The Active Line Source Temperature Logging Technique and its Application in Fractured Rock Hydrogeology

Transcription

1 307 The Active Line Source Temperature Logging Technique and its Application in Fractured Rock Hydrogeology Peeter E. Pehme Department of Earth Sciences, University of Waterloo and Waterloo Geophysics Incorporated 386 Park Green Place, Waterloo, ON N2L 5S6 Canada John P. Greenhouse Waterloo Geophysics Incorporated P.O. Box 122, Tobermory, ON N0H 2R0 Canada Beth L. Parker Department of Earth Sciences, University of Waterloo School of Engineering, University of Guelph, 50 Stone Road East, Guelph, ON N1G 2W1 Canada ABSTRACT We present a technique for placing a borehole into thermal dis-equilibrium, and thereby interpreting groundwater flow through fractures where it may have been previously undetected. Denoted as Active Line Source (ALS) logging, the method consists of temperature logging while a borehole is heated by the cable and/or during cooling after the heating. With two or more logs collected during either heating or cooling, an estimate of thermal conductivity is obtained. The basic theory, widely used for such things as thermal conductivity probes, is shown to fit the recorded data well. The mechanics of ALS logging are described, and the practical challenges are outlined. In the absence of groundwater flow in or around the borehole, variations in the thermal conductivity of the rock are largely due to variable water content and the ALS log provides a reasonable surrogate for the neutron log. When groundwater flow dominates the dissipation of thermal energy from the borehole, however, the apparent thermal conductivity is increased. In open boreholes this flow can be both ambient (within the formation itself) and connecting (vertical flow between fractures intersected by the borehole). In cased or lined holes with no connecting flow, ALS logs are particularly useful as detectors of ambient groundwater flow. Alternative methods for flow detection, such as chemical dilution or flow-meters, require an open borehole and either have poor vertical resolution or require multiple stationary measurements, often with packers to minimize the effects of connecting flow. The ALS technique is a comparatively simple tool, useful in both open and cased or lined boreholes, run continuously down the length of the borehole, with fracture resolution on the order of a few centimeters. We describe ALS logging of a 75-meter section of a borehole through fractured dolomite which has been lined with a FLUTe sleeve. The ALS results are compared to the geologic units encountered, conventional geophysical logging techniques, time-lapse passive temperature logging, heat pulse flowmeter data and packer testing. Introduction The ALS Concept Heat flowing from Earth s center towards its surface is routinely harnessed to estimate the thermal conductivity of the rocks through which it passes. The vertical gradient of temperature DT/Dz, measured in boreholes or mines, is linearly related to the upward heat flow q through the thermal conductivity K by: K ~{q= ðdt=dzþ: ð1þ This simple picture ignores local heat sources or sinks in the rock. More important in the hydrogeological studies of interest here is the advective and conductive transport of heat with groundwater flow through these rocks which result in locally anomalous JEEG, December 2007, Volume 12, Issue 4, pp

2 308 borehole temperature gradients. Temperature logs are routinely used to identify groundwater flow into and out of an uncased well bore, or through fractures around a cased borehole. Temperature logging, therefore, has an established role in fractured rock hydrogeology (e.g., Conaway and Beck, 1977; Drury, 1984; Howard, 1990). Temperature logs are relatively simple to collect and the data are straightforward to process. Temperature variations of a few 10,000ths of a degree Centigrade can be measured with modern probes. These logs provide a cost-effective reconnaissance of the borehole flow environment that is sufficient for some applications, or may be used to guide the implementation of more definitive flow measurements (packer testing, flow meters, etc.). Modern fractured rock contaminant hydrogeology provides particularly difficult challenges to geophysical borehole logging. Contaminants can be transported through small hydraulically active fractures by very low volume groundwater flows. Fractures cannot always be conclusively identified through the examination of cores and conventional techniques for detecting small amounts of flow are not, in general, sufficiently sensitive for the task. We have experimented over the past decade with a number of different approaches to this problem. For example, we have carefully monitored the temperature of a borehole with successive runs over several days ( time-lapse logging), using the milli-degree and centimeter resolution of the probe to identify areas where temperatures vary slightly with time, on the assumption that these variations were related to varying flow (Greenhouse et al., 1998). A modification of this approach was to run the logs with and without a removable FLUTe liner 1 installed in the borehole. Temperature anomalies on logs recorded within the liner can be assumed to reflect the addition or removal of heat from the stationary water column by the ambient flow around the borehole, in principle allowing when compared to the open borehole logs the separation of vertical (borehole induced or connecting ) and horizontal (ambient) flow influences. Cherry et al. (2007) provide details of the FLUTe system and its benefits. 1 This removable sleeve (also referred to as a borehole liner) used to seal a borehole is a relatively thin polyurethane-coated nylon sock pressurized by having a higher water level than the surrounding formation. The sleeve forms a tight seal on the borehole wall preventing vertical flow in the hole caused by hydraulic conditions. It is manufactured by Flexible Liner Underground Technologies Ltd (FLUTe), 6 Easy Street, Santa Fe, NM, Journal of Environmental and Engineering Geophysics Inspired by a winter visit to our local hardware store, we began using eaves-trough de-icing cables suspended down the length of the borehole to apply heat uniformly, and then monitored the rise of temperature in the borehole as heating progressed. We reasoned that those areas where cooler water flowed into the borehole would be accentuated by increasing the contrast in temperature with the water column. A zone from which water flowed away, either vertically or laterally, would be difficult to heat (hence cooler) because the added heat would be advected away. The technique could be applied in either an open or sleeved borehole. Run with and without the FLUTe sleeve, the technique should differentiate between borehole-created vertical flow and ambient (mainly horizontal or subhorizontal) flow. We refer to the technique as Active Line Source (ALS) logging. The simple geometry of the line heating source allows the application of some basic theory to estimate thermal properties of the formation. The rate of temperature rise following the commencement of heating, or the decay of temperature following the cessation of heating, can both be used to construct continuous logs of an apparent thermal conductivity K app, apparent in the sense that both conduction and advection/convection by groundwater may play a role. Competing Methodologies The ALS method has parallels in applications such as the heat pulse flowmeter (e.g., Lister, 1979). The most comparable thermally based borehole flow measurement system may be the KVA flowmeter originally described by Kerfoot (1982) and later modified for operation in fractured rock (Kerfoot, 1992). The KVA flowmeter creates a pulse of heat and measures the time required for the warm water to reach a surrounding ring of thermistors to estimate horizontal flow velocity within a borehole. The probe is oriented by a compass. The velocity of the water measured within the borehole is corrected to a fracture velocity based on laboratory calibrations. No information regarding formation thermal conductivity is inferred. Data are collected at discrete fixed points and although a fuzzy packer with a plastic baffle is used to limit vertical flow, reduce the fracture-borehole velocity contrast, and minimize turbulence, it is still an open borehole system. Other systems for measuring horizontal flow velocity and direction within an open borehole include particle (or colloid) tracking devices. The detection of the particle movement is either achieved optically, in the case of the Colloidal Borescope (Kearl, 1997), or with the use of Doppler technologies in the Acoustic Doppler Velocimeter (ADV) (Wilson et. al., 2000). As is the case with the KVA flowmeter, both of these tracking devices

3 measure at discrete locations within the borehole rather than along the entire length. The approach described by Tsang et al. (1990) is a chemical diffusion method intended to detect horizontal flow along the entire length of the borehole. In their approach, the borehole fluid is replaced with either deionized or saline water to create an electrical conductivity contrast with the groundwater in the formation and fractures. The fluid electrical resistivity is repeatedly measured as the relatively conductive (or resistive) formation (fracture) water moves into the borehole. The rate at which the water conductivity changes is related to the amount of water entering the borehole from the formation. This approach is clearly analogous to the ALS method in that it relies on diffusion (chemical instead of heat) with time to measure horizontal flow. It is arguably more quantifiable than the ALS method, but it does require the introduction of foreign fluids into the groundwater regime and packers to minimize vertical flow. The Borescope, ADV, KVA and chemical dilution techniques were compared by the USGS at two locations in limestone aquifers (Wilson et al., 2001). The four approaches usually provided very different estimates of velocity. Estimates of flow direction (the chemical dilution technique does not infer direction) were no more consistent, even when pumping from a nearby well presumably controlled the local flow direction. The USGS field study did not benefit from knowledge of a true velocity against which to rank the success of the four approaches and it is possible that flow varied between the tests. All four techniques suffered where vertical flow occurred within the borehole and it should again be emphasized that, unlike ALS, all are designed to operate only in an open borehole. There appears to be a role for a simple technique that can work in both an open or sleeved (cased) borehole. This paper describes our experience with the ALS logging concept. The basic theory of line source heating and cooling in a borehole is summarized in the next section. The subsequent sections describe the implementation of the ALS method, show typical examples of data obtained, and discuss their usefulness. Basic Theory The theory for the dissipation of heat around a vertical line source in a cylindrically symmetric, vertically uniform medium can be found in standard texts and is only summarized briefly here. Heating Beck et al. (1971) derive temperature as a function of time after the initiation of heating for a point on the surface of the heating cable suspended in the center of Pehme et al.: Active Line Source Logging 309 Figure 1. Schematic of a water-filled borehole with a FLUTe liner and a heating cable. Note that position of cable may vary from center to a borehole wall. a borehole. The limitation of this approach for ALS logging arises from the fact that the heating cable and temperature probe are suspended freely within the borehole. The sensing thermistor is therefore not on the surface of the cable but displaced from it by a variable distance potentially ranging from half the probe diameter to almost the diameter of the borehole. The water-filled, cased borehole is shown schematically in Fig. 1. The thermal conductivities (K) and thermal diffusivities (k) are constant within the cylindrically symmetric shells representing the line source, the cable, the water column, the casing and, if present, the grout (uniform grout thickness is assumed). The thermal properties for the formation, K f and k f, will vary primarily as a function of porosity and, to a more limited degree, of the matrix material. Beck et al. (1971) show that after a sufficiently long time (t) the rise in cable temperature DT(t), plotted as a function of ln(t) will approach an asymptotic straight line with slope: DTðÞ? t ðq=4pk f Þ ð2þ

4 310 where Q is the power supplied to the cable in watts per meter. The thermal conductivity of the formation K f can be estimated from this slope. This expression does not depend on the thermal properties of the borehole environment (fluid, casing, grout) assuming they are constant. However, K f can still be influenced by vertical flow and convection within the borehole as well as fracture or formation flow and is usually dubbed apparent as a result. This formalism can in principle be extended to estimating the thermal diffusivity K f of the formation from the intercept of the asymptotic straight line DT vs. ln(t), although we have not done so here. Simply put, for a given heat input Q the amplitude of DT at a given time in the asymptotic region of the curve reflects the thermal diffusivity of the formation and the borehole environment, whereas the gradient ddt/dt depends only on the thermal conductivity. Cooling There is a complementary theory for the cooling of a borehole once it has been heated. Shen and Beck (1986), in their investigation of the temperature reequilibration after drilling, show that the process of heating and then cooling the borehole can be treated as the application of heat Q at time t 5 0, followed by a superimposed addition of negative heat Q at time t h when heating ceases. The superimposed solutions result in the asymptotic solution DTðt c,t h Þ? ðq=4pk f Þlnððt c z t h Þ=t c Þ, ð3þ where t c is the cooling time t 2 t h. The conditions of this solution require that measurements be made on the axis of the borehole (at the cable) and that the heating time t h is short compared to the cooling time t c. At sufficiently long time t the thermal conductivity of the formation can therefore be found from the slope of a plot of DT vs. ln(t/t c ). Sample Calculations We have coded the relevant equations using MATHCAD TM to simulate the heating and subsequent cooling of a typical borehole, shown in Fig. 2. The borehole is assumed to be 15 cm in diameter, water- filled with a PVC casing and no grout layer 2. The cable has a heat output of Q 5 15 watts per meter. Typical published values have been used for the various thermal properties of the cable, insulation, water, and casing. The formation thermal conductivity K f is watts/(muc). Thermal diffusivity k is m 2 /s for curves a,b and c, m 2 /s for curves a9, b9 and c9. The borehole is heated for t h h, then allowed to cool, mimicking an actual field trial to be described in a later section. Plots 2 The particulars or number of relatively thin cylinders around the borehole have minimal influence on the calculations. Journal of Environmental and Engineering Geophysics Figure 2. Curves a and a9 show modeled temperature as a function of time following heating cable turn-on. Time since turn-on (t) is in hours. Curves b and b9 plot temperature during cooling as a function of log(t) following the heating turn-off at 300 h. Curves c and c9 plot the cooling temperature as a function of the log (t/t c ) where t c is the (cooling) time since turn-off. In all cases the formation thermal conductivity is The curves a, b and c have a formation diffusivity of ; for a9, b9 and c9 it is a and a9 show temperature change DT during heating, while plots b and b9 show the subsequent cooling, both as a function of log(t). Plots c and c9 display the cooling temperatures as a function of log(t/t c ). Note that during heating both curves a and a9 in Fig. 2 reach their asymptotic slopes in less than 1 h after turn-on, corresponding to a temperature rise in the borehole of approximately 1 degree. Their asymptotic slopes are identical, but the intercepts of the asymptotic straight lines reflect the different diffusivities. During cooling, the responses for the two diffusivities are indistinguishable at the scale of this plot. When the cooling data are plotted as a function of log(t/tc), the temperatures fall on a remarkably straight line once the cooling time t c is as little as 1/100 of the total time t. Sample Data The basic theory is useful as a guide to the behaviour of the heated borehole, but application of these formulae to the observed data needs to be leavened by common sense. To this end, in dealing with actual borehole data we will refer to the parameter determined from the asymptotic slopes of the heating and cooling plots as the apparent thermal conductivity K app, recognizing that it may not be uniquely related to the properties of the formation matrix itself. In the case of the thermal conductivity estimate, the primary uncertainty arises from the role of water flow. Figure 3 shows the passive (background; a temperature log collected prior to heating) log and two logs

5 Pehme et al.: Active Line Source Logging this paper we concentrate on estimation of an apparent thermal conductivity from multiple logs obtained during a heating or cooling phase, but the usefulness of single ALS logs should not be underestimated. An ALS methodology based on cooling would seem less practical than one based on heating. More time is required overall for the measurements, and only thermal conductivity can be estimated. However, as will be demonstrated in the next section, the cooling data are much less noisy than the heating data and yield more consistent estimates of K app. Implementing ALS Logging 311 Figure 3. Background passive (left, smooth) and two ALS logs run in a sleeved borehole in Cambridge, Ontario. The ALS logs were recorded beginning 22 and 530 h following the onset of heating. obtained after sevceral hours of heating for a borehole in a fractured dolomite aquifer described in detail in a later section. Note that the heating is clearly more effective in some parts of the borehole than others, reflecting variable thermal diffusivity of the borehole environment. There are clearly visible variations in the spacing of the two ALS curves (for example, above and below 212 m) reflecting variations in the ability of the formation (K app ) to remove the heat supplied. With the two ALS logs it is possible, in principle at least, to estimate apparent thermal conductivity as a continuous function of depth. Where flow is not a factor, Eq. (2) should yield thermal conductivities that lie within the normal range of the rocks being traversed. Clearly several ALS logs equi-spaced logarithmically in time are better than two, both for estimating the slope and intercept of T vs. ln(t) as well as for ascertaining that asymptotic straight line conditions have been reached. If time (and/or funding) limit one to a single ALS log following the onset of heating, thermal conductivity cannot be determined and only the relative diffusivities of different sections of the borehole can be estimated. This information can still be very useful. Markedly variable ability to heat the borehole at different depths within a single rock formation may reflect different groundwater flow conditions. While flow-related temperature anomalies are often evident in the passive temperature log, heating the borehole amplifies their effects. In This section examines aspects of ALS data, including noise levels, the variation in borehole temperature as a function of time during heating and cooling, the coherence between multiple runs, and the reliability of rock thermal parameters estimated from these temperature variations. The data have been taken from sections of two 7.5- cm diameter boreholes drilled to over 100 m and traversing a predominantly dolostone sequence of Paleozoic rocks in Cambridge, Ontario. The first, UW13, was logged in 2001, both with and without a FLUTe sleeve. The second, UW1, was logged with the sleeve in place as part of thermal tracer experiments in 2004 and These are two of many boreholes used to study a local contamination problem (Carter et al., 1995). The groundwater flow in the area is quite active, driven by several pumping water-supply wells that regionally surround the site. Pumping rates vary with demand, generally increasing through the work week and dropping over weekends. As a result, temperatures could vary significantly in the vicinity of flowing fractures over the course of short time spans, as documented by Greenhouse and Pehme (2002). The boreholes are heated with one or two (side-byside) cables, using 110 or 220 volts AC, depending on the situation. The de-icing cables used come in fixed lengths (15 m, 30 m, 60 m and 73 m) and all put out approximately 15 watts/m. This requires that the heating wire in each length category have a different resistivity, and so longer cables must be created by joining two or more wires from the same length category. Deep boreholes, where two or more cables must be spliced end-to end, can require 220 volts in two side-by-side cables to generate sufficient heat. Shallow boreholes usually require only one cable and 110 volts. Noise Levels in Temperatures Measured During Heating and Cooling of a Borehole As stated earlier, the ALS technique does not actually measure the temperature on the surface of the

6 312 Journal of Environmental and Engineering Geophysics Figure 4. Heating (curve segments a) and cooling (segments b and c) recorded at a fixed depth of 35 m in borehole UW1, as a function of time t since heating began (a and b) and also, for the cooling data, as a function of the ratio t/t c (c). The borehole was heated for 11 h and then allowed to cool for a further 110 h. Temperature readings were made with the probe stationary for five short periods during heating (a 1 a 5 ) and six during cooling (b 1 b 6,c 1 c 6 ). Parallel lines emphasize the similarity of heating and cooling slopes at later times as required by theory. cable, but rather at varying distances from it within the borehole. Assuming the measurement probe is 4-cm in diameter, with the thermistor on its central axis, the measurement in a 10-cm diameter borehole is made anywhere from 2 to 8 cm from the cables, and this distance is continuously varying. This section examines the variability of borehole temperature measurements while the cable is heating the borehole and while the borehole is cooling, made both with the probe stationary and during normal logging. Stationary tests. The example is from UW-1. This borehole is cased through the overburden and the highly fractured bedrock surface to a depth of 25 m, and continues uncased through fractured dolomitic rock to a depth of 150 m. UW-1 was fitted with a FLUTe sleeve at the time these data were collected in August of In August, 2005, UW1 was heated for 11 h with side-by-side cables operating at 240 volts, then allowed to cool for a further 100 h. Temperature logging of the borehole proceeded during the heating and cooling process and, in addition, the temperature was monitored every 0.5 s at a fixed depth of 35 m as time allowed between runs. The same routine was followed during cooling, with the cable in place but inactive. Figure 4 shows the results of that monitoring, with heating and cooling data plotted as a function of time t from their onset. The heating process is represented by Figure 5. (a). Forty minute segments of heating and cooling in borehole UW1 (from sections a3 and b2 in Fig. 4). Cooling data are shown as dashed line and offset vertically for the comparison. (b) A 5-minute segment of the record in Fig. 5a. five segments of approximately 1 h denoted a 1 to a 5. The cooling is represented by six short segments of recording labeled b 1 to b 6. As with Fig. 2, the cooling data are also plotted as a function of t/tc, labeled c 1 to c 3. Note that, on these semi-logarithmic plots, the heating and cooling series a and c describe fairly straight lines for sufficiently large times, as predicted by Eqs. (2) and (3). Real parallelism of these lines is not achieved until somewhat later than predicted by the model of Fig. 2, in the vicinity of segments a 3 a 5 for heating and c 4 c 6 for cooling (as indicated by the two parallel lines in Fig. 4). This probably reflects the fact that, in the borehole, temperature is not actually measured at the cable surface. Figure 4 also shows that the noise on heating is considerable, and increases with time. On cooling, however, the noise levels are low and decrease with time. In Fig. 5, sections a 3 and b 2 of the heating and cooling data from Fig. 4, each approximately 7 h after their respective onsets, are compared at two different time scales. Figure 5a compares 2000 s (33 min) segments of the heating and cooling records. The heating curve is much noisier than the cooling, with excursions from the average of as much as +/20.2uC. Figure 5b

7 shows a 400 s section of this record. Here it is evident that the heating record is smooth on a scale of a few seconds, that the noise in Fig. 5a does not extend across the frequency spectrum but rather has a dominant periodicity in the general range of 50 to 200 s. The heating and cooling noise levels vary and are typically less than uC on time scales less than 50 s. This behaviour is typical of all sections of the stationary record, in this borehole and others in the vicinity. Pehme et al.: Active Line Source Logging 313 Noise levels in open and sleeved holes during heating. Figure 6 compares segments of continuous downward ALS logs collected during heating in the sleeved and open borehole UW13. The passive (background) log is also shown for comparison. In all cases the probe moves down the hole at 1 m/min. Figure 6a shows data interpolated 3 from the raw data (but not smoothed) every 2 cm for an 11-m section of the borehole between 55- and 66-m depth. Figure 6b shows a 1-meter segment of the same data. The sleeved and open hole logs were recorded several weeks apart, but both were recorded starting roughly 3 h after the initiation of heating. Because of the variable temperature structure of the borehole with time, a comparison of the absolute temperatures of the open and sleeved runs is not strictly meaningful. The noise levels here are marginally lower in the sleeve than in the open hole, but in general the temperature variability in both cases is consistent with the stationary data shown in Fig. 5 (,0.6uC). Spectral analysis of the records shows a significant peak in the region of wavelengths 20 cm to 2 m, comparable (at these logging speeds) to the time variations observed in the stationary tests. We conclude that the noise on ALS logs during heating is not a function of the presence or absence of the sleeve. The source of the noise on the heating data may be either a periodic swinging of the suspended probe relative to the cables, convective effects, or both. The amplitude of the noise is considerable and its suppression will require that the ALS heating data be smoothed over vertical windows of one meter or greater during normal data collection speeds of 1 1K m per minute. Coherence between successive runs. Visual or mathematical analysis of the ALS logs consists of comparing logs run at two or more times in the borehole. It is important to understand in view of the noise described in the previous section on what scale (e.g., mm, cm or m) these comparisons are meaningful. Figure 7a shows two 3 The IFG logging system collects raw data (including depth information) continuously on a time basis and conversion to depth-based measurements is an early step in post processing. Figure 6. (a) Temperature variation measured between 55 and 65 m depth in heated borehole UW13. From left to right the logs are: passive in open hole; ALS in open hole; ALS in sleeved hole. Logging speed is 1 m/min and both ALS logs were recorded roughly 3 h after the initiation of heating. (b) A 1-meter segment of the logs in Fig. 5a. Interpolated 2-cm data points are indicated on the passive trace.

8 314 Journal of Environmental and Engineering Geophysics heating runs of the ALS log in borehole UW1 (sleeved with a FLUTe liner), recorded roughly 22 and 43 h after the onset of heating. The passive (background) log has been subtracted from the active logs to show only the temperature rise DT in the borehole due to heating. All logs were recorded at logging speeds of 1 m/min. In Fig. 7, the long wavelength character of the temperature rise above background is very similar in the two logs. As might have been expected from the previous discussion, however, spectral analysis of the two records shows that coherence is weak (,0.5) at wavelengths less than 1 2 m. All these data suggest that ALS logging during borehole heating is unlikely to be useful for estimating formation parameters on scales of less than one meter. It is, of course, still possible that a large temperature contrast in a thin zone could be detected. As Fig. 4 shows, the cooling logs are much quieter than the heating logs. A coherence analysis of cooling logs shows that successive logs are highly coherent right down to scales of a few tens of centimeters. These logs do not need to be smoothed over vertical intervals of depth. This smoothness will be seen below to result in much more stable estimates of K app than can be obtained from the heating logs. The implied advantage in vertical resolution of cooling over heating for thermal parameter estimation is, we suspect, mitigated by natural blurring through vertical conduction and convection of detailed temperature variations that could be maintained during the heating process. Figure 7. The two ALS logs of Fig. 3 with the passive log subtracted. Determining Thermal Parameters From ALS Logs During Heating The data for this section and the next are taken from a heat tracer test conducted in the fall of 2004 using borehole UW1 as a source, with nearby downgradient boreholes UW2 and UW16b logged repeatedly for evidence of a breakthrough of warm water. Two side-by-side heating cables attached to a 220 volt generator produced an estimated (from cable specifications) 5.65 watts/m to heat the borehole; however, the output of these cables was not actually tested on site, leading to some uncertainty in the data as described below. The results of the tracer test itself are beyond the scope of this paper, however some very limited presentation is provided by Burns (2005). During the course of the test the heated borehole UW1 was also temperature logged repeatedly, starting at 22:20 h and then at 43:35, 94:50, 168:03, 190:45, 214:12, 238:17, 266:17 and 334:45 h following the onset of heating. At 337 h heating was stopped and the cooling process monitored for an additional 262 h (see next section). Although recorded over an impractically long time for routine ALS measurements, this test provides an exceptional data set for examining the heating and cooling process. Figure 8a shows the suite of eight logs recorded during heating at borehole UW1. The passive (or background) log has been subtracted. Figure 8b shows these logs smoothed with a 150 point triangular filter (an effective window of about 2 m). Logs 1,2,3,4, and 9 (in order of time, from left to right) have been highlighted for discussion below. The white trace in Fig. 8a is an average of the eight traces, referred to below as DTBAR. This parameter is a simple but effective means of reducing the noise on the individual traces, and it highlights the three anomalous zones at 211, 228 and 238 m above sea level (masl) to be discussed below. The three dashed horizontal lines on Fig. 8b indicate those depths at which the DT vs. ln(t) plots of Fig. 9 have been made. It is clear in Fig. 9 that the data at each depth are well fitted by a straight line; actual goodness of fit (R 2 ) estimates are all greater than The heavy line in Fig. 9 is the theoretical response for a simulated borehole environment with formation thermal conductivity and thermal diffusivity 2.5 and respectively, with a source output of 5.65 watts/m. This analysis can be extended to all data points in the set of ALS logs to determine K app (z) by fitting a trend line to the temperatures and heating times at each depth z.

9 Pehme et al.: Active Line Source Logging 315 Figure 8. (a) Series of ALS logs run in borehole UW1, Cambridge, Ontario, at 22, 43, 94, 168, 190, 214, 238, 266 and 334 h after the initiation of heating. The passive (background) temperature log has been subtracted from the ALS logs to show the effect of heating. White trace is an average of the 8 logs. (b) The data of Fig. 8a smoothed with a 1.5-m triangular window. The horizontal dotted lines indicate depths at which the parameter estimate plots of Fig. 9 have been made. The first, second and last logs have been emphasized. Figure 9. Temperature rise (plotted as points) versus heating time for the three depth locations in borehole UW1 indicated in Fig. 8b. The light lines are least squares fits of straight lines to the nine data points at each depth. The heavy solid line is the theoretical asymptotic response for a formation conductivity K = 2.5 W/(m/uC) and diffusivity k = m 2 /s. See the text for further details. From a practical point of view, logging a borehole slowly many times would be very expensive, even if spread over several hours as opposed to the several days presented in this case. To examine how to reduce the number of logs, we first choose five logs (numbers 1,2,3,4 and 9 from Figs. 7b and 8) that are roughly equi-spaced on the logarithmic time scale. The results are shown in Fig. 10a, where K app (z) and R 2 (z) estimates are shown for both the equi-spaced 5-point set and for the original 9-point set. We note that the two estimates are reasonably coherent, that they define broad zones of varying conductivity and perhaps even finer structures on the scale of 3 to 5 m. The R 2 value is uniformly high, but with a dip at 228 masl where the K app estimate is unstable, possibly owing to variable groundwater flow at that depth over the time logging took place. Figure 10b shows K app and R 2 estimates based on two combinations of three adjacent logs (1,2,3, 2,3,4) from the 7-log data set, and compares them to the 5-log estimates of Fig. 10a. These 3-log estimates confirm the broad trends of the 5-log estimates, but they vary widely with it and between themselves on much of the shortscale detail. Reasonable coincidence between the 3- and 5-point estimates is not achieved until smoothing windows of 10 m or more are used. As might be expected, 2-log estimates are even more variable.

10 316 Journal of Environmental and Engineering Geophysics Figure 11. Cooling following the shut-off of heat to the cable in borehole UW1. The rightmost curve was recorded 2 h before turn-off, 338 h after the heating commenced. The leftmost (heavy) curve is the passive or background borehole temperature recorded before heating began. In between are the seven logs recorded at 21, 44, 68, 92, 167, 235 and 262 h following turn-off. No smoothing has been applied. UW1 is a sleeved hole so we do not expect large and sharp variations caused by groundwater moving in or out of the borehole, variations that might cut through the noise on the ALS logs. Without that, we conclude for now that the ALS heating-mode method, as practiced, can identify thermal property variations on scales of several meters and greater, but cannot be relied upon to distinguish finer scale variations. Figure 10. (a) Estimates of K app and R 2 for borehole UW1 made by determining the slope of a trend line at each depth for the logs of Fig. 7b. The heavy line is the estimate using five logs (logs 1, 2, 3, 4, 9 in Figs. 7b and 8) roughly equi-spaced in time, the lighter line using all nine logs. (b) The heavy lines are K app and R 2 taken from Fig. 10a. The lighter lines show two 3 data point estimates (123 solid, 234 dotted) taken from the 5 equi-spaced (in time) logs. Determining Thermal Parameters From Logs During Borehole Cooling Figure 11 shows a suite of seven logs recorded during the cooling of sleeved borehole UW1 during the 2004 tracer test. Heat was turned off after t h 5 337:45 h and recordings made starting at t c 5 11:19, 45:09, 68:27, 92:55, 162:11, 236:08 and 262:54 h following turn-off. Also plotted are the passive log (extreme left, heavy line) and the last log recorded during heating (extreme right). Compared to the logs obtained during heating, the cooling logs are very smooth, and coherences between consecutive logs are well above 0.5 at all frequencies sampled. Figure 12 shows samples of polynomial fits to the semi-log plots of T versus ln(t/tc) for the seven logs (where t 5 t h + t c ), taken at the three elevations in the

11 Pehme et al.: Active Line Source Logging 317 Figure 12. Plots of temperature versus log of the ratio t/ t c, for the three elevations of Figs. 8b and 11. Trend lines are fitted to late time (low t/t c ) data to emphasize the slight curvature of earlier data. borehole indicated by the horizontal lines in Fig. 9. No smoothing has been used. The data at each elevation fall close to a trend line, but the slope of T vs. ln(t/t c ) does decrease slightly with increasing time (decreasing abscissa), as the conditions (t c. t h ) for the application of Eq. (3) come into effect. Figure 13 shows estimates of K app (z) for the cooling log suite of Fig. 11, made from straight line fits to T vs. ln(t/t c ) at all depths. The heavier curve at the center is derived from all seven logs. To test the stability of the cooling parameter estimates, two subsets of three logs have been chosen from the seven available; specifically, logs 1 to 3 and 5 to 7 have been used to make independent 3-log estimates of K app at each depth. K app estimates based on the later logs (5 7) are significantly higher than those based on the earlier logs (1 3), the result of the above mentioned curvature to the T vs. ln(t/t c ) plot. Note, however, that apart from a horizontal offset, the shapes of the three K app (z) estimates are very similar to vertical scales of at least a meter, suggesting that the relative values of K app are faithfully rendered at earlier times. R 2 values in Fig. 13 are indistinguishable and close to unity for all three estimates. Comparison of the Heating and Cooling Data Figure 14 compares the K app (z) and R 2 (z) estimates obtained from the cooling logs with estimates from the 5 equi-spaced heating logs of Fig. 9b. The Figure 13. Estimates of K app for subsets of the seven cooling logs. The heavy curve at the center uses all seven logs. The two lighter lines are 3-log estimates, from logs 1 to 3 on the left and logs 5 to 7 on the right. The R 2 values for these estimates are almost unity and indistinguishable at this scale. heating data have been smoothed as described above; the cooling data are unfiltered. R 2 measures of the T vs. ln(t) fit are generally higher for the cooling data, evident also in a comparison of Figs. 9 and 12, but both data sets are well matched to the heating/cooling models. The broad shapes of the two thermal conductivity curves are similar on scales of tens of metres; both show a broad high in the vicinity of masl although the amplitude of this high is much less for the cooling than the heating data. Detail at shorter wavelengths is not obviously coherent between the two data sets. Estimates of K app are based on the difference between logs with time; when the data are noisy (as with the heating logs) this estimate is not particularly stable. There is a second and perhaps more intuitive way of comparing these two data sets, referred to briefly in Fig. 8a, which emphasizes the common features of the temperature logs. Each log in a suite has the background passive log subtracted and is then normalized to a mean

12 318 Journal of Environmental and Engineering Geophysics Figure 14. Estimates of K app and associated R 2 obtained from the cooling logs of Fig. 13 (dark line, 7-log estimate) compared to the estimates of the same quantities made from heating logs (Fig. 10, lighter lines). value of unity. The normalized logs are then added and averaged. The result is a single curve in which uncorrelated noise on the individual logs is dampened and common features are emphasized. These so-called DTBAR curves for the cooling and heating logs at UW1 are shown in Fig. 15. If the thermal properties of the borehole are constant down its length, then variations in the ability to heat or cool the water column reflects variations in formation thermal diffusivity, the ratio of thermal conductivity and specific heat. Compared to K app, variations in DTBAR(z) are better correlated between the heating and cooling curves on scales between roughly 1 and 5 m. The heating DTBAR shows better vertical resolution than either the heating or cooling K app estimates, and its form is fairly stable (though noisier) when applied to smaller subsets of the heating curves. In short, it is probably a better parameter than K app to summarize the heating data. Recommended Procedures The measurement of thermal parameters using logs run during the heating of the borehole is attractive because (a) it can be done relatively quickly and (b) the greatest amplification of contrasts in thermal parameters would be expected to be observed in that time. However, the experience at UW1 suggests that the cooling method, though requiring more time on site, is far more effective at providing the information required. Figure 15. DTBAR curves, constructed by summing the individual logs in the cooling (darker line) and heating cycles. Each curve has been normalized to have a range of 0.5 units. Our experience suggests that a practical approach to ALS logging is to heat the borehole for 4 to 24 h (depending on the number of data sets required and borehole depth) and to monitor the decay at least three times (with progressively longer intervals) over a near equal length of time following shutoff. If the absolute value of K app is required, then logging should be done at later times, such that t c /t h.51, although this will require working with very small changes in temperature between successive logs. If only relative variations in K app are required, earlier measurements will suffice (Fig. 12). Extensive monitoring during the heating process itself is probably not cost effective, but clearly can be of use. If several boreholes are to be surveyed at a site, a number of cables can be used for the overnight heating and the monitoring performed on a rotating basis during the following day. Hydrogeological Significance of the ALS Data at UW1 The data used here to describe the ALS technique were taken from a 60 m ( masl) section of

13 Figure 16. The ALS parameters K and DTBAR compared to conventional geophysical and hydrogeological borehole data at UW1. All data are normalized to a horizontal range of 0.5 units for easy comparison and offset for ease of viewing. Horizontal dashed lines show boundaries between (from top to bottom) the Guelph, Lockport Eramosa and Lockport Gasport formations. From left to right the plotted logs are: hydraulic conductivity k; laboratory porosity W data points and dotted average curve; K app derived from heating and cooling; neutron; cc (density) and c (natural gamma) logs, DTBAR cooling and heating. Substantial flow in the fracture zone of the upper shaded zone is suggested as an explanation for its having an opposite ALS response to the lower shaded zone. Pehme et al.: Active Line Source Logging 319 borehole UW1. This is a 110-m borehole through (predominantly) fractured dolostone as described thoroughly by Burns (2005) and lined with a FLUTe sleeve. In this section we compare the ALS data with other geophysical and core data in order to examine the role the ALS can play in developing a geologic and hydrogeological model of this particular environment. This comparison is made in two steps. The first, based on Fig. 16, compares the ALS logs to the geologic log and porosity measurements from core, hydraulic conductivity estimates from packer testing, and bulk properties as determined by neutron, gamma and gamma-gamma (density) geophysical logs. The second (Fig. 17) compares the ALS logs to temperature logging methods in the sleeved hole, and thermal flowmeter data in the open hole. Bulk Properties: ALS Logs Versus Core Data, Natural Gamma, Neutron, and Density Logs (Fig. 16) Independently measured thermal conductivity (K) measurements are not available for borehole UW1; superficially the apparent values from ALS measurements shown in Fig. 14 are reasonable (e.g., Anderson, 2005) considering the uncertainty in the cable s power output in this particular experiment. Geology. Burns (2005) indicates that this section of borehole traverses the Guelph and Lockport dolostone formations, with their boundary (at 241 masl) indicated by the upper horizontal line in Fig. 16, and the upper Figure 17. ALS cooling parameters K app and DTBAR (third box from left, each normalized to a range of 0.5 for comparison) compared with conventional temperature logging techniques at UW1. At left, passive temperature logs a, b and c recorded in the sleeved hole in February, March and April of 2004, respectively. Vertical temperature gradient log d (second box) is derived from passive log c. Thermal flow meter data (right box) was measured in the open hole. Arrow pattern at right shows, very schematically, interpreted horizontal flow in the formation.

14 320 portion of the Rochester dolomitic shale at masl. The uppermost (above 230 masl) member of the Lockport formation, the Eramosa, is designate as Le in the figures, while the lower member of the Lockport formation, the Gasport, is designated as Lg. The Gasport and Guelph dolostones are not of themselves hydrogeologically very distinct (but sections within them are) while the Eramosa is described (Johnson et al., 1992) as a unit with lower porosity than the rock above and below. Burns (2005) has identified two zones of higher than normal fracture frequency, 5 to 10 m in thickness, centered on 240 and 210 masl. The first may be associated with the Guelph Eramosa boundary. The second marks the approximate boundary between what Burns describes as a change in matrix composition from pinpoint and vuggy porosity to predominantly pinpoint porosity below. The central portion of these zones where geophysical logs identified fractures are identified with shading in Fig. 16. From the 204 to 190 masl the Gasport is fairly uniform and is described as having densely stylolitic zones with few fractures. Hydraulic conductivity and porosity. The hydraulic conductivity estimates from packer testing and laboratory porosity measurements (with a smoothed average) in Fig. 16 are taken from Burns (2005, Figures 1 32 and 2 20, respectively). Hydraulic conductivity estimates vary from to m/s. These measurements combine both matrix and effective fracture permeability. Peaks are observed in the vicinity of Burns fracture zones (237 and 210 masl) with a lesser peak at 228 masl. The matrix porosity values in Fig. 16, which vary from a high of 11% to a low of 3.5%, show the Lockport formation to be less porous in general than the Guelph, with the Eramosa member and the upper portion of the Gasport having the lowest matrix porosities. Geophysical logs. The natural gamma log shows that the dolostone above the Eramosa Gasport boundary (230 masl) is generally higher in clay content than the rock below. The lower two thirds of the Eramosa is relatively clay-rich, and higher clay content layers at 212 and 206 masl bound a slight increase in clay content through the section corresponding to a fractured core zone within the Gasport centered on 210 masl (Burns, 2005). The density (gamma-gamma) and neutron logs, as portrayed, respond inversely to density and moisture (free or bound) content, respectively. The density log shows strong evidence of fractures at 237 and 228 masl, as well as less pronounced features at 220 masl and in the vicinity of the 210 masl fracture zone. The neutron log identifies events at the 228 masl and 210 masl levels, but not at 237 masl. Televiewer data collected in the Journal of Environmental and Engineering Geophysics borehole show a near horizontal fracture with a 3-cm aperture at masl. This is narrow compared to the source-receiver separation on the particular neutron sonde used (25 cm), which may explain its poor response. The ALS logs. The K app logs and DTBAR logs determined for both the heating and cooling cycles are shown in Fig. 16. Examining the more consistent of the K app logs, K cool in the figure, we note that the broad shape of the trace mirrors a smoothed line through the porosity values determined from samples. In other surveys, particularly in sedimentary environments, we have found K app to be a reasonable alternative to the neutron and density logs as a detector of moisture content. The K cool, DTBAR and density logs also show a distinct change in character across the fracture zone at 210 m, which separates the predominantly vuggy and predominantly pinpoint porosity lithologies described by Burns (2005). The typical reaction to the higher water content of a fracture zone would be lower density, higher neutron porosity (the log responses themselves are inversely proportional to these quantities), higher DTBAR temperatures and lower thermal conductivity. This typical reaction is seen in the vicinity of the lower shaded fracture/high hydraulic conductivity zone near 211 masl. Near 228 masl there are strong neutron and density log responses of the expected polarity, but despite the R 2 anomaly of Fig. 10 no distinct response in the ALS logs. Of most interest is the high hydraulic conductivity (upper shaded) zone at 238 masl. Here there is a strong density log response, a very small, if any, neutron response, but a large ALS response and of the opposite polarity to what would be expected; that is, this is a zone of higher than normal thermal conductivity and lowered DTBAR temperature. We propose that this response results from groundwater flow in the fracture(s) of this zone, advecting away the heat supplied by the cable. The negligible neutron response to this fracture suggests (with the density log) that it is very narrow and does not present a large moisture content variation over the volume sampled by that log. If this is the case, one would expect a weak response from the ALS logs if flow was not involved. The possibility to detect ambient formation flow around the sleeve is, in our view, the most important contribution that the ALS log can make to investigations of this type. ALS Logs Versus Passive Temperature and Flowmeter Logs (Fig. 17) Figure 17 compares the ALS logs (cooling version only) with traditional and time-lapse passive tempera-

15 ture logging of the sleeved hole, and with a thermal flow meter log of the open borehole. Time-lapse temperature logging is particularly useful in an environment like Cambridge where local pumping can create significant temperature changes on well-connected fractures with time (Greenhouse and Pehme, 2002). Curves a,b and c in Fig. 17 are passive temperature logs taken over a three month period. Figure 17d is the vertical temperature gradient determined from log c, and highlights smallscale variability in temperature. There is a striking change in the temperature of the water column with time above a depth of about 220 masl, almost certainly the result of groundwater of varying temperature flowing in the formation past the sleeved borehole. There is a pronounced peak around the 237 masl fracture zone, suggesting a source in that area from which heat is distributed up and down the water column through conduction and/or convection. (The logs suggest there may also be another source zone above the section being studied.) It is also possible that the vertical distribution of heat about the 237 masl zone is due to weaker fracture and/or matrix flow above and below the 237 masl fracture. The temperature gradient log (d) is taken from the noisiest and warmest of the three passive logs, and is disturbed over a broad zone mainly below the 237 masl fracture. This again suggests that there is some weaker ambient flows of warmer water in the section above 220 masl and below the major fracture at 237 masl. The heat pulse flow meter data in Fig. 17 show another and more direct view of the flow, albeit of vertical flow in the open borehole. These data were collected at one meter intervals in the open borehole. At each measurement point the probe was held stationary and 2 to 4 individual tests were performed to ensure representative results were collected. The scale extends from zero to minus six l/min; all flow is downwards. Above 238 masl depth no discernible vertical flow was measured. Immediately below 238 masl, the flow is at the highest measurable value observed in the borehole (1.4 gal/min downward), presumably entering the borehole from the fracture. With the exception of abrupt changes in level between tests at 236, 215 and to a lesser degree at 204 masl, the flow exhibits a steady gradual decrease with depth, suggesting that the water is primarily leaving the borehole by way of numerous small individual, indistinct, discrete fractures and/or non discrete (indistinct) matrix permeability. While flow in the open borehole cannot be directly related to ambient flow, these data do suggest that the fracture at 237 masl is a major conduit of groundwater flow in this section of the borehole. A simple schematic of the proposed natural distribution of flow in the formation (ambient) flow is Pehme et al.: Active Line Source Logging shown at right in Fig. 17. The contribution of the ALS logs in this case is to strongly suggest that the predominant source of the heating observed in the time-lapse passive logging is in fact high flow of groundwater in the narrow zone at or in the immediate vicinity of 237 masl. Note that the passive logs do not react significantly to the fracture zones identified in the vicinity of 220 and 210 masl, corroborating the interpretation from the ALS logs that these are not significantly flowing fractures. Discussion 321 The ALS logs provide a complementary and largely independent perspective when used in conjunction with the larger database in hydrogeological investigations. On a broad scale the ALS data respond to the water content in the formation, emulating the porosity variations measured by neutron and density logging. Although clearly an expensive data set in terms of field time, the ALS approach avoids the potential liabilities and expense associated with storage and handling of active nuclear sources. Most importantly, the technique has the potential to detect flow in a fracture through a liner or casing, based on the ability of the cable(s) to heat or the subsequent cooling of the water column in its vicinity. Combined with other data, the ALS data helps identify hydrogeologically active features and rank their significance. The interpretation of data collected through the FLUTe sleeve minimizes the influence of the open borehole. At the Cambridge site we have the luxury of a database that spans considerable time. Few site investigations include information collected over days or weeks, let alone months or years. Therefore the information we could glean from, for example, timelapse passive temperature logs is rarely available. Like time-lapse logging the ALS technique examines the system in disequilibrium, but the disequilibrium is controlled and measurements made over a much shorter period of time. The data obtained through cooling of the heated borehole tend to be smoother and more consistent than the data acquired during the heating cycle. The heating data are potentially better at resolving narrow zones of horizontal flow, but the noise in this mode makes stable estimates of K problematic. We are currently attempting to reduce this noise by averaging the temperature over multiple thermistors in the horizontal plane of the borehole. Nonetheless, even with the single thermistor it is well worth logging the borehole during both the heating and cooling cycles.

16 322 Acknowledgments We (Pehme and Greenhouse) have been experimenting with the ALS method since In the early years we received a small grant from the (then) Waterloo Centre for Groundwater Research and in more recent years advanced through collaboration and financial support provided by a CFI infrastructure grant and an NSERC CRD project (No ) and Syngenta Crop Protection Canada Inc. awarded to Dr. John Cherry and Dr. Beth Parker at the Waterloo Institute for Groundwater Research, University of Waterloo. This funding supports a major field research program in fractured rock hydrogeology and has established the Cambridge, Ontario site as a field laboratory for borehole method development and testing, providing the data sets used in this paper. Chris Turner, James Plett and especially Leanne Burns have provided data, analysis and insights from their own thesis research. Otherwise our work on the ALS technique has been sandwiched between or gratuitously added onto consulting projects. 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