INDUCED POLARIZATION AND RESISTIVITY LOGGING IN PERMAFROST

Transcription

1 INDUCED POLARIZATION AND RESISTIVITY LOGGING IN PERMAFROST Richard Fortier 1, Michel Allard 2 1. DŽpartement de gžologie et de gžnie gžologique Pavillon Pouliot, UniversitŽ Laval Sainte-Foy (QuŽbec) Canada G1K 7P4 rfortier@ggl.ulaval.ca 2. Centre dõžtudes nordiques Pavillon Abitibi-Price, UniversitŽ Laval Sainte-Foy (QuŽbec) Canada G1K 7P4 michel.allard@cen.ulaval.ca Abstract Induced polarization (IP) and resistivity logs were carried out in a permafrost plateau near Umiujaq, Northern QuŽbec, to outline the permafrost dynamics. A permanently buried multiconductor cable into permafrost, with regularly spaced electrodes and thermistors, was used for IP, resistivity and temperature logging. The cryofronts in the active layer and at the permafrost base corresponded to resistivity values around 500 ohm-m. A basal cryopeg was clearly delimited between 21 and 22.7 m in depth by a sharp decrease in resistivity from 2000 to 500 ohm-m close to 21 m and a sharp increase in total chargeability from -4 mv/v in permafrost to values over -16 mv/v in the basal cryopeg. The freezing-point depression due to overburden pressure at the base of perennially frozen layer is evaluated to be -0.2 C. Resistivity values higher than 60,000 ohmm were measured in permafrost. Decrease in resistivity with time was monitored due to permafrost warming during summer Introduction Variations in ground temperature, which follow the seasonal warming-cooling cycle in cold regions, affect the physical properties of permafrost such as unfrozen water content and ice content. Moreover, aggradation and degradation of permafrost induce changes in physical properties and in the cryostratigraphic profile. According to Allard et al. (1996), the reconstruction of the evolution of permafrost can be made by using information from cryostratigraphic profiles. Geophysical logs, collected for this study, provide information on the variation of physical properties in permafrost and in the cryostratigraphic profile. Results from induced polarization (IP) and resistivity logging in permafrost are presented. They demonstrate that these are useful geophysical tools for mapping permafrost discontinuities, for monitoring changes in electrical properties of permafrost due to warming or cooling, and for understanding the dynamics of permafrost. Study site The study site is located in a deep valley, called ÒVallŽe des TroisÓ, at the north end of Lac Guillaume- Delisle and near the Inuit community of Umuijaq, Northern QuŽbec, Canada. The mean annual air temperature of Umiujaq is about -6.5 C. Temperature, IP and resistivity logs were carried out in a permafrost plateau. The permafrost has aggraded after emergence in a marine deposit of silt of the postglacial Tyrrell Sea. Induced polarization IP EFFECT The effect of IP can be illustrated with a standard four-electrode resistivity array. It occurs by interrupting abruptly the current flow between the current electrodes. The voltage across the potential electrodes generally does not drop to zero instantaneously (Figure 1). After an initial large decrease from the original steady state value, the voltage decays rather slowly. The decay time is in the order of a few seconds or even minutes. If Richard Fortier, Michel Allard 275

2 the current is turned on again, the potential increases over a similar time interval to the original steady state after a sudden initial increase (Figure 1). The IP effect is evaluated by comparison of the residual voltage V(t) existing at time t after the current flow is interrupted with the steady voltage Vp during the current flow (Figure 1). Chargeability is the parameter defining the IP effect. The time-domain chargeability m (expressed in mv/v) is given by the following integral over time ta to tb after the cut off of current: m = 1 tb Vt ( t - t ) ò () V dt ta b a p The time limits (t a and t b ) define a given fraction of the entire off-time cycle for the measurement of partial chargeability (Figure 1). Three sources of the IP effect may be recognized: membrane polarization, electrode polarization and surface polarization. The membrane polarization and the surface polarization are the most important sources in IP logging within permafrost. The decay curve of potential represents a return to the original state following the disturbance induced by the current flow. During the period of current flow, storage of chemical energy occurs due to the variations in the mobility of ions in fluids throughout the material structure (membrane polarization and surface polarization) and the variations between ionic and electronic conductivity [1] where metallic minerals are present (electrode polarization). The electrode polarization is not discussed further since this IP effect is insignificant in the study of permafrost. The membrane polarization takes place in the capillary pores of material filled with electrolyte. Most minerals such as clay have a net negative charge and, therefore, the positive ions in the electrolyte are attracted towards the mineralõs surface producing a diffuse cloud of cations in the vicinity of the minerals. When a current flows in the material, the remaining positive ions in the electrolyte easily pass through the diffuse cloud but the negative ions accumulate around it. Therefore, an ion-selective membrane exists. This ion concentration impedes the current flow. When the current flow is interrupted, the ions move back to their initial steady state. Membrane polarization is most pronounced in the presence of clay minerals. In the case of frozen soil where the pores are filled with ice and the unfrozen water films around soil particles are discontinuous, the electrolytic conduction can be very low and the IP effect is almost non-existent (values of chargeability are close to zero). The surface polarization of cryogenic boundaries is another important IP effect in permafrost environment as first mentioned by MelÕnikov (1973). At the contact between thawed and frozen layers, the frozen layer acts as a barrier and when current flows across this boundary, ionic charges accumulate and polarization occurs. MelÕnikov (1973) established the existence of surface polarization at the boundary of the active layer and permafrost. The surface polarization is characterized by negative values of chargeability. IP effects normally produce positive values of chargeability (Figure 1). The negative chargeability occurs at the boundary between polarizable layers of materials with a large contrast in resistivity. Thus, the surface polarization should occur not only at the boundary between permafrost and the active layer but also at the boundary between a resistive ice-rich layer and a more conductive ice-poor layer. IP AND RESISTIVITY LOGGING The dipole-dipole array was used for the measurement of resistivity and IP (Figure 2). This array consists of four electrodes along the multiconductor cable. The top pair of electrodes forms the current dipole AB, while the bottom pair forms the potential dipole MN. The spacing (a) between each pair of electrodes is constant. Spacing (na) between the two dipoles is variable and the spacing increment (n) is varied from 1 to 5. The polarity of electrodes is shown in Figure 2. Figure 1. Time domain IP waveform. At the beginning of the dipole-dipole IP log, the first four electrodes are selected on the multiconductor cable. Initially, the current dipoles AB remains fixed and 276 The 7th International Permafrost Conference

3 Values of apparent resistivity depend on the true electrical resistivities of all the components in the materials inside a ellipsoidal volume of half-axis equal to the radial distance and roughly delimited by the electrodes A and M (Figure 2). For the dipole-dipole array, the apparent resistivity is given by: ( )( + ) p = 2p dd an n + 1 n 2 DV I where r dd apparent resistivity (ohm-m), ÆV potential difference across the electrodes MN (V), and I current flow between the electrodes AB (A). Electrical resistivity of permafrost depends on the temperature, pore water salinity, porosity, material types, unfrozen water and ice contents, cryostructure and electrode array geometry (Fortier et al., 1994). [3] Figure 2.IP logging Ð Dipole-dipole array. the potential dipole MN is moved downwards for each measurement by increment of one electrode from one to five increments. Then the current dipole is moved down one electrode spacing, the spacing between dipoles is reduced to one electrode spacing, and the sequence is repeated until the last electrode is reached at the end of the cable. The dipole-dipole array combines logging and profiling aspects. This array serves to analyze not only resistivity and chargeability variations as a function of depth but also the lateral variations as a function of distance from the hole. Results are plotted at the intersection of the midpoint of the spread and the radial distance of investigation from the multiconductor cable. The radial distance (r) is controlled in part by the spacing between each pair of electrodes (a) and between the dipoles (na). In first approximation, the radial distance of investigation (r in meters) is given by (Barker, 1989): ( ) r = 0. 25L = n+ 2 a Since the midpoint of the spread varies in depth and the radial distance of investigation varies with the spacing between dipoles, the dipole-dipole IP log provides pseudo-sections of resistivity and chargeability as shown by the grid of x symbols in Figure 2. [2] INSTRUMENTATION A two-dipole electrical receiver, SYSCAL R2E manufactured by BRGM, and a 100 W transmitter were used to measure resistivity and time-domain chargeability. A pulse-current waveform (+I, 0, -I, 0) is transmitted with a pulse length of 2000 ms for a complete cycle of 8000 ms (Figure 1). Four partial chargeabilities (m 1, m 2, m 3 and m 4 ) are measured after the interruption of current flow (Figure 1). From the four partial chargeabilities, two new parameters are defined: 1. the total chargeability (a weighting of the four partial chargeabilities in mv/v): m t = 120m + 220m + 420m + 820m the gradient of chargeability (a average slope of the variation of chargeability with time in mv/v/s): m - m m m m m Dm = 1000 æ ö è ø 3 Negative values of the gradient of chargeability are characteristic of a decrease in chargeability (decrease in V(t) as illustrated in Figure 1) with time after the interruption of current flow between the current dipole electrodes. This decrease in chargeability is the normal IP effect. MULTICONDUCTOR CABLE A 25 m long multiconductor cable with 51 stainless steel electrodes spaced every 0.5 m was permanently [5] [4] Richard Fortier, Michel Allard 277

4 buried into permafrost. The electrodes consisted of stainless steel bands fixed around the cable. Each electrode was linked to a contact box at the ground surface by an individual electric conductor. In parallel with the electrode cable, 11 thermistors regularly spaced along another cable provided the temperature control. The access hole for the insertion of the multiconductor cable into permafrost was bored in August 1989 by water jet drilling. A hook at the end of the multiconductor cable fastened to the bottom of the first pipe allowed to drive the multiconductor cable to be driven into permafrost. At the end of drilling, the train of pipes was removed. Drilling rates were highly variable and dependent on the ice content. A slower rate was associated with icerich layers encountered during drilling. The drilling rate was very slow close to the surface, particularly between 2 and 4 m, and around 10 m depth. A depth of 23.5 m was reached. At a depth of about 21 m, the water circulation at the surface was lost because the water jet broke through the base of the perennially frozen layer, and water injection occurred in the water table below the permafrost base. METHODOLOGY Temperature, IP and resistivity were first logged two weeks after the installation of the multiconductor cable to evaluate the thermal disturbance of water jet drilling. During the study period in 1990, the same logs were carried out weekly over a two-month period from May 5 to July 4 to monitor the variations in electrical properties and temperature with time. The time interval between the cable installation and the study period was enough to refreeze the slurry in the borehole and to reach a new thermal equilibrium in the permafrost plateau after one complete winter. Discussion THERMAL DISTURBANCE OF DRILLING AND CRYOSTRATIGRAPHIC PROFILE Pseudo-sections of resistivity are given in Figure 3 for the IP logs of August 19, 1989, and May 24, The Ò+Ó symbols in Figure 3 indicate the intersection where parameter values were measured. The temperature profiles are also given. For the first profile (Figure 3A), the ground temperature is close to 0 C due to the large thermal disturbance induced by the water injected during drilling. At radial distances from the multiconductor cable less than 0.5 m in the pseudo-section, the resistivity is below 500 ohm-m for the major part of the log except for zones at 6, 11 and between 16 to 21 m in depth. This resistivity value marks the boundary between unfrozen and frozen zones. Therefore, the soil close to the cable (or drilling axis) was still unfrozen two weeks after drilling. The cone shape of the unfrozen soil due to a larger disturbance increasing toward the surface is perceptible in this pseudo-section with smaller resistivity values covering a larger area close to the surface. At 10 m in depth, the diameter of the unfrozen soil can be estimated to be about 1.2 m. Nine months after drilling, after one full freezing cycle, the permafrost plateau reached a new thermal equilibrium and recovered from the drilling disturbance (Figure 3B). The cooler permafrost is characterized by higher resistivity. However, the resistivity values for small radial distance remain lower than fur- Figure 3. Temperature and resistivity logs (Dipole-dipole array, a = 0.50 m, n = 1 to 5) (A- 08/19/89; B- 05/24/90). 278 The 7th International Permafrost Conference

5 Figure 4. Temperature, resistivity and chargeability as a function of depth and time (Dipole-dipole array, a = 0.50 m, n = 3). ther away from the cable. These resistivity values are characteristic of the disturbed zone. Close to the drilling axis, the water jet destroyed entirely the sequence of cryofacies and the original cryostratigraphic profile was lost. For radial distance larger than 0.60 m or for spacing increment larger than 3, two layers with resistivity value in excess of ohm-m are visible at a depth of 4 and 11 m. They are associated with ice-rich layers as detected during drilling. They are also visible in Figure 3A but to a lesser extent. The first layer near the top of permafrost is composed of aggradational ice slowly formed by downward migration of water Richard Fortier, Michel Allard 279

6 (Allard et al., 1996). The deeper layer is associated with a stabilization of the thermal gradient over a long period of time allowing the formation of ice lenses during the period of permafrost aggradation. Between these layers, the resistivity values are lower and, therefore, the ice content is also lower. These ice-poor layers formed when thermal gradients were too large and the cooling rate was too fast to allow much ice lensing during permafrost aggradation (Allard et al., 1996). RESISTIVITY AT CRYOFRONTS AND FREEZING-POINT DEPRESSION Figure 4 gives the distributions of temperature, resistivity, total chargeability and chargeability gradient with depth and time over a two-month period during the study period in These distributions were plotted from the IP and resistivity logs and for a spacing increment (n) of 3. The variation of air temperature is also plotted in Figure 4A. The cryofronts in the active layer and at the permafrost base are underlined by thick contour lines. The warming of shallow permafrost is clearly shown. The depth of zero annual amplitude change is approximately 10 m and the permafrost base is located at a depth of 22.7 m (Figure 4A). The resistivity at the cryofront in the active layer, delimiting the unfrozen from the frozen zones, is evaluated at 500 ohm-m (Figure 4B). Therefore, using this resistivity value as a reference, a basal cryopeg can be delimited between 21 m, where a contrast of resistivity from 2000 to 500 ohm-m occurs over 0.5 m (from 20.5 to 21 m), and 22.7 m at the permafrost base delimited with the temperature profile (Figure 4A). This resistivity contrast is due to the phase change from frozen to unfrozen layers at the base of the perennially frozen layer even if the temperature is still below 0 C. The freezing-point depression at the base of the perennially frozen layer (at 21 m depth) due to the overburden pressure is then evaluated to be -0.2 C. The warming of shallow permafrost is also perceptible through a decrease in resistivity from to ohm-m for a warming from -6.5 to -3.5 C at a depth of 3 m over the two-month period. Below 5 m, a small increase of resistivity was monitored with time. For example, the layer at a depth of 11 m defined by high values of resistivity increases in thickness with time due to the delayed penetration of the cold wave. migration by gravity of snowmelt water (the thawing period began around May 15 as observed with the variations of air temperature in Figure 4A) along the side of the valley and in the marine deposit washed and modified the ionic content of the groundwater. After the snowmelt period, the conditions in water table stabilized and slight chargeability variations were observed after June 4. Negative values of chargeability (Figure 4C) and positive values of chargeability gradient (Figure 4D) were measured in permafrost. This effect has the opposite sign of normal IP effect. It is due to the surface polarization previously discussed. The boundary between the ice-poor layer ( ohm-m at a depth of 7.5 m) and ice-rich layer ( ohm-m at a depth of 11 m) is delimited by a total chargeability close to 0 mv/v (Figure 4C) and a negative chargeability gradient of -4 mv/v/s (Figure 4D) at a depth of 9 m. Conclusions Based on the results from temperature, IP and resistivity logs carried out in a permafrost plateau in Northern QuŽbec, the following conclusions can be drawn: (1) IP and resistivity logging allows detection and mapping of permafrost discontinuities such as alternating ice-rich and ice-poor layers, and the basal cryopeg, and evaluation of the freezing-point depression at the permafrost base. (2) The repeated measurement of IP and resistivity logging can provide information on the permafrost dynamics. (3) The surface polarization at the boundary between frozen and unfrozen layers, or between icerich and ice-poor layers, is an important IP effect in permafrost environment allowing boundaries between these layers to be located. Variations of chargeability with time are visible in the active layer and below the base of perennially frozen layer (Figures 4C and 4D). In the active layer, they are due to the increase in unfrozen water before thaw and to the advance of the thawing front in the active layer. Large variations of chargeability and chargeability gradient in the basal cryopeg and below the permafrost base between May 10 and June 4 are associated with leaching by groundwater flow in the water table. The 280 The 7th International Permafrost Conference

7 References Allard, M, Caron, S. and BŽgin, Y. (1996). Climatic and ecological controls on ice segregation and thermokarst: the case history of a permafrost plateau in Northern QuŽbec. Permafrost and Periglacial Processes, 7, Barker, R. D. (1989). Depth of investigation of colinear symmetrical four-electrode arrays. Geophysics, 54, Fortier, R., Allard, M. and Seguin, M.-K. (1994). Effect of physical properties of frozen ground on electrical resistivity logging. Cold Regions Science and Technology, 22, MelÕnikov, V. P. (1973). Concerning the nature of an induced polarization field in connection with the surface polarization at the boundary between thawed and frozen soils. In: Proceedings 2nd International Conference on Permafrost, Yakutsk, USSR, USSR Contribution Volume, pp Richard Fortier, Michel Allard 281