Lima Project: Seismic Refraction and Resistivity Survey. Alten du Plessis Global Geophysical

Transcription

1 Lima Project: Seismic Refraction and Resistivity Survey Alten du Plessis Global Geophysical Report no 0706/ December 2006

2 Lima Project: Seismic Refraction and Resistivity Survey by Alten du Plessis (Pr.Sci.Nat) Global Geophysical PO Box Erasmuskloof 0048 Tel: +27 (0) Fax: +27 (0) Cellular + 27 (0) alten@globalgeo.co.za Internet: Final Report presented to Larry Pringle Associate (Geotechnical) BKS Engineering and Management Tel. +27 (0) Fax. +27 (0) Cell. +27 (0) Report no 0706/ December 2006

3 List of Figures Figure 1 - Site Locality Map Figure 2 - Traverse T1 Seismic Refraction & Resistivity Results Figure 3 - Traverse T2 Seismic Refraction & Resistivity Results Figure 4 - Traverse T3 Seismic Refraction & Resistivity Results Figure 5 - Traverse T4 Seismic Refraction & Resistivity Results Figure 6 - Traverse T5 Seismic Refraction & Resistivity Results Global Geophysical 1

4 Executive Summary A geophysical investigation comprising a combination of Seismic Refraction and Resistivity was conducted for the Lima Escom Pumped Storage Scheme Project. The main objectives of the investigation were the following: Mapping of possible faults and/or fracture zones Mapping of depth of weathering and bedrock quality The geophysical investigation was conducted on the upper dam site, and a total linear distance of 2,106 metres was acquired. Seismic Refraction and Resistivity traverses were conducted on the same location for improved interpretation of anomalous features. Seismic Refraction is an industry accepted geophysical technique for mapping depth to bedrock and bedrock quality and is typically conducted during dam and quarry site investigations. It produces seismic velocity as a function of depth and chainage, and is very sensitive to the change in depth to bedrock. Resistivity measures subsurface resistivity and can also be used to map depth of weathering and different geological stratigraphical layers. Good data quality was obtained for both the Seismic Refraction and Resistivity surveys, and a very good correlation between the two data sets were also observed. The results could be used to infer changes in the depth-of-weathering, and the location of potential structural features like faults and/or fracture zones could also be inferred. Global Geophysical 2

5 1. Introduction Global Geophysical was appointed to conduct a geophysical investigation for the Lima Escom Pumped Storage Scheme in Mpumalanga. The main objective of the investigation was the mapping of depth-of-weathering and structural features like faults and fracture zones. The site is underlain by granite and numerous granite outcrops and boulders and outcrops were visible on site. However, the quality of the rock and the extent of quality rock as a function of depth and distance could not be visually estimated, and the difference between boulders and actual rock outcrop could not be unambiguously identified. A geophysical investigation comprising a combination of Seismic Refraction and Resistivity were employed. Seismic Refraction is an industry accepted geophysical technique for mapping depth to bedrock and bedrock quality and is usually conducted during dam and quarry site investigations. Resistivity measures subsurface resistivity and can also be used to map depth of weathering, stratigraphy, and structures such as faults, dykes and fracture zones. Seismic Refraction and Resistivity were conducted on the same traverse lines to assist in the interpretation of the results. Global Geophysical 3

6 2. Data Acquisition and Survey Methodology The use of a multi-geophysical approach is recommended as the measurement of two or more subsurface properties, such as velocity and resistivity, can assist in a more quantitative interpretation of geophysical features and anomalies. Seismic Refraction is conventionally used in dam site investigation as seismic velocity as an excellent tool to map depth to bedrock, and bedrock quality. 2.1 Description of Geophysical Techniques The following geophysical techniques were used/evaluated in the investigation: Seismic Refraction Resistivity Seismic Refraction The Seismic Refraction method utilizes seismic waves traveling through different parts of the subsurface. A seismic source is used to generate compressional waves, which is measured by a seismograph and a series of evenly spaced sensors (typically 12, 24, 48 or more geophones). Typical sources include a hammer and plate (for imaging depths up to 10's of metres), as well as explosive sources such as dynamite for deeper penetration. Seismic refraction is a quantitative method as it produces depths of various geological layers, as well as the seismic velocities of these various layers. Seismic velocities can assist in the interpretation of geological layers as well as determining the rippability of bedrock. The geophysical property that is measured in seismic refraction is seismic velocity. In seismic refraction surveys, two kinds of waves are of importance, namely the P-wave (a compressional, longitudinal wave) and the S-wave (a shear, transverse wave). P-waves propagate at the highest velocity of any seismic waves and are therefore commonly used to pick the first breaks of seismic waves that propagated through earth materials. Since travel time equations can be derived as a function of velocity, depth to a refractor such as bedrock can be determined in a seismic refraction survey. Global Geophysical 4

7 Resistivity With the resistivity technique, a current is transmitted into the ground by two current electrodes, and a resulting secondary voltage is measured by two separate potential electrodes. Measurement of the voltage and current allows determination of the average subsurface resistivity in a volume greatly determined by the separation of the electrodes. Depth of investigation is a function of the distance between electrodes as well as the configuration between current and potential electrodes, and can be increased by increasing the electrode separation. Resistivity allows the mapping of the lateral and depth variation in subsurface resistivity and can thus be used to discriminate between geological units of different resistivities. 2.2 Instrumentation & Field Procedures The table below lists the equipment used for the different techniques as well as the field procedures and settings: Table 1. Geophysical Instrumentation and Field Procedures Geophysical Technique Instrumentation Field Procedures Seismic Refraction 24 channel Geometrics SmartSeis Hammer and Plate Energy Source 3 m geophone separation 7-9 shots per spread Record length = 1024 milliseconds Shot stacking: No filters applied during acquisition Resistivity 41 channel Abem Lund Imaging System Stainless steel electrodes. 2-m electrode separation Schlumberger array. 2.3 Surveying and Referencing A sub-metre accuracy Omnistar GPS system was used for referencing of geophysical traverses, pegs and topographical variations. The positions of existing borehole UR-3 was used to confirm the GPS accuracy. X and Y coordinates were found to be within sub-metre accuracy; however an elevation shift of metres was applied to the GPS elevations by comparing the GPS elevation to elevation contours provided on the Client Locality Map. Global Geophysical 5

8 Pegs were inserted at 69-m intervals. Survey system used was WGS-84 LO-29 (WG-29). A total of five (5) traverses were conducted as shown in Figure 1. The positions of previously drilled boreholes are also indicated. Global Geophysical 6

9 3. Data Processing The following data processing packages were used to process the geophysical data. Table 1. Processing of Geophysical Data Technique Software Comments Seismic Refraction Inversion program, no layer identification necessary. Resistivity RES2DINV Inversion programme. Global Geophysical 7

10 4. Results The seismic refraction and resistivity results are presented in Figures 2 to 6 respectively. The seismic results are presented at the top of each figure, with the corresponding resistivity results below. Seismic velocities (compressional) were found to vary between 300 to 6000 m/s, with resistivity variations of between 200 and Ohm.m in general. The following table describes a general relationship between rock type and seismic velocity: # Description Seismic velocity 1 Overburden consisting of transported material m/s 2 Highly weathered/fractured to moderately weathered/fractured rock m/s 3 Moderately to slightly weathered/fractured rock m/s 4 Slightly weathered/fractured to unweathered/fractured rock > 3000 m/s The seismic refraction technique measures either compressional (p-wave) or shear (s-wave) wave velocities; in this particular study compressional p-wave velocities were measured. Compressional wave velocity of a material depends on the density as well as the elastic moduli, and can in general be used to classify geological material. However, in general, it is not possible to accurate distinguish between highly weathered rock and highly fractured rock, or slightly weathered rock and slightly fractured rock. The degree of water saturation also increases the compressional wave velocity of material. The software used to model the subsurface velocities generated a model where velocities increase continuously as a function of depth. This is a more acceptable model in areas where the weathering profile changes gradually with depth than other refraction interpretation models where a 2 or 3-layer model with discreet velocities are used. The gradient of velocity change with depth is an indication of the sharpness of the transition zone. Figure 2. Traverse T1 A very good correlation can be observed between the seismic and resistivity results for T1. The seismic results are characterized by the presence of a shallow high velocity layer between 0 and 150 metres chainage where granite outcrop could be visually observed on site, inferring the presence of Global Geophysical 8

11 shallow competent bedrock (Slightly weathered and/or fractured granite) at depths shallower than 10 metres. Similarly, the resistivity results show a high resistive zone at this chainage, also inferring the presence of shallow granite. A similar zone of shallow bedrock can be observed between 300 and 450 metres chainage on both the seismic and resistivity results. Two very pronounced zones of inferred deeper weathering can be observed as indicated. These zones may represent zones of faulting and/or fracturing with associated preferential weathering. Depth to mediumly weathered and fractured rock is expected to exceed 30 metres in these areas. Figure 3. Traverse T2 The area covered by traverse T2 is characterized by the presence of numerous outcrops of granite bedrock with expected shallow weathering. The seismic results confirm this shallow weathering, with velocities exceeding 2000 m/s observed at depths between 0 and 10 metres below surface. Fresh unweathered bedrock is interpreted at an elevation of approximately metres (depth of 15 metres on average) with some variations at 50 to 100, and also possible between 175 and 200 metres chainage. Figure 4. Traverse T3 A very good correlation can again be observed between the seismic and the resistivity results. Shallow, and predominantly horizontal bedrock is inferred between 0 and 200 metres chainage, with a potential zone of deeper weathering (possible fault/fracture zone) observed at 240 to 280 metres chainage). Figure 5. Traverse T4 Very uniform seismic and resistivity sections are observed on Traverse T3, with a well-defined change in the character of the data. The seismic results now show a prominent zone of very low seismic velocities, and a deeper weathering profile is interpreted compared to the previous traverses. Global Geophysical 9

12 Figure 6. Traverse T5 Traverse T5 suggest even deeper weathering, with the resistivity section dominated by the presence of a very low resistivity zone at depth. The resistivity method also could not see higher resistivities at depths of approximately 30 metres, suggesting that the resistive bedrock is at least at depths exceeding 25 to possibly 30 metres below surface. This is also confirmed by the seismic results which suggest that fresh unweathered bedrock vary between 30 and 40 metres below surface. A zone of potentially even deeper weathering can be observed between 300 and 450 metres chainage. Global Geophysical 10

13 5. Coordinates Table 1. Geophysical Peg Coordinates X Y Z Peg SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT SUT5+552 Global Geophysical 11

14 Figure 1. Lima Project Locality Map Survey System: WGS-84 LO SU05 UR SU06 UR UR SU SU UR SU SU03 SU UR Global Geophysical October SU

15 Elevation in metres Seismic Refraction Results Zone of very shallow weathering Figure 2. Preliminary Seismic Refraction & Resistivity Results Traverse 1 (T1) Resistivity Results Preferential weathering - possible fault / fracture zones Seismic velocity (compressional) in metres per second Elevation in metres Global Geophysical October 2006

16 Elevation in metres Figure 3. Preliminary Seismic Refraction & Resistivity Results Traverse 2 (T2) Seismic Refraction Results Seismic velocity (compressional) in metres per second 1700 Resistivity Results Elevation in metres Global Geophysical October 2006

17 Figure 4. Preliminary Seismic Refraction & Resistivity Results Traverse 3 (T3) Seismic Refraction Results Elevation in metres Shallow bedrock Resistivity Results Seismic velocity (compressional) in metres per second Preferential weathering - possible fault / fracture zones Elevation in metres Global Geophysical October 2006

18 Figure 5. Preliminary Seismic Refraction & Resistivity Results Elevation in metres 1700 Traverse 4 (T4) Seismic Refraction Results Seismic velocity (compressional) in metres per second 1700 Resistivity Results Elevation in metres Global Geophysical October 2006

19 Figure 6. Preliminary Seismic Refraction & Resistivity Results Elevation in metres 1700 Traverse 5 (T5) Seismic Refraction Results Resistivity Results Zone of deeper weathering Seismic velocity (compressional) in metres per second 1700 Elevation in metres Global Geophysical October 2006