Publication - Research and analysis

Hill of Banchory geothermal energy project: feasibility report

Published: 23 Mar 2016
Directorate:
Energy and Climate Change Directorate
Part of:
Environment and climate change
ISBN:
9781786520753

Report of a study which explored the potential for a deep geothermal heat project at the Hill of Banchory, Aberdeenshire.

Hill of Banchory geothermal energy project: feasibility report
6. Expanding the database: additional survey work on the Hill of Fare granite

6. Expanding the database: additional survey work on the Hill of Fare granite

Although the existing data for the region have provided the bulk of the evidence used to develop this analysis, the lifetime of the feasibility project provided an opportunity to collect some more detailed site-specific data for the Hill of Fare pluton, to help constrain geothermal resource estimates. Results of this survey work are summarised in this chapter.

6.1. Heat Production

What is heat production?

Heat production is the heat energy produced in each cubic metre of rock due to radioactive decay of naturally occurring radio-elements. Many granites, such as the Hill of Fare Pluton, have elevated concentrations of uranium (U) and thorium (Th) compared to other rocks, as well as a significant component of potassium (K). These elements have radioactive isotopes which produce heat during decay. If present in large enough quantities then the heat produced by the decay of K, U, and Th can significantly elevate the geothermal gradient making higher temperatures accessible at shallower depths.

The heat production rate of the Hill of Fare has previously been estimated at 3.9μW/m 3 (Downing and Gray 1986). However only two samples were used to make this estimate; as such there would be significant risk when using this value for predicting the geothermal gradient of the Hill of Fare granite. We conducted a new survey of the Hill of Fare granite to produce a robust value of heat production.

Method

Field Methods

The method used is this study is consistent with that shown in McCay et al. (2014). That paper also provides a comprehensive but accessible introduction to gamma-ray spectrometry in geothermal exploration.

A hand held portable gamma-ray spectrometer was used to measure concentrations of K(wt%), U (ppm), and Th (ppm.) The equipment used was a GSII model spectrometer produced by GF instruments in Brno, Czech Republic. This model of spectrometer automatically produces concentration of K, U, and Th without the need for calibration or manual calculation.

To take a measurement the gamma-ray spectrometer is placed onto the rock and held there for several minutes. In this study, each measurement lasted five minutes as this has been found to be a sufficient trade-off between accuracy and speed when using a GSII on such moderately radioactive granites. Measurements were repeated until two consistent values were found. Consistent readings were judged as being less than 1ppm difference in uranium or 10nGy/hr difference in total dose rate.

Measurement locations are ideally flat surfaces with good rock exposure. This minimises any over or under-estimation that may affect the results, due to local topography, vegetation cover, or preferential erosion of grains that contain K, U, or Th. The Hill of Fare granite is poorly exposed and highly weathered in most locations. As such, most measurement locations were either likely underestimates or overestimates of the true concentrations of K, U, and Th; the following section outlines how these were dealt with.

Data Processing

In total 38 sample locations were measured of the Hill of Fare granite, 3 of the microgranites within the Hill of Fare, and 6 measurements of the surrounding Crathes granite. At least two consistent measurements were collected at each sample location. When just two measurements were taken then the first of these was used for the representative K, U, and Th concentrations. If three or more measurements were taken then the middle value was used.

Many sample locations were taken from abandoned quarries which are located around the Hill of Fare granite. These quarries have the advantage of being relatively fresh faces so have not undergone erosion to the same degree as natural exposures. However 10 of these sample locations had the disadvantage that the nearby quarry walls and ledges created overestimated results. There are no standard methods detailed in the gamma-ray spectrometry literature to deal with such problems, so a novel solution is proposed here. K% is often the most consistent of the three radio-elements across granites and it is the change in U and Th concentrations that generally determine higher and lower heat production zones. K is typically between 3.8 and 4.5% in granite intrusions of the Cairngorm Suite. However K% values in the quarries were consistently higher than this indicating that they are likely overestimates due to the other quarry walls and surfaces. This was tested by taking a reading in the corner of a quarry wall which should lead to an overestimation by 50%. The K% value from this test was 7.14 which is 58% higher than an assumed reasonable value of 4.5. This test shows that the assumption of using K% as an indicator of overestimation is reasonably sound. Therefore, all the heat production values taken from the quarries have been lowered by the ratio that their K% estimate is higher than 4.5.

Nine values have not been used for the Hill of Fare granite because they were identified as likely underestimates. This underestimation results from topography of the rock or partially exposed rock surfaces where it isn't clear if we are sampling bedrock or a large boulder. Due to the small number of measurements in the micro-granite and Hill of Fare granites all of these values were used to estimate heat production of those units.

The following equation was used to calculate the heat production value of each of the measurements of K, U, and Th:

HP = ρ{(0.035C K)+(0.097C U)+(0.026C Th)}

Where ρ is the granite density in (kg m -3) C K is concentration of potassium by % weight, and C U and C Th are concentrations of uranium and thorium in parts per million.

Data and Results

The raw data are contained within Appendix 4, which also explains which samples were used to calculate the averages shown below, and which samples were discarded due to underestimation errors, or else corrected for overestimation.

Table 1: Heat Production Values for the Hill of Fare Granite, Hill of Fare Microgranite and the neighbouring Crathes Granodiorite

Sample Location

Mean Heat Production (μW/m 3)

Hill of Fare Granite

4.04

Hill of Fare Microgranite

3.13

Crathes Granodiorite

2.09

The average value of 4.04μW/m 3 is slightly higher than the previous estimate of 3.9μW/m 3, however the new value is taken from 29 samples across the Hill of Fare granite so is much more robust than the earlier figure. The Hill of Fare granite has a lower heat production compared with other high heat production granites in the East Grampians, i.e. Cairngorm (7.3), Ballater (6.8), Mount Battock (4.8), and Bennachie (7.0), however the heat production of Hill of Fare is significantly higher than several other granites (e.g. Crathes Pluton (2.09) or Aberdeen granite (2.2)) and the surrounding Dalradian metasedimentary rocks.

6.2. Temperature Projections

The method used for predicting subsurface temperatures in the granites is consistent with previous geothermal studies into the East Grampian granites (e.g. Downing and Gray (1986)). The use of consistent methods allows direct comparison between the Hill of Fare and the other granite bodies of the East Grampians, as well as to radiothermal granites elsewhere. These predictions have also been tested against other methods for predicting subsurface temperatures to add robustness and confidence in the predictions; these other methods are discussed in Westaway and Younger (2013).

The following equation has been used for the temperature predictions.

T z = a' exp[(q 0z - f(z))/a'λ 0)]-b'

Where: T z is the temperature at a given depth (⁰C), q 0 is surface heat flow (mW/m 2), z is depth (m), λ 0 is surface thermal conductivity (W/m/k). b' is a constant of 823.33 and a' = b' - θ g where θ g is mean surface temperature which is taken here at 7.5 ⁰C.

The function of f(z) depends on whether the heat production of the granite is assumed to decline exponentially or linearly.

When heat production is assumed to decline exponentially the function is:

f(z) = A 0D[z - D(1-exp(-z/D))]

When heat production is assumed to decline linearly the function is:

F(z) = [(A 0z 2)/2] - [(uz 3)/6]

Where A 0 is surface heat production (μW/m 3) and u is a constant with the value of 0.3.

These predictions provide a one dimensional heat profile. However the granite will be a complicated three dimensional structure. In reality, areas such as the microgranite with lower thermal conductivity will create a more complicated temperature profile.

The key inputs to the temperature equation are heat flow, heat production and the thermal conductivity. The new robust estimates of heat flow and thermal conductivity (see section 6.3) will be used as inputs. However heat flow is best estimated from temperature data in boreholes several hundred metres deep, preferably 500m - 1 km deep. Lack of suitable boreholes meant that this could not be done during the present project. Fortunately, four neighbouring high heat production granites have previously had heat flow boreholes drilled as part of the original East Grampian geothermal exploration in the 1980s. The four heat flow boreholes were drilled in the Cairngorm, Bennachie, Ballater, and Mount Battock granites. As previously noted, heat flow estimates in these boreholes were found to be surprisingly low. However, it is now clear that these low values were due to routine corrections to palaeo-climate not being applied. Two recent publications have added the palaeo-climate corrections. Westaway and Younger (2013) corrected the Ballater borehole as part of a wider study of the UK's heat flow as well as applying a new topographical correction. Additionally, Busby et al. (2015) applied corrections to all four East Grampian boreholes and significantly upgraded the heat flow estimates. We have also applied the methods of Westaway and Younger (2013) to the Cairngorm, Bennachie, and Ballater granites (Table 2) to independently verify the corrected results presented by Busby et al. (2015). The relatively close correspondence of these two recent suites of estimates is highly encouraging.

Table 2: Heat Flow Estimates of the High Heat Production East Grampian Granites

Granite

Original heat flow estimates (Downing and Gray 1986) (mW/m 2)

Busby et al (2015) heat flow estimates (mW/m 2)

New Heat flow estimates using the method of Westaway and Younger (2013) (mW/m 2)

Cairngorm

69.5

88.9

95

Bennachie

75.8

89.9

87

Ballater

71.4

87.8

89.5*

Mount Battock

58.7

76.3

87

The Hill of Fare granite is in the same suite as these four high heat-producing granites and is geochemically similar. Therefore we can expect the heat flow in the Hill of Fare granite to at least be similar to that of the other four HHP granites, so that we can use these heat flow values to guide the Hill of Fare temperatures prediction.

Table 3: Heat Flow, Heat Production and Thermal Conductivity Estimates for Hill of Fare Granite

Scenario

Heat Flow (mW/m 2)

Heat Production (μW/m 3)

Thermal Conductivity (W/m/k)

Most favourable

90

4.5

3.00

Intermediate

85

4.04

3.16

Pessimistic

75

3.6

3.33

Table 3 shows the values that have gone into creating the three downhole temperature scenarios for the Hill of Fare granite. The intermediate scenario for heat production and thermal conductivity was selected as the baseline for this project. The 'most favourable' scenario was calculated by using the best estimate of heat production from measured values, increased by double the standard error of the mean. The 'pessimistic' scenario for heat production used the same best estimate, this time reduced by double the standard error of the mean. (For the thermal conductivity the favourable and unfavourable scenarios were developed by lowering and raising the maximum density Gaussian kernel by 5%, which is a reasonable range for this relatively invariant parameter). Each of these scenarios was also modelled with two assumptions relating to the decline of heat production with depth, as outlined above (i.e. exponential and linear).

Figure 14: Geothermal Gradient Prediction Scenarios for Hill of Fare Granite

Figure 14: Geothermal Gradient Prediction Scenarios for Hill of Fare Granite

Figure 14 shows the predicted temperatures for both heat production assumptions for each of the three scenarios. The heat production assumptions do not seem to make a significant difference down to 5km depth. The favourable scenario has a geothermal gradient of 29.0⁰C/km, the modest scenario of 25.9⁰C/km, and the unfavourable scenario of 21.1⁰C/km. At 3km depth there is already a 22⁰C difference between the favourable and unfavourable scenarios.

Table 4 indicates the depth each scenario predicts for two possible target temperatures for a heat only geothermal scheme. For a 90⁰C temperature to be reached in the granite there will be a 1km difference in target depths between the favourable and unfavourable scenarios. Even for the 75⁰C temperature the difference remains large at 0.8km. Generally, the difference in depths between the favourable and unfavourable scenarios means that drilling will be around 30% deeper for the unfavourable scenario for a specific targeted temperature.

Table 4: Depth to Typical Target Temperatures

Target Temperature (°C)

Depth to target temperature (km)

Favourable

Modest

Unfavourable

75

2.2

2.4

2.9

90

2.8

3.2

3.8

6.3. Thermal Conductivity

Calculation of heat flow requires knowledge of the thermal conductivity of the rocks. Thermal conductivity is a material's capacity to conduct or transmit heat and is measured in W/mK. Being rich in quartz (the most thermally conductive of the common rock forming minerals), granites are usually good heat conductors. However, site-specific data for the Hill of Fare Granite were not available, so 18 plug samples from the Hill of Fare Granite were analysed using a Portable Electronic Divided Bar.

Methods

Sample Collection

The 18 samples were collected from the Hill of Fare granite and microgranite at three key localities. They were collected using a handheld 1-inch diameter diamond-edged plug drill. Full details of the samples are listed in Appendix 1.

Sample Preparation

The drill core samples (roughly 24mm in diameter) were cut into roughly 1 cm thick slices. During the rock sawing process some of the samples chipped along the edges, most commonly due to pre-existing fractures within the core. These features are noted in Appendix 1.

Sample preparation was an important part of the process as irregularities along the sample / plate contact can impede the heat flow across the sample and can produce lower thermal conductivity values. These irregularities can come in the form of grooved sample faces from the rock sawing process, chips along the edges of the samples, and fractures within the samples. To create the best sample/plate contact they underwent two polishing processes. First, both faces and the edges of each sample were smoothed using grinding wheels of roughness 175 and 40. Another round of polishing was then completed using 300 grit.

Thermal Conductivity Analysis

Data were gathered using a Portable Electronic Divided Bar, which produces a temperature gradient across rock samples and allows thermal conductivity of the samples to be calculated using Fourier's Law.

Once the samples had been prepared the top and bottom faces were coated in petroleum jelly to facilitate good thermal contact. They were then clamped into the divided bar until the sample equilibrated. ∆T (a unitless software calculated parameter) versus time was graphed using a Pico Logger coupled with the divided bar, allowing us to visualise when the sample had equilibrated. Thermal conductivity values were then calculated by inputting sample thickness, sample surface area, sample diameter and ∆T at equilibrium into the master spreadsheet. The following equation was used;

Thermal Conductivity = d/R

R= (A (∆T - c))/ (a (diameter + b))

A = surface area of sample in mm 2

D = diameter of sample in mm

a, b, c = calibration constants

Standards were run every two samples. The standards, provided by HDR, were gabbro, sericite and granodiorite. Readings from these samples during this project were combined with measurements of the standards taken over the lifetime of the instrument. This allowed us to produce "lifetime corrected" thermal conductivity values.

The granites were put into the divided bar with the outermost surface facing up. This was done to allow for future research into weather conductivity values vary when heat flows in different directions through the rock.

Each sample was run three times to increase confidence in calculating an accurate average and to highlight anomalies.

Results

Table 5 details lifetime corrected K values.

Table 5: Lifetime Corrected K Values

Lifetime Corrected K (W/mK) Summary

HoF#

Run 1

Run 2

Run 3

Average

1

3.21

3.22

3.42

3.284

2

3.27

3.26

2.46

2.995

3

3.12

3.10

3.41

3.213

4

2.24

2.24

2.56

2.349

5

2.81

2.82

2.77

2.803

6

2.92

2.94

3.15

3.004

7

2.92

2.90

3.04

2.955

8

2.74

2.71

2.65

2.697

9

3.08

3.06

3.26

3.133

10

2.57

2.56

2.67

2.601

11

3.05

3.03

3.14

3.072

12

3.23

3.21

3.33

3.256

13

3.33

3.32

3.39

3.346

14

3.33

3.30

3.21

3.279

15

3.11

3.10

3.00

3.071

16

2.04

2.02

2.46

2.176

17

3.54

3.53

3.97

3.682

18

3.19

3.15

3.58

3.305

Univariate statistics were deemed inappropriate for this type of data due to the number of variables, so it was fitted to a Kernel Density Estimator ( KDE). Results show that the data show maximum likelihood at a thermal conductivity value of 3.16 W/mK.

Figure 15: Estimated thermal conductivity, Hill of Fare

Figure 15: Estimated thermal conductivity, Hill of Fare

This statistical analysis was carried out by using the following online software - http://www.wessa.net/rwasp_density.wasp.

Discussion

Average thermal conductivity values for the Hill of Fare Granites range between 2.176 and 3.682 W/mK, which is consistent with the range of 2.9 and 3.6 W/mK given for other High Heat Production Granites ( HHP) in Scotland in the Scottish Government paper 'Study into the Potential for Deep Geothermal Energy in Scotland' (Gillespie et al. 2013).

The highest, most consistent results were found amongst the Craigton Main Quarry samples, which yielded values that were consistently above 3 W/mK.

The lowest results were found in the Raemoir North Quarry samples with values consistently at or below 3 W/mK.

No real correlation was seen between thickness and thermal conductivity. Chips, tapers and other minor irregularities in the samples also didn't seem to have a noticeable impact on the results.

The third round of readings show, in general, slightly higher values than the first two. However, within the set they show relatively similar patterns to the first two sets. This systematic discrepancy may have arisen as a result of the laboratory being of a significantly lower temperature than when the first two sets were run. The effect of room temperature on readings given by the Divided Bar is now being interrogated by Town Rock Energy and the University of St Andrews, where the tool is held. For the purpose of this study, the effects on the results are negligible and within the 5% degree of uncertainty.

6.4. Gravity and Magnetic Surveying

Background

Potential field methods are geophysical methods that use variations in the Earth's gravity and magnetic fields ("anomalies" - differences in what are measured from some reference value) to infer variations in the geometry and properties of rocks below the surface that are not otherwise directly observable (as in outcrop geology). In the case of gravity and magnetic methods the rock properties that vary are density and magnetic susceptibility respectively. Regional gravity and magnetic anomalies for an area of north eastern Scotland centred on the Hill of Fare pluton are shown in Figure 16.

Figure 16: (a) Regional Gravity [left-side image], (b) Magnetic Anomalies [right-side image] for a part of north-eastern Scotland ( BGS Data (2002))

Figure 16: (a) Regional Gravity [left-side image], (b) Magnetic Anomalies [right-side image] for a part of north-eastern Scotland (BGS Data (2002))
Contains British Geological Survey materials © NERC 2016

Figure 16 is based on measurements approximately one per 2 km 2 ( BGS, 2002), which gives an indication of its resolving capability. The magnetic map is compiled from aeromagnetic data observed on E-W flight lines with 2 km spacing. The location of the Hill of Fare pluton is schematically indicated by the white ellipse and the white tick mark the location of the local gravity and magnetic surveys carried out as part of this project.

Figure 16 demonstrates that the gravity and magnetic fields of north-eastern Scotland correlate to a large degree with the distribution of igneous plutons (cf. McGregor and Wilson, 1967; Johnston, 2015). Gravity anomalies associated with the Hill of Fare pluton are generally less than surrounding areas and magnetic anomalies generally greater; the latter is most evident although in both cases, for the low resolution, regional data available, any singular Hill of Fare anomaly coalesces with those sourced from surrounding igneous (and possibly other geological) bodies.

Bulk density of upper crustal rocks varies in the approximate range 2,000-3,000 kgm -3. This depends on the constituent minerals of rocks with common rock-forming minerals that are "light-coloured" (quartz, white and pink feldspars such as those that dominate in "felsic" rocks like granites) being less dense than minerals that are "dark-coloured" (amphibole, pyroxene, grey and black feldspars such as found in "mafic" rocks like basalt and amphibolite) are denser. A typical density for the Hill of Fare granite ("biotite granite, pink, leucocratic"; BGS, 2002) would be in the range 2,600-2,700 kgm -3 and according to measurements made by McGregor and Wilson (1967) it is 2,630 kgm -3. The country rocks to the south of the Hill of Fare granite are "psammitic" and "pelitic" (metamorphosed sandstones and shales) metasedimentary rocks of the Argyll Group ( BGS, 2002) and such rocks are likely characterised by higher density than the granite of the Hill of Fare; a value of 2,740 kgm -3 is reported by McGregor and Wilson (1967) for the Dalradian Supergroup, of which the Argyll Group forms one subdivision. Accordingly, the Hill of Fare granite (and associated igneous plutons) should display negative gravity anomalies relative to surrounding areas as is the case in Figure 16.

Bulk magnetic susceptibility of upper crustal rocks lies in the range 0-0.2 ( SI units) and depends on the presence of magnetic minerals such as, especially, magnetite. Magnetite is generally most prevalent in dark-coloured mafic crystalline rocks, less so in light-coloured crystalline felsic rocks and relatively uncommon in (meta) sedimentary rocks. According to McGregor and Wilson (1967) suitable values of magnetic susceptibility are 0.025 ( SI units) for Hill of Fare granite and nil or very small for Dalradian strata. Accordingly, the Hill of Fare granite (and associated igneous plutons) should display positive magnetic anomalies relative to surrounding areas as is the case in Figure 16.

Objectives

The project activity was a small-scale pilot survey aimed at testing whether high resolution (tightly-spaced) gravity and magnetic field measurements along a profile crossing the geologically inferred boundary of the Hill of Fare granite pluton and Dalradian country rocks would be capable of pinpointing its location exactly, possibly with inferences about its subsurface geometry (e.g. dip direction, thickness). The results were expected to provide an indication of whether density and/or susceptibility contrasts would be sufficient for delineation of this boundary in three dimensions from high resolution gravity and magnetics surveying on a two dimensional grid in an area of specific interest such as a potential future geothermal well site.

Data acquisition, reduction and results

The scope of the work and logistical considerations dictated that surveying was limited to several days in the field and at one locality with good road access crossing the inferred Hill of Fare granite-country rock contact. Figure 17 shows the chosen location of the surveying, which took place on the southern slopes of the Hill of Fare on and near the Burnhead Farm road off highway A980 just north and west of the Raemoir House Hotel.

Figure 17 shows the location of the observed gravity and magnetic profiles. Gravity observations were made on 21-23 January 2016 using a Scintrex CG-5 "Autograv" gravity meter. The instrument is extremely sensitive to levelling and therefore ground stability. For this reason measurements were made as much as possible along hard roadbed or other surfaces. Measurements in fields was also possible though more time-consuming, by reinforcing the ground surface with harder materials before measuring. Some 40 stations were observed as shown in Figure 18. Weather conditions prevented measurements being made in the later part of the afternoon of 21 January and all of the morning of 22 January. Suitable daylight was available from about 9 am until 4 pm.

Figure 17: Location of the observed gravity and magnetic profiles

Figure 17: Location of the observed gravity and magnetic profiles
image: OS Digimap

Gravity observations were made every 15-20 m along the Burnhead farm road and in the field north of Burnhead cottage; the western "murky green" solid line represents a profile projection of the gravity data. Magnetic observations were made every ~10 m on a line roughly coincident with the eastern "murky green" solid line. The dotted line indicates the inferred contact between the Hill of Fare granite to the north and Dalradian (Argyll Group) metasedimentary rocks to the south (from BGS, 2002).

Figure 18: Locations of gravity stations along the track from the main road (A980) to Burnhead Farm

Figure 18: Locations of gravity stations along the track from the main road (A980) to Burnhead Farm
Image: Google Earth

Figure 18 shows the locations of gravity stations along the main road (A980) to Burnhead Farm. The Base Station (labelled 8 here) was near the farmhouse and several additional stations, not plotted here, were measured between it and Station 14.

The Earth's gravity field is sensitive to height (altitude) as well as latitude and variability associated with these is not of geological interest so corrections are made to account for them. In particular the relative altitudes of stations must be known very accurately. In the present case, with geological variability in the order of 0.1 mGal, elevations should be known to within 30-50 cm. Differential GNSS equipment was used to determine horizontal as well as sufficient vertical co-ordinates at each measurement station with each observation point requiring some 20 minutes of data accumulation. Analyses during and after the fieldwork, however, revealed that the vertical co-ordinate was not likely recorded with the intended accuracy of ± 1-10 cm although the reason(s) for this are not clear. It is estimated that elevations are known only to ± 1 m at best and this has resulted in some degradation of the resolution of the calculated gravity anomalies.

Figure 19 shows the observed gravity anomalies corrected for latitude and elevation changes, along the projected profile shown in Figure 17. The absolute values of the gravity anomalies do not have significance (they are calculated relative to the base station value at Burnhead cottage) but the change in anomaly value along the line does. The observed anomalies have an absolute range of about 2.5 mGal (milligals). "Noise" levels related to height measurement inaccuracy are in the range 0.5 mGal.

Figure 19: Gravity anomalies along the (westerly) profile shown in Figure 17

Figure 19: Gravity anomalies along the (westerly) profile shown in Figure 17

The arrow in Figure 19 shows the approximate location of the inferred Hill of Fare granite-country rock contact from BGS (2002).

Magnetic observations were made on 9 October 2015 using a Geometrics G-857 magnetometer. Measurements of the magnetic field are sensitive to nearby metal and electrical currents so they must be made at least 10 m from fences and overhead wiring. For this reason measurements were taken in the middle of fields. Access was easy and non-problematic as crops had already been harvested. Some 90 stations were observed roughly along the line shown in Figure 17 with gaps between stations being greater where fences between fields were encountered. (An additional line of observations was made running roughly in an E-W direction in fields north of highway 977 east of "The Green" farm [cf. Fig. 17] but these are not discussed further here.)

Besides the profile magnetic measurements being made a second magnetometer is used as a "base station", remaining stationary but recording continuously throughout the survey period so that diurnal variations in the Earth's magnetic field (related, for example, to electrical activity in the atmosphere and/or solar activity) are known and can be removed from the station observations.

Figure 20 shows magnetic anomalies, corrected for the diurnal variations, along the profile located in Figure 17. The observed anomalies lie in the range -160-250 nT (nanotesla). A correction for latitudinal variation, which in the UK is about 2 nT/km increasing to the north-west, was not applied but the effect is clearly very small compared to the size of the observed anomalies.

Figure 20: Magnetic anomalies along the (easterly) profile shown in Figure 17

Figure 20: Magnetic anomalies along the (easterly) profile shown in Figure 17

The arrow in Figure 20 shows the approximate location of the inferred Hill of Fare granite-country rock contact from BGS (2002).

Discussion

The gravity field along the observed profile (Fig. 19) is dominated by a gradient derived from sources that are more regionally and more deeply distributed than the small scale of the ~1 km long survey itself. The polarity of the observed local gradient, with gravity decreasing to the north, appears to be consistent with what can be inferred from the regional field as seen in Figure 16 and what is indicated on the BGS (2002) geological map of the area. Given that the density of the Hill of Fare granite is thought to be less than the adjoining metasedimentary country rocks (McGregor and Wilson, 1967) it can be inferred that the (regional) source of the gradient is the large Hill of Fare pluton itself along with associated granitic bodies occupying the crust north of the survey area. There is a suggestion of an increase in (negative) gradient just to the north of the BGS (2002) inferred contact (in the distance range ~400-600 m), which would be expected since this contact lies near the ground surface and suggesting that the actual contact is some tens of metres further north than where it has been mapped. The negative perturbation in the distance range 500-600 m is the local feature that is greater than perturbations that are likely noise (altitude measurement) related. It may be related to a buried channel on the soil-bedrock surface, something that is suggested by the physiography of the area.

The magnetic field along the observed profile (Figure 20) clearly shows the signature anomaly of the edge of a buried magnetic body (such as an igneous intrusion) adjacent to relatively non-magnetic country rocks (e.g. Kearey et al. 2002). This is in the distance range~ 250-650 m with a positive peak of about 250 nT and probably a negative lobe to the south of some -50 nT. A theoretical example of the same effect - which is the magnetic anomaly of the magnetic body induced by the Earth's own magnetic field superimposed upon the Earth's own field, is shown in Figure 21. The observed data were complicated by superimposed regional or other intermediate and local effects as well as noise. The last may include "cultural noise" derived from buried pipes and cables, since the survey took place near a built area with dwellings and other (farm) buildings, although typically these would have an identifiable effect. A second magnetic anomaly, centred at around 1000 m, is also evident. If the larger of the two anomalies is the signature of the geological contact of interest then it is near its location inferred by BGS (2002), though perhaps some tens of metres further north of the arrow in Figure 20, but the second, smaller, anomaly indicates that there may be structural complications and/or compositional heterogeneity within the Hill of Fare granite at the scale of the survey.

Figure 21: Theoretical effect in the induced magnetic field of the contact of a magnetic body and a non-magnetic one

Figure 21: Theoretical effect in the induced magnetic field of the contact of a magnetic body and a non-magnetic one

Conclusions and prognosis

High resolution gravity and magnetic surveying is possible in the fields at the base of the Hill of Fare granite in the vicinity of its contact with Dalradian Supergroup country rocks. There are significant magnetic field anomalies that appear very likely related to the granite-country rock contact and they suggest that there is local compositional heterogeneity within the upper hundreds of metres (below the present ground surface) of the igneous pluton. In contrast, there are no specific small scale local effects in the gravity field associated with the granite-country rock contact although there is a suggestion of an increased negative (to the north) gradient in the gravity field that could be indicative of the country rock-pluton contact. One likely significant local anomaly is probably related to the underlying bedrock topography. Otherwise, the gravity field is dominated by the effects of larger, more regional structures such as the Hill of Fare body itself as a whole.

A high resolution magnetic survey on a 2-3 km 2 grid would provide sufficient data and data redundancy to allow three-dimensional modelling of the subsurface structure and composition on a local scale. These data could be augmented with new gravity and magnetic observations at lower resolution spacing in the surrounding area (say ~15-20 km 2) to provide further context for the locally inferred structure and strengthen its robustness.


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