Magnetic Mapping of the Butterton Dyke: an Example of Detailed Geophysical Surveying
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Journal of the Geological Society, London, Vol. 144, 1987, pp. 29-33, 5 figs. Printed in Northern Ireland Magnetic mapping of the Butterton Dyke: an example of detailed geophysical surveying W.T. C. SOWERBUTTS Department of Geology, University of Manchester, Manchester M13 9PL, UK Abstract: A detailed vertical gradient magnetic survey of part of a small intrusion known as the Butterton Dyke has been made with the aid of a microcomputer-based data gathering system. The results of over 16500 magnetic measurements made by one person in less than a day have given results which reveal a detailed pattern of magnetic anomalies. From these it is possible to trace the course of two dykes for a distance of over 300 m, and identify places where they change direction and showsmall offsets. The advantage of making vertical gradient rather than total fieldmagnetic measurements include a faster surveying speed and better resolution of near-surface anomalies. This paper describes the execution and results of a detailed seconds. One reason for making a magnetic survey of part magnetic survey made over part of a Tertiary intrusion in of the Butterton Dyke was to see if useful information could Staffordshire, England. The surveywas made in order to be obtained using another type of magnetometer, a assesspossible geological applications of a magnetic magnetic gradiometer, whenused with a microcomputer gradiometer connected to a microcomputerbased data based data gathering system. The aimwas to find out if gathering system. measurements of magnetic gradient wouldbe effective at Twodeeply weathered olivine dolerite dykes are defining the course of a shallow igneous intrusion, and if a exposed in a disused quarry at the southern end of Church more detailed survey, than isfeasible with a proton Wood, Butterton, 4 km south of Newcastle-under-Lyme, magnetometer, wouldreveal information about fine England (Grid Ref. SJ84; 434420, Fig. 1). The host rock is structure. Upper Coal Measures sandstone. In the quarry face the The magnetometer usedwas a PhilpotModel AM0 dykes are about 1 and 1.2m wide, about 3 m apart, and vertical magnetic gradiometer. The instrument contains two trend approximatelyNNW. Samples of dolerite fromthis fluxgate sensors mounted at opposite ends of a 0.5 m-long quarry have been radiometrically dated and their palaeo- vertical tube. Electronics provides a measure of the magnetism studied. The dolerite gives a K-Ar date of difference in the vertical component of the magnetic field at 52 f2 Ma (Evans 1969), and is reverselymagnetized the two sensors and this is taken as the vertical magnetic (Dagley1969), giving a magnetic pole positionconsistent gradient, in units of nT/0.5 m. This instrument can be used with the Tertiary age. Other exposures of the Butterton to make spot readings, but since its response is effectively Dyke have been recorded. One is about 1 km NNW of the instantaneous, it can alsobe used to give continuous Church Wood Quarry, the others are all to the SSE, the measurements when carried along. When used in thisway, it nearest 2 km distant (Gibson 1925). The area between these is advantageous to use an automatic method to read and sites is mostly gently undulating farmland with few bedrock record its output rather than try and do so manually. exposures. Magnetic surveys suggest the dyke is present at For thissurvey, values of magnetic gradient were Keele University, 5 kmNNW of the quarry (N. J. Kuznir automatically read and recorded by a geophysical data pers. comm.). It is assumed that these isolated occurrences gathering systembased on a battery poweredmicrocom- represent part of a single intrusion. It is not known if it puter (Sowerbutts & Mason 1984). The survey was made by subcrops beneath soil and glacial drift at all points between carrying the magnetometer along a total of317 parallel these known occurrences, or if the top of the intrusion traverses 1m apart, with the computer recording the occurs deeper at someplaces. A number ofNE-SW- magnetometer output at 0.5 m intervals along each traverse. trending faults are shown crossing the expected course of Distances along traverses were determined automatically by the intrusion on the Geological Survey map (Gibson 1925). an ultrasonic rangingdevice, also connected to the The intrusion isshown terminating against one of these computer. The survey entailed recording a total of 16865 faults about 40 m south of the ChurchWood Quarry separate magnetic readings. It took about 6 h to complete, exposure. The magnetic survey reported here was made on with the author working alone. About half the time was farmland to the south of the quarry, in the vicinity of this spent setting out and moving a surveygrid and the conjectured fault. equipment, and the other half takingmeasurements. Measurements were started close to the quarry outcrop of the dykes,and the surveygrid shifted assurveying Magnetic survey proceeded and the course of the intrusions was revealed. One of the commonestgeophysical methods used for The results were stored in the field on floppy disc and later, locating and tracing igneous dykes is the magnetic survey, for convenience, transferred to a mainframe computer for e.g. Goulty et al. (1984). For land magneticsurveys the plotting. anIn attempt to determine the detailed proton magnetometer is probably the instrument most often relationship between the vertical gradient anomalies and the used at the present time. With this instrument, spot readings positionand width of the dykes, magnetic gradiometer of the Earth’s total magnetic field can be made in a few measurements weremade manually on a single traverse 29 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/1/29/4888786/gsjgs.144.1.0029.pdf by guest on 30 September 2021 30 W. T. C. SOWERBUTTS 2' 15'W 2' 14' which canbe described as a single sharp peak with a 52' 59'N shoulder on its eastern flank.Clearly the total field anomalies due to the two dykes are interfering to the extent that the two anomalies can barely be resolved. The vertical gradient profile,although somewhat more complex, does show the anomalies due to the two dykes resolved. Interpretation In simple terms the results of the magnetic gradient survey are interpreted as due to two separate dykes, approximately 52' 58' parallel, which on a large scale form two broad arcs. One dyke, hereafter referred to as Dyke A, extends continuously from the northern limit of the survey area to a point about 200 m further south. The other dyke, Dyke B, is interpreted asbeing discontinuous, occurring on the eastern side of Dyke A in the northern part of the area, on the western side in the south, and with a gap in between. Dyke B extends about 85 m further south than Dyke A. Fig. 1. Location map showing known exposuresof the Butterton Dyke and the area where the magnetic surveywas made. The overall extent of the dykes can be deduced readily from the plots of vertical magnetic gradient. A close study along thetop of the quarry facewhere the dykes are of the contour map shows that on a small scale the dykes do exposed. Also, to compare the form of total magnetic field nothave a smooth continuous arcuate outline but show and vertical magnetic gradient anomaliesover the same considerablecomplexity and many irregularities. A map feature, measurements of the total magnetic field were made showing a more detailed interpretation of the magnetic on this traverse using a proton magnetometer. anomalies isshown in Fig. 4. In a number of places the dykes appear to show sudden changes in direction and small lateral offsets. The most pronounced offset occurs at about Survey results the 160 m position. This is not a simple offset because the The results of the main magnetic survey are presented as a dyke separation on either side of the offsetposition is contour map and isometric plot (Fig. 2). The main feature is different and the displacement directions across the offset a pair of linear magnetic anomalies, approximately parallel, are different for the two dykes. The magneticanomalies extending southward for a distance of over 200m. On a decrease inmagnitude and increasein width abruptly on large scale the pair of anomalies form two broad arcs which passing south acrossthis offset. Continuing south the intersect in a cusp. Each anomaly consists of a narrow zone magnitude of the anomaliesgradually increases and their of positive values of vertical magnetic gradient flanked on width decreases, so that 25 m south of the offset they show the eastern side by a zone of negativevalues. In a few values similar to those north of the offset. This behaviour places, notably in the northernmost 25 m of the area, there suggests that the depth to the top of the dykes increases is a flanking zone of negative values on the western side abruptly passing south acrossthis offset, then gradually also. The total width of each linear anomaly is generally decreases over the next 25 m to valuessimilar to those between 6 and 8 m, the width of the zones of positive and elsewhere. negative values being about equal. Their magnitudes differ, When interpreting magneticsurvey results, numerical the positivevalues being mostly 3-4 times greater than modelling isoften applied to determine the approximate size corresponding negative values. The maximum magnitude of and depth to the top of magnetized geological features. Such the positivevalues mostlyis in the range +40 to modelling can also give information about the nature of the +60 nT/0.5 m (+80 to +l20 nT m-') whereasnegative magnetization. Techniques and computer programsfor values reach only 15 to 20nT/O.5 m (-30 to -40nTm-l). dealing with total magnetic field anomalies over geological Largest values occur at the northern end of the survey area, features are readily available, and these can be adapted for and are +270 and -140 nT/0.5 m (+540 and -280 nT m-').