TESTING GEOPHYSICAL METHODS FOR EXPLORATION OF ECONOMIC ROCK

TESTING GEOPHYSICAL METHODS FOR EXPLORATION OF ECONOMIC ROCK SOURCES IN TIMBER SALE SITES IN WESTERN WASHINGTON, PACIFIC NORTHWEST OF THE UNITED STATES Survey Locations CAKIR, Recep(1); JENKINS, John E. (2); HAYASHI, Koichi(3); SCHILTER, Joseph(1); GUFLER, Terran(1); GOETZ, Venice(2); BENSON, Matt(4); CUMMINGS, Laura(2); SHAFER, Ana(2); WALSH, Timothy J. (1); HANELL, Casey(2); and NEWMAN, Patricia(5) (1) Geology and Earth Resources, Washington State Department of Natural Resources, Olympia, WA, USA, (2) Forest Resources Division, Washington State Department of Natural Resources, Olympia, WA, USA (3) Geometrics Inc, San Jose, CA, USA, (4) Northwest Geophysics LLC, Redmond, WA, USA, (5) GSI Water Solution Inc, Kennewick, WA, USA Tiger Mountain (BB- Pit) Eastern Panhandle Perry Creek A’ A A B Abstract Three million acres of federally-endowed trust lands in Washington State are managed by the Washington State Department of Natural Resources (WADNR) State Lands Division to provide revenue for the construction of public schools, colleges and other state buildings as well as support for certain state services. Road rock sources are needed for timber harvest and can be challenging to locate in rugged terrain conditions of heavily forested areas. If the desired quality rock is not found close to a timber harvest area, then the timber sale project can fail and represent a revenue loss to the trust beneficiaries. For this reason we tested geophysical methods for exploration of desired rock in the timber sale areas. The methods selected are reliable, relatively inexpensive, unobtrusive, ecologically sound, and portable for use in remote locations. The primary objectives of our work are to provide a cost effective and practical geophysical method or combination of methods to 1) locate new bedrock sources, and 2) expand existing quarries to obtain quality aggregate required for road construction. In addition, it is desired to a) identify the extent and thickness of overburden soils, b) characterize the rock quality beneath the overburden, and c) identify if groundwater is present and a significant concern. Our approach includes geologic reconnaissance in combination with one or more of the following geophysical methods: active/passive shallow seismic, single-station passive seismic, electric resistivity (OHM-Mapper), Ground Penetration Radar (GPR), and Electromagnetic Induction (EMI). Our main goal is to evaluate the use of various geophysical methods to identify feasible rock sources. To meet this goal, we tested the geophysical methods at three sites in western Washington. Results highlight the feasibility of each geophysical method used for rock exploration in planned timber harvest areas. Based on our experimental study we recommend P-wave seismic refraction and GPR surveys for the rapid exploration of the optimum source rock, and electromagnetic induction and/or electric resistivity survey methods (supplementary to seismic and GPR surveys) to explore the ground water and/or subsurface fracture conditions. B B’ A’ A’ A’- A A A’ A B B’ I Hard: Non-rippable; requires blasting Very hard to hard (pings with hammer); fresh to slightly weathered; massive or very wide fracture spacing: tight or slightly open and clean or very thin filling II Intermediate: Rippable depending on size of excavator and fracture characteristics Medium hard to hard; moderately weathered; close to moderately close fracture spacing; moderately thin to moderately thick filling; coarse breccia Soft: Dig-able with a shovel Extremely soft to soft; intensely weathered or decomposed rock; very close to close fracture spacing and moderately thick to thick filling; Note: this category includes soil overburden (colluvium and topsoil); highly weathered decomposed rock III 2. 4 -3. 6 0. 8 - 1. 8; 0. 8 -2. 4 Mineable rock with blasting; typically clean with few fines generated (gray area at lower velocities may be rippable) B B’ III II 0 - 0. 8 Waste material (unusable rock or soil overburden) A- A’ A’ Line 2000: Active seismic P-wave velocity profile. Comparing the DNR P-wave velocity to Caterpillar (2010) chart*, dashed lines delineate hypothetical boundaries between: III Dig-able: 0 -800 m/s II Rippable: 800 -1800 m/s* II or I Rippable/non-rippable intermediate/marginal zone: 1800 -2400 m/s* A’ Waypoints 785 -784: GPR profile obtained with 270 MHz A antenna. Solid line shows possible layer boundary between overburden (topsoil, soft dig-able rock), and harder but still rippable rock. GPR profile data were collected on a line parallel to active seismic line 2000. III II Line 3000: Active seismic P-wave velocity profile. Comparing the DNR P-wave velocity to Caterpillar (2010) chart*, dashed lines delineate hypothetical boundaries between: III Dig-able: 0 -800 m/s II Rippable: 800 -1800 m/s* Comments Gray area exists between low end of rippable and what is dig-able; low end of rippable may not be usable because it is too soft and has too many fines A B’- B Figure 2: Mineable basalt rock classification based on seismic velocities. The higher the velocity the denser/harder the rock. Characteristics Line 1000: Active seismic P-wave velocity profile. Comparing the DNR P-wave velocity to Caterpillar (2010) chart*, dashed lines delineate hypothetical boundaries between: III Dig-able: 0 -800 m/s II Rippable: 800 -1800 m/s* II or I Rippable/non-rippable intermediate/marginal zone: 1800 -2400 m/s* I Non-rippable: >2400 m/s* Line F_189: GPR profile with 270 MHz antenna. Solid line shows possible layer boundary between overburden (topsoil, soft dig-able rock), and harder, but still rippable rock. GPR data were collected on a line parallel to active seismic line 1000. II Mineability Electric Resistivity (OHM-Mapper) II Line F_190: GPR profile with 270 MHz antenna. Solid line shows possible layer boundary between overburden (topsoil, soft dig-able rock, ), and harder, but still rippable rock. GPR data were collected on a line parallel to active seismic line 3000. B’ Line PSTA_015: GPR profile obtained with 400 MHz antenna. Solid line shows possible layer boundary between overburden (topsoil, soft dig-able rock), and harder but still rippable rock. GPR profile data were collected on a line parallel to active seismic line 1000. B B’ Line PSTA_016: GPR profile obtained with 100 MHz antenna. The 100 MHz antenna allows for deeper ground penetration at a loss of high detail. GPR profile data were collected on a line parallel to active seismic line 1000. Electromagnetic Induction (EMI) Survey using the Geonics EM 31 -MK 2 Fast Mapping of Conductivity - 5 x 5 m grids established to accommodate the 3. 7 meter length of the coil spread. Waypoint 795: Single station passive seismic data taken with a Tromino unit. The Tromino results show estimated P-wave velocities (Vp) of: • 539 m/s @1. 0 m • 1158 m/s @3. 9 m These results correlate with velocities seen at similar depths on active seismic line 2000. References Cakir, R. and Walsh, T. J. (2011) Shallow seismic site characterizations at 23 strong-motion station sites in and near Washington State. U. S. Geological Survey Award No. G 10 AP 00027. [http: //earthquake. usgs. gov/research/external/reports/G 10 AP 00027. pdf] OHM-Mapper Survey B B’ B’- B Line 3001: Active seismic P-wave velocity profile. Comparing the DNR P-wave velocity to Caterpillar (2010) chart*, dashed lines delineate hypothetical boundaries between: III Dig-able: 0 -800 m/s II Rippable: 800 -1800 m/s* II or I Rippable/non-rippable intermediate/marginal zone: 1800 -2400 m/s* Electromagnetic Single-station Passive Active/Passive Multi-channel Analysis of Surface Waves Induction (EMI) Seismic (Tromino) -channel Shallow (MASW) field survey setup and surfacewave data processing steps Seismic B’ B III II Waypoints 778 -779: GPR Profile obtained with 270 MHz antenna. Solid line shows possible layer boundary between overburden (topsoil, soft dig-able rock), and harder, but still rippable rock. GPR profile data were taken on a line parallel to active seismic line 3001. 1) Geophysical methods can be used to identify the location of mineable rock for road aggregate. 2) Study area source rocks include basalt, basalt breccia, and intrusive basalt. 3) Our rock quality criteria can be compared with the Caterpillar (2010) results that may be correlated to seismic P-wave velocity to mining method (rippable vs. non-rippable). 4) Recommendations for Geophysical Survey Methods: a. Best: Active-source seismic P-wave refraction method yields the most detail in the fastest timeframe for shallow depths up to 10 m where it is needed for surface mining b. Second best: Ground Penetrating Radar (GPR) is the preferred method for ease of use but surface conditions must be optimum to run the survey c. Good: Electric Resistivity(ER) and/or Electromagnetic Induction (EMI) indicates the presence of water which suggests deep weathering or abundant fractures. Advantages: rapid implementation; optimum conditions not necessary d. Poor: Single-Station Passive Seismic (SSPS) does not show details near surface; affords a look at deep structure or soil/rock interface between 100 -200 m. This is not an optimum method for rock exploration which needs shallow data – up to 10 m. However, this method may be used for a preliminary rapid survey to understand the rough shear-wave velocity distribution in the area. This method is not reliable if there are high horizontal variations of soil/rock or weathered rock layers. For example, at the Tiger Mountain BB-Pit area we found that the SSPS values correlated well with Vp values in Line 2000. 5) More studies are needed in other rock types to relate geology (hardness, density, and porosity) to geophysical parameters (P- and S-wave velocities, resistivity/conductivity) for future quantitative analyses. Line A: Electric Resistivity model Survey Methods Ground Penetrating Radar (GPR) B’ B B III Type Line 1000: Active seismic P-wave velocity III Dig-able: 0 -800 m/s II Rippable: 800 -1800 m/s* profile. Comparing the DNR P-wave velocity to II or I Rippable/non-rippable Caterpillar (2010) chart*, dashed lines intermediate/marginal* zone: 1800 -2400 m/s delineate hypothetical boundaries I Non-rippable: >2400 m/s* between: III A’ Seismic P-wave velocities in correlation with rippability chart (Caterpillar Inc. , 2010) (1000 m/sec) A A’ Figure 1: Rippability chart displaying the correlation between seismic compressional-wave velocities, lithological types, and rippability classification (Caterpillar Inc. , 2010). Conclusions B’ A Electric Resistivity Results Caterpillar Inc. , 2010, Caterpillar performance handbook (40 th ed): Caterpillar, Inc. , Peoria, III. , 1, 442 p. Geometrics Inc. (2009 a) Seis. Imager/SW software manual—Windows software for analysis of surface waves: Geometrics Inc. , version 3. 3, 314 pp. [http: //www. geometrics. com] Geometrics Inc. (2009 b) Seis. Imager/2 D software manual; version 3. 3, 257 pp. [http: //www. geometrics. com] Powers, M. H. , and Burton, B. L. , 2012, Measurement of near-surface seismic compressional wave velocities using refraction tomography at a proposed construction site on the Presidio of Monterey, California; U. S. Geological Survey Open-File Report 2012 -1191, 17 p. High Speed Capacitively-Coupled Resistivity System (www. Geometrics. com) Tzanis, A. , 2010. mat. GPR Release 2: A freeware MATLAB® package for the analysis & interpretation of common and single offset GPR data, Fast. Times, 15 (1), 17 – 43.