ELECTRICAL RESISTIVITY SURVEYS AT THE ANDERSON RESIDENCE SITE, PORT CLYDE, ME For: St.Germain-Collins 4 Union Street, Suite 3 Bangor, Maine 441 July, 218
ELECTRICAL RESISTIVITY SURVEYS AT THE ANDERSON RESIDENCE SITE, PORT CLYDE, ME INTRODUCTION At the request of St.Germain-Collins electrical resistivity surveys were completed by Northeast Geophysical Services (NGS) at the Anderson residence at 31 Glenmere Road in Port Clyde, Saint George, Maine. The purpose of the survey work was to locate possible bedrock fracture zones beneath a portion of the site. The survey work consisted of three survey lines having a total length of 1, feet. The field work was completed on June 29 th, 218 by Mike Scully, Jack Rawcliffe and Colby Rand of NGS. This report describes the equipment and methods used and the results of the surveys. A line location map and vertical profiles showing the modeled 2-D resistivity for each survey line are included with the report. LOCATION AND SITE CONDITIONS The Anderson residence is located at 31 Glenmere Road near the village of Port Clyde, town of Saint George, Maine. Figure 1 shows the layout of the site and the locations of the resistivity survey lines. In the fall of 217 a heating oil supply truck tipped over on the south side of the driveway and spilled oil onto the surface of the wooded ground that slopes down away from the home. Subsequent to that a soil removal operation was eventually conducted over an approximately ¼ acre area of the spill site. Vegetation and soil were completely removed down to the bedrock surface which is generally less than a few feet deep in the area. The exposed bedrock surface is somewhat hummocky and appears to show some fracturing at the surface. Three resistivity survey lines were conducted at the site having a total length of 1, feet. Lines 1 and 2 are both 4 feet long and trend generally south to north. Line 3 is 2 feet long and trends west to east. Weather conditions were clear and warm during the field survey. METHODS AND INSTRUMENTATION Electrical Resistivity: Electrical resistivity is the resistance (in ohms) to the flow of electricity across a volume of material. Resistivity values are commonly expressed in ohmmeters. The resistivity of earth material is determined by measuring the voltage drop between two electrodes when current is applied into the earth through two other electrodes located a set distance away. Resistivity is calculated by dividing the voltage by the current multiplied by a constant. This constant is determined by the electrode spacing and configuration. The resistivity of earth material is primarily determined by its water content and the ionic content of the water. Lower resistivities can be caused by increasing the water content or by increasing the ions in the water or both. Thus, dry soil or rock typically has a higher resistivity than if it is saturated. And generally, the more porous or highly fractured that saturated material is, the lower its resistivity will be. The resistivity data were collected using an ABEM Terrameter SAS 4 resistivity meter with an ABEM LUND ES464 electrode selector. This is an automated multi-electrode resistivity system. With this equipment one standard field spread can consist of up to 4 1-meter cables with 21 electrode connections at 5 or 1 meter spacings on each cable. The initial spread length is thus commonly 2 or 4 meters (656 or 1,312 feet). The survey line length can be extended by successively leap-frogging the first cable to the far end of the line as each set of readings is recorded. Shorter electrode spacings can be used where more detail is preferred in the shallow subsurface. The Lund electronic switching mechanism allows the instrument to measure resistivities between electrode pairs along the line spaced progressively further apart. In this manner several hundred resistivity measurements are made on each spread. The modeled depth
of investigation is typically about 2% of the initial spread length, e.g., 13 feet for the 2- meter setup and 26 feet for the 4-meter setup. At this site Lines 1 and 2 were surveyed using 4 cables with the electrodes spaced 5 feet apart for a total line length of 4 feet. Line 3 was surveyed using 2 cables with the electrodes also spaced 5 feet apart for a line length of 2 feet. The surveys were conducted using the Dipole-Dipole array type. The Dipole-Dipole array consists of a pair of current electrodes (C1 & C2) that are followed in line by a pair of potential electrodes (P1 & P2). The positions and spacings within and between the electrode pairs are varied in order to measure the resistance at several depth levels along the survey line. Depth of investigation is determined by the spacing width between the electrode pairs with the wider electrode spacings penetrating more deeply into the earth. INTERPRETATION OF SURVEY DATA The data were interpreted using the RES2DINV interpretation software written by M.H. Loke. This program creates a 2-dimensional model of the subsurface resistivity based on the apparent resistivities measured at the surface. The effectiveness of the model to match the surface measurements is calculated as a percentage of the root-mean-square (% RMS) difference between the modeled and actual measurements. In general, a RMS value of 1% or less is considered a close match between the model and field measurements. At this site the RMS values for the models were generally fair ranging from 11% to 18%. The interpreted data was then contoured using the Surfer contouring program by Golden Software and presented as color-contoured vertical sections of apparent resistivity for each line. The colors on these sections depict the modeled resistivity with dark orange-red colors representing areas of higher resistivity (>2, ohmmeters) and low resistivities (below 25 ohmmeters) shown in blue shades. Resistivities from 25 to 2, ohmmeters are represented by green and yellow shades. The areas of high resistivity (greater than 2, ohmmeters) are interpreted to represent relatively massive, un-fractured bedrock. Very low resistivities of below 25 ohmmeters are interpreted to represent saturated fine-grained soils or saturated fractured bedrock. Areas of intermediate resistivity (25 to 2, ohmmeters) are interpreted to represent intermediate conditions such as saturated sandy or gravely soil or partially saturated fractured bedrock. This interpretation assumes that the lithology of the bedrock is consistent throughout the survey area. Low to intermediate resistivity values within the bedrock can also be caused by lithologic changes. Sulfide-bearing formations and some sedimentary units such as graphitic shale will have low resistivity values relative to most other types of bedrock. However these types of lithologic changes were not observed within the exposed bedrock at this site. SURVEY RESULTS Figures 2 through 4 show the modeled resistivity profiles for Lines 1 through 3. The survey results are presented as color-contoured vertical profiles of the modeled resistivity for each of the survey lines. The lowest resistivities, which are shown in blue shades, are interpreted to represent moist or water-saturated soil or saturated bedrock fractures. The highest resistivities, shown in orange to dark red, are interpreted to represent dry soil or massive, relatively unfractured bedrock. Intermediate resistivities, shown as greens and yellows, are interpreted to represent intermediate conditions such as saturated sandy or gravely soil or partially saturated fractured bedrock. Low to intermediate resistivity in bedrock can also be caused by changes in the rock composition. A Division of NGS, Inc.
Lines 1 and 2 show very similar patterns of modeled resistivity with both lines showing two distinct low to intermediate resistivity zones in similar positions along the lines. Line 3 also shows two fairly distinct resistivity anomalies within the bedrock at about 5 and 145 along the line. These low to intermediate resistivity zones could be caused by saturated or partially saturated fracturing within the bedrock. It is also possible that they could be caused by changes in the bedrock lithology as described above. It is observed that the northerly anomalies within the bedrock on both Lines 1 and 2 appear to extend from the bottom of the profile to the top of the bedrock surface. However the southerly anomalies on Lines 1 and 2 do not extend to the top of the profiles, they are both overlain by areas of high resistivity. Assuming that the anomalies are caused by fracturing, this difference in the modelling results for the anomalies is likely due to the nature of the material overlying the bedrock (e.g., Line 1 near RB-11 where up to 1 feet of overburden is present) or the lack of water within the fractures near the surface. The northerly anomaly on Line 1 occurs where the bedrock has been exposed by the soil removal so the modelling simply continued the anomaly to the ground surface. The northerly anomaly on Line 2 occurs where the bedrock surface is overlain by the low resistivity material of the driveway fill. Both of the northerly anomalies appear to be at least partially water-saturated near the ground surface, probably reflecting the saturated overburden in this area. The southerly bedrock anomalies on both Lines 1 and 2, however are overlain by areas of high resistivity. This situation could be caused by a layer of dry soil and/or dry bedrock overlying the saturated or partially saturated bedrock underneath. Shallow areas of low to intermediate resistivity are observed on Lines 1 and 2 where the lines pass over the gravel driveway that wraps around three sides of the house. This is likely caused by fine material within the gravel fill and would be accentuated by any application of salt to the driveway during the winter months. We did not observe any significant cultural features that could cause the resistivity anomalies seen. However, Line 2 does pass close to the house, which may be responsible for some of the low resistivity response near 3 on that line. Figure 1 shows the locations of the possible bedrock fracture zones along each survey line as magenta colored diamond shaped symbols. The anomalies have been classified as being either weak, moderate or strong as shown on the figure. Two dashed lines have been drawn on Figure 1 to suggest possible alignments connecting the anomalies. The northerly dashed line connects anomalies on all three lines while the southerly dashed line connects anomalies on lines 1 and 2. However, in discussions with St. Germain Collins, the east-northeast orientation of these alignments does not match the predominant fracture or foliation trends measured at the site, so these alignments are speculative. A Division of NGS, Inc.
4 Ft. 4 Ft. Resistivity Line 1 Resistivity Line 2 Anderson Ft. Resistivity Line 3 RB-13 RB-15 RW-2 RB-14 2 Ft. RB-12 RW-1 RB-11 EXPLANATION Weak Anomaly Moderate Anomaly Strong Anomaly Boring Ft. Ft. FIGURE 1 Distance in Feet 5 1 15 Surveyed: 6/29/218 by: RESISTIVITY LINE LOCATION MAP ANDERSON RESIDENCE SITE PORT CLYDE, ME For: St.Germain Collins
South Modeled Electrical Resistivity Anderson Residence Site, Port Clyde, ME Line 1 North 5 1 15 2 25 3 35 4 12 12 1 1 Approximate Elevation (Feet) 8 6 4 2-2 5 1 RB-11 15 Exposed Rock RB-13 Line 3 2 25 Anderson Well 3 35 4 8 6 4 2-2 -4-4 -6 Looking West -8 5 1 15 2 25 3 35 4 Distance in Feet Vertical Exaggeration: None Modeled Earth Resistivity in Ohm-Meters -6-8 Surveyed: 6/29/218 by:
Modeled Electrical Resistivity Anderson Residence Site, Port Clyde, ME South Line 2 North 5 1 15 2 25 3 35 4 12 12 1 1 8 Driveway Adjacent to House 8 Approximate Elevation (Feet) 6 4 2-2 5 1 15 Line 3 2 25 3 35 4 6 4 2-2 -4-4 -6 Looking West -8 5 1 15 2 25 3 35 4 Distance in Feet Vertical Exaggeration: None Modeled Earth Resistivity in Ohm-Meters -6-8 Surveyed: 6/29/218 by:
West Modeled Electrical Resistivity Anderson Residence Site, Port Clyde, ME Line 3 East 5 1 15 2 12 12 1 1 8 8 Approximate Elevation (Feet) 6 4 2-2 5 1 RB-15 Line 1 Line 2 15 2 6 4 2-2 -4-4 -6-6 Looking North -8 5 1 15 2-8 Vertical Exaggeration: None Distance in Feet Modeled Earth Resistivity in Ohm-Meters Surveyed: 6/29/218 by: