted.Many river valleys filled with glacial materials mapped by

AMS.Other river valleys in an adjacent county show well on

shade-relief magnetics. AMSDrumlins cause multiple anomalies on AMS.Both present-day and older, buried drainages with glacialmaterials cause AMS anomalies.Lateral moraines of a mtn. glacier are mapped by AMS.Glacial till anomalies mapped.Mag. lows on AMS caused by removal of glacial till in riverbeds.Drumlin covered area is blanketed by magnetic anomalies.

River drainages coincide with mag. anomalies similar toothers shown here, although the author believed in anonglacial source.

Multiple short-wavelength anomalies drilled for kimberlitesproved to be glacial till.

Same results as 16.

Same results as 16 and 17.

Same results as 16, 17, and 18.

Magnetic anomalies along river systems—not mentionedby authors as due to glacial till.

“...surficial magnetic layer [of]...glacial deposits.”

End(?) morraine from mtn. glacier causes very stronganomaly, possibly from gabbro boulders.

Wide valleys infilled with glacial deposits. AMSMultiple narrow river drainages are infilled with glacial

deposits. AMSDrainage system over glacially infilled rivers on formerly

high and dry British shelf, now inundated by North Sea.AMS.

Many river drainages, both old and present-day, are wellmapped by properly processed AMS.

SourceC. Ludwig, pers.comm. 2001

E. deRidder, pers.comm. 2001

Pub. by Onesti and Hinze, 1970P. Millegan, pers.comm. 2003

pers.comm. 2002 Anon

pers.comm. 2002 Anon

Pub. by V. Chandler, 1985

Pub. by Boerboom, 2001

R. Ikola, pers.comm. 2002S. P. Gay, 1990

T. Krebs, pers.comm. 2002S. Hogg, pub.on Web site, 2002S. Hogg, pers.comm. 2002

Pub. by Kornic, 1983

Pub. by T. Glenn, 1997

T. Rich, pers.comm. 2003

Shear Minerals, 2001 An.Report

Montello Resources, AGS, 2000

Keith Jones, pers.comm.2003

Pub. by LeBlanc and Morris,1997

Thurston et al., 1997

Schnepfe, pers. comm., 1990

J. Cady, pers.comm. 2002J. Cady, pers.comm. 2002

J. Rowe, pers.comm. 2002

S. P. Gay, 2002

*Abbreviations: AMS = airborne magnetic survey; GMS = ground magnetic survey

periods had been recognized in North America (“old divi-sions”), but newer studies, based on modern age-dating tech-niques, reveal at least 10 different glacial pulses going backapproximately 800 000 years into the Pleistocene (Martini etal., 2001).

Continental glaciers remove rock from the underlyingbedrock by a process called “plucking,” or “quarrying.” Asthe glacier moves, these quarried blocks and boulders, as wellas soil and alluvial materials lying on the surface of the ground,especially in river drainages, lakes, etc., including previousglacial materials, become incorporated into the glacier. Sincethe accumulation centers of ice in North America are under-lain by igneous and metamorphic rocks of the Canadian Shield(“basement” in the oil patch), all glacial materials contain acomponent of these rocks and are therefore magnetic. Figure2 shows a glacial esker with typical unsorted, unstratifiedpebbles and boulders of these basement rocks in a matrix ofclay and silt.

Figure 3 shows the extent of glacial drift in the northernUnited States as per the old division of glacial epochs. A com-parison of this figure with a map of petroleum basins in NorthAmerica reveals that till covers the following basins (from eastto west):

1) the northern one-third of the Appalachian Basin2) 100% of the Michigan Basin3) the northern two-thirds of the Illinois Basin4) the northern two-thirds of the Williston Basin5) 100% of the Western Canada Basin, and, in fact, all of

Canada.

Types of glacial deposits and landforms and their magneticresponse. Glacial deposits take on many forms because of thevaried nature of glacial movement and because of later rework-

ing of the glacial deposits by water. The materials depositedby the glaciers are generally termed “till” and their landformsare termed “moraine,” although some geologists use the termsinterchangeably. Ground moraine is a blanket of till of vary-ing thickness laid down beneath a moving glacier. End moraineis deposited at the end of a moving glacier where melting andforward movement are in balance.

A “knob and kettle” topography is left by a glacier melt-ing in place, where till sloughs off large blocks of unmeltedice and is deposited on the sides in “knobs,” leaving a lowarea, a “kettle,” where the block of ice once sat. The famouscranberry bogs of New England are such glacial kettles.

Eskers are long, narrow, raised deposits left by streamsflowing beneath, through, or on top of glaciers. Drumlins aredeposits of “bottom” till in the form of long, low moundsshaped by the glacier moving over them. A number of otherglacial landlords are recognized, but the ones I’ve mentionedwill suffice for this brief discussion of typical magnetic anom-alies. In Figure 4, I show the magnetic responses of seven ofthe most common glacial landlords, or combinations thereof.A brief perusal of this chart by the interested reader is rec-ommended.

Examples of magnetic anomalies mapped over glacialdeposits. To demonstrate the common occurrence of magneticanomalies over glacial deposits, I thought it useful to gatheras many field examples of these types of anomalies as possi-ble.

Some came from my own files and some from the litera-ture, but the majority came by way of telephone calls fromgeophysical colleagues in the United States and Canada. Inall, 29 examples, or example areas, have so far been obtained.(Additional areas are solicited.) Their locations are shown inFigure 5 and listed in Table 1. In 11 of the 29 examples I show,

544 THE LEADING EDGE JUNE 2004

Figure 3. Glacialdrift sheets of thecentral andnorthern UnitedStates. (From vonEngeln, 1949,after Richard F.Flint, 1947,Glacial geologyand thePleistoceneepoch.) This 57-year-old mapshows the agedivisions of theglacial drift asworked out in theearly to mid-20thcentury.

there was controversy as to the source of the magnetic anom-alies; that is, the anomalies were at first incorrectly ascribedto sources other than glacial till. Such controversies clearlyaccentuate the need for an up-to-date paper on this subject,with examples like the ones shown.

Space limitations permit discussion and illustration ofonly five of the examples obtained. (The complete text of thispaper, with all examples, may be found at www.appliedgeo-physics.com, as paper 47 under “Papers and Publications.”) Thefirst example I show here (Figure 6) is an aeromagnetic sur-vey over a lake adjacent to the famed Balmat zinc mine inupper New York state (example 1 in Table 1). Project geolo-gists in the 1980s believed that the aeromagnetic high coinci-

dent with Lake Sylvia might be associated with an intrusivebody containing similar undiscovered deposits. However,consulting geophysicist Chris Ludwig (who sent this mater-ial in 2002) pointed out that the magnetic high could be mod-eled by a magnetic source at shallow depth beneath the lakebottom. The log of an earlier drill hole located in the mine’sfiles proved this interpretation to be correct. The hole had pen-etrated a thick section of glacially deposited igneous and meta-morphic boulders before being abandoned in these boulders.

Other lakes and river drainages in upper New York state,for example the well-known Finger Lakes, show similar coin-cident magnetic anomalies, and comprise example 2 in Table1 (provided by the late Ed deRidder). Both these correspondto type example GL3 in Figure 4.

Figure 7 is an example (No. 10) from the author’s files thatis similar to E1, except that it occurs in an area of high fre-quency noise (0.2-0.5 nT) over a ground moraine in NEMontana. The anomaly over Big Muddy Creek clearly showsthrough this noise, and can, in fact, be traced north into Canada(on another survey) for a distance of 80 km more to (andthrough) Big Muddy Lake in southern Saskatchewan (No. 26in the appendix). This example corresponds to GL3 in Figure4, and is highly relevant to the theme of this paper in that thespectral signature of background glacial noise overlaps thatof magnetic anomalies that would arise in the underlyingshallow or mid-level sedimentary section. The inset in Figure7 shows a ground magnetic profile in this same area that mim-ics the topographic profile, also illustrating the pervasive mag-netic nature of the till and the pervasive magnetic anomaliesthe till causes. As Thurston et al. (“Structural and lithologicalmapping of Archean and Helikian Basement underlying theLiard Basin, northeastern British Columbia” in Program withAbstracts, CSEG HRAM Forum, 1997) state: “The problem ofseparating magnetic responses from [underlying rocks] iscomplicated by the presence of a surficial magnetic layer ...glacial deposits [that] have wavelengths on the order of 200m ... these may be aliased into the frequency band dominatedby low-amplitude responses in the ... underlying [sedimen-tary section].”

Just north of the area of Figure 7 is a magnetic anomalyin the shape of an oxbow (not shown) that does not correspondto a present-day drainage. It would be a buried or “paleo-drainage,” GL5 in Figure 4, and would have resulted from anearlier glaciation, not the last, or “Wisconsin,” glaciation. Suchox-bows are “common” on the magnetic data in parts of

JUNE 2004 THE LEADING EDGE 545

Figure 5. Locations of documented magnetic anomalies over glacial tillcollected for this report. Numbers by each location may be used to refer-ence the examples in Table 1 and in the text.

Figure 6. Left: Aeromagnetic survey over Lake Sylvia, Balmat mine area,St. Lawrence Co., New York. The drill hole encountered scattered igneousand metamorphic boulders down to TD at 365 ft resulting from rework-ing of glacial till in this paleo-river channel. Right: Ground magneticprofile and modeling results by Chris Ludwig, Denver, show that thegeometry of the interpreted source matches that of a paleo-channel underthe lake.

Figure 4. Types of magnetic anomalies over various glacial landforms.Shapes of the anomalies vary not only with the type of landform but alsowith its strike.

Alberta and Saskatchewan (J. Genereux, personal communi-cation, 2002).

A different type of example (No. 14) is shown on the leftof Figure 8. This is from a magnetic survey over theNEA/IAEA geophysical test area in Saskatchewan, Canada.The published map shows myriad high frequency anomaliestrending northeast that are attributed to glacial sources. Theright side of the figure is an air photo from a Canadian text-book of a “drumlinized” area in Canada (no location given)that would be similar to the area of aeromagnetic anomalies

on the left.Figure 9 shows another unpublished example (No. 13), this

one from the files of consulting geophysicist Scott Hogg ofToronto. Aseries of succinct aeromagnetic profiles over a riverin the James Bay lowlands in northern Ontario, Canada, clearlyshow magnetic lows coincident with a river. Here the glacialtill would have been removed by the river without any sub-sequent redeposition of boulders in the bottom, my exampleGL4 in Figure 4.

Aunique and spectacular example (No. 25) of a glacial river

546 THE LEADING EDGE JUNE 2004

Figure 7. Left: Short wavelength 2nd derivative magnetic contour map over the Early Wisconsin drift sheet in the Plentywood area of NE Montana.Right: Note that the persistent magnetic high is located over the drainage valley of Big Muddy Creek due to deeper glacial till here, i.e. infilling ofreworked glacial boulders. (Thanks to Richard Hansen, of Pearson, deRidder, and Johnson, Denver, for the release of this data.) The inset shows groundmagnetic and topographic profiles of “knob and kettle” topography in a nearby area. Modeling gives a figure of 560 � 10-6 cgs for the magnetic suscep-tibility of the till here.

drainage system mapped by aeromagnetics is shown in Figure10, unique because this area is located under the North Seaand spectacular because of the detail obtained and the opti-mal processing employed by the contractor. Water depthshere are on the order of only 100 m, and the area was dry landduring the glacial epoch due to the accumulation of water fromthe oceans on the earth’s various ice caps. Without knowledgeof glaciation and its geologic and geophysical effects, thisexample would have been difficult to explain.

The remaining 24 examples of magnetic glacial till anom-alies may be viewed on the Web site.

Conclusion. Multiple examples of magnetic anomalies overglacial deposits of various kinds, and an understanding of thecauses and effects of continental glaciation, demonstrate thewidespread occurrence of glacial deposits in northern NorthAmerica and their deleterious effects on magnetic data. Thismuch was known to geophysicists in the 1940s and 1950s, butit needs restating at the present time due to the recent empha-sis on flying magnetic surveys in glacial till covered areas atlow ground clearances (“HRAM”), which causes the anom-alies from till to be greatly enhanced (up to 10 times or more).This problem occurs in all or parts of five major petroleumbasins in North America. In these basins, ground clearancesshould be increased to 300-400 m or more to attenuate theeffects of the till, thus allowing more accurate magnetic inter-pretations to be made.

Suggested reading. “Cenozoic history of northeastern Montanaand northwest North Dakota with emphasis on the Pleistocene”by Howard (USGS Professional Paper 326, 1960). “Vertical mag-netic gradiometer survey and interpretation,NEA/IAEAAthabasca test area” by Kornic (in Uranium Exploration in AthabascaBasin, Saskatchewan, Canada, 1983). Principles of GlacialGeomorphology by Martini et al. (Prentice-Hall, 2001). Glacial andFluvioglacial Landforms by Price (Oliver & Boyd, 1973). TLE

Dedication: I dedicate this paper to two deceased colleagues who contributedvaluable material to the present study; both were excellent, skilled, knowl-edgeable geophysicists: Robert Schnepfe (1933-1991) and Edward de Ridder(1939-2002).

Corresponding author: benagi@aol.com

JUNE 2004 THE LEADING EDGE 547

Figure 8. Left: A busy aeromagnetic map of short wavelength (i.e. nar-row) anomalies from glacial deposits in the NEA/IAEA Athabasca testarea, Saskatchewan. (From Kornic, 1983, Geological Survey of Canada.)Right: Air photo of a “drumlinized area” in Canada. The magnetic mapwould have been flown over such an area. (From Thornbury, Principlesof Geomorphology, 1966, no location given.)

Figure 9. Stacked aeromagnetic profiles over river carved in glacial till,James Bay lowlands, Ontario, Canada. (Data from Scott Hogg, consult-ing geophysicist, Toronto, Canada.)

Figure 10. High-frequency residual magnetic map of an area in the NorthSea showing a drainage system infilled with glacial materials. (Dataprovided by Jeff Rowe, Fugro Airborne Surveys, 2002.)