IMAGING A SOIL FRAGIPAN USING A HIGH-FREQUENCY MASW METHOD. Abstract

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1 SAGEEP 2014 Boston, MA USA IMAGING A SOIL FRAGIPAN USING A HIGH-FREQUENCY MASW METHOD Zhiqu Lu, National Center for Physical Acoustics, The University of Mississippi, University, MS Glenn V. Wilson, USDA-ARS National Sedimentation Laboratory, Oxford, MS Craig J. Hickey, National Center for Physical Acoustics, The University of Mississippi, University, MS Abstract The objective of this study is to noninvasively image a fragipan layer, a naturally occurring dense soil layer, using a high-frequency (HF) multi-channel analysis of surface wave (MASW) method. The HF-MASW is developed to measure a soil profile in terms of shear (S-wave) wave velocity at depths up to a few meters. While conventional MASWs use geophones as surface vibration sensors, the present MASW uses an accelerometer as a sensor to detect Rayleigh wave propagation generated by an electromechanical shaker operating in a chirp mode to achieve high frequency and high spatial resolution. With the method, the subsurface soil properties at a test site were measured, visualized, and evaluated. A 2-dimensional S-wave velocity image was obtained and from the contrast of the image, the presence, depth, and extent of a fragipan were identified. The HF-MASW result was compared with those of site characterization made by invasive methods and a 2-dimensional image obtained by a penetration test. The results from the HF-MASW and soil characterization were in good agreement. The study demonstrates the capability of the HF-MASW technique for detection and imaging subsurface layers such as a fragipan. Introduction Soil is highly complex and spatially variable. Typical soil profiles are comprised of layers with distinct soil properties. It is these layers that define a soil and its impact on the Critical Zone. One such layer, a fragipan, has proven to be particularly important to soil behavior. Ubiquitous, yet spatially variable across landscapes, fragipans are dense, brittle when moist, and restrictive to root and water penetration (Lindbo et al., 1995). By restricting water movement, fragipans play a critical role in hydrologic behavior, erosion, and land use (Lindbo et al., 1995). It is defined as a layer "with very low organic matter, high bulk density, and/or mechanical strength..., has hard or very hard consistence (seemingly cemented) when dry, but moderate to weak brittleness when moist" (SSSA 2008). Despite its significance, this unique soil feature is difficult to identify in the field (Smeck and Ciolkosz, 1989) and better tools are needed for mapping fragipans. In the past, a so-called acoustic to seismic coupling technique was employed to determine the depth of soil-fragipan interface and the mechanical properties of the soil-fragipan system (Howard and Hickey, 2009). In this study, a well-developed non-invasive method known as multi-channel analysis of surface wave (MASW) method is adopted and modified. The MASW method is a seismic technique based on spectra analysis of one type of seismic surface waves, i.e. Rayleigh waves, to determine S- wave velocity profile (Park et al., 1999; Xia et al., 1999; Park et al., 2007). This method has been increasingly applied to many geotechnical and civil engineering projects. The MASWs aim is exploring subsurface properties at depths from several meters to tens of meters due to the usages of low frequency sources, such as sledgehammers and weight drops. The soil, typically with a thickness of a couple of meters of weathered parent material, is usually treated as an effective layer with average properties.

2 Therefore, the detail structure of the upper few meters of soil cannot be determined by the conventional MASWs. Recently, a HF-MASW method has been developed using high frequencies in a range from Hz or above to measure soil profile properties in the vadose zone for applications in the areas of agriculture, environment, and military (Lu, 2014a; 2014b). The objective of this study was to test an accelerometer-based HF-MASW method for its ability to identify the presence, depth, and extent of a fragipan at a test site in Holly Springs, MS. The HF- MASW result will be compared to site characterization using standard profile description methods and a 2-dimensional image obtained by a penetration test. High-frequency MASW Method A standard MASW procedure consists of several parts: (1) generating and recording surface waves, (2) determination of dispersion curve, and (3) inversion, as described briefly as follows. In the study, the HF-MASW method consists of an electromechanical shaker (Vibration Test System, Model VG-100-6) as a seismic source and an accelerometer with a spike (PCB Piezotronics, Model 352B) as a seismic vibration sensor. The shaker operates in a frequency-sweeping mode to generate a chirp signal to excite surface waves. The surface vibrations are measured by consecutively inserting the accelerometer into the ground at multiple locations along a straight line. Due to Nyquist sampling theorem and the need to distribute seismic energy uniformly over entire frequency range, the source frequency is divided into three overlapping bands: Hz, Hz, and Hz, with corresponding chirp durations of 3.2 s, 1.6 s, and 1.3 s and sampling frequencies of 8 khz, 15 khz, and 20 khz, respectively. A program written by LabView (National Instruments, Inc.) is used for chirp signal generation, data acquisition, signal processing, and data analysis. To determine dispersion curve, two transformations are performed with the LabView program. The time traces are transformed through FFT, as expressed in the following. 1 F (, x) f ( t, x) exp( i t) dt 2 where t and ω are time and angular frequency, f(t, x) and F(ω, x) are the received signal and complex FFT spectrum, respectively, at distance x from the source. The second is a wavefield transformation, expressed as, x F( c, ) F(, x) exp( i ) dx (2) c where F(c, ω) is the amplitude in c-ω space and c is an arbitrary phase velocity. The outcome of the two transformations is a so-called overtone image plotted in an intensity graph. One of the major advantages of the MASW is its ability to deal with all the information of seismic wave signals, which usually consist of the fundamental-mode and higher-modes of surface waves, body waves, reflected and scattered waves, and ambient noises (Park et al., 2007). These seismic wave components can be effectively identified or separated from the energy patterns because of their unique wave speeds, attenuations, and propagation paths. The dispersion curve can be obtained by picking up the identified fundamental-mode and possible higher modes of Rayleigh wave components. The shear wave velocity profile is determined from the measured dispersion curve by an inversion process with an iterative algorithm. An intermediate dispersion curve is calculated using forward modeling of Rayleigh wave propagation starting with an initial earth model that includes frequency and four soil parameters: the thicknesses of layers, the shear and longitudinal wave velocities, and the density at each layer. The estimated dispersion curve is compared with the measured one. If the (1)

3 comparison yields a difference greater than a threshold value, the earth model is modified. The estimation and comparison are iterated until the results converge to a pre-determined value. The shear wave velocity profile is determined from the final earth model (Xia et al., 1999; 2003). In the study, SurfSeis3 (Version , Kansas Geological Survey) was used for inversion and 2D imaging. Site Characterization Site characterization tests were conducted on the North Mississippi Experiment Station at Holly Springs, MS. The site selected is mapped as Providence (fine-silty, mixed, active, thermic Oxyaquic Fragiudalf) with fragipan layers expected to occur between 58 and 135 cm deep. A soil pit was dug to 150 cm depth and the soil profile was described in Table 1. Undisturbed soil cores were taken in triplicate from each horizon for bulk density measurement. Shear strength was measured in triplicate in 10 cm increments from 10 cm depth to depth of refusal using a Geonor shear tester (H-60 vane tester by Geonor Inc.) and at the soil surface using a Torvane shear vane (Durham Geo Slope Indicator). In addition, the shear strength of each horizon was determined on the pit face using the Torvane shear vane. Table 1: Soil profile characterization Geonor Depth Shear Horizon (cm) Strength (KPa) Torvane Shear Strength (KPa) Bulk density (Mg m 3 ) Structure description A 0 to weak fine granular E 7 to weak medium granular Bt 1 14 to subangular blocky Bt 2 35 to subangular blocky Btx 1 50 to 90 refusal too hard 1.65 prismatic Btx refusal too hard 1.46 prismatic Site characterization showed conclusively that fragipan horizons existed at the site with two fragipan layers (Btx 1 and Btx 2 ) having upper boundaries at 50, and 90 cm depths, as seen in Table 1. In addition, two argillic (Bt1 and Bt2) horizons, i.e. layers exhibiting accumulation of clay that have been translocated from layers above, existed above the fragipan layers. The boundary between the lower argillic and upper fragipan was irregular due to the nature of the prisms of fragic material that extended up into the argillic layer. The dense fragic material must occupy 60% by volume of the soil to be classified as a fragipan layer. Thus, the lower argillic horizon (Bt 2 ) contained significant volume of fragic material that was insufficient to qualify as a fragipan layer. The Torvane shear strength increased to 85 and 94 kpa, respectively, for the argillic horizons at 14 to 35 and 35 to 50 cm depths and to 224 and 225 kpa, respectively, for the Geonor shear strength. However, the shear strength of the two fragipan layers exceeded the limits of both techniques. The USDA-NRCS had proposed to define fragipans based upon their bulk density being greater than 1.60 to 1.65 Mg m-3 depending upon the clay content (Glocker and Quandt, 1993). The bulk density increased to 1.65 Mg m-3 for the upper fragipan layer (50 to 90 cm depth) but decreased to 1.46 for the lower fragipan. Soil penetration tests were conducted using a penetrometer (Eijkelkamp, Agrisearch Equipment, Giesbeek, Netherlands). Twenty penetrations were made at the mid-points of MASW scanning lines with an equal separation of 0.2 meter. They were used to construct a vertical cross-section image

covering an area of 4.0 0.8 m 2, as shown in Fig.1.

4 covering an area of m 2, as shown in Fig.1. Each penetration recorded penetration resistance readings in units of MPa with the maximum penetration depth of 80 cm and penetration force of 1000 N. At depths of cm the soil became so hard that the maximum penetration force was exceeded and no reading was available below these depths. In this case the penetration resistances were assigned a value of 11 MPa as shown as black pixels in Fig. 1. The image of Fig. 1 represents a typical soil stratum formation in the vadose Figure 1: The result of the penetration test zone. Three distinctive layers can be identified. The topsoil from the surface to the depth of 10 cm manifests a high resistance region indicating the existence of a hard top layer. This thin surface layer is often caused by rainfall impacts that form a surface crust (Leary et al., 2009) and by vehicle traffic (Lu et al., 2004). In addition, dry surface condition increases soil suction that leads to an increase in the effective stress (Lu and Sabatier, 2009; Lu, 2014a) and hardens the surface soil. The middle layer around 10 cm to 30 cm exhibits low value of penetration resistance, showing an opposite hydrological effect on the soil stiffness, i.e. the increased water content softens the soil. The third layer, from depths of 30 cm to 50 cm, manifests a gradually increasing trend in the penetration resistance with depth. This zone represents the transition from the argillic to fragipan layers (see Table 1). From Figure 1, it is clear from the high resistance to penetration that a dense restrictive layer exists but given the natural occurring irregular boundary between the argillic and fragipan layers it is difficult to determine the thickness and extent of the fragipan layer. Results of the High-frequency MASW Test At the same site of the penetration test, the HF-MASW test was conducted. The geometric parameters for 1D MASW data acquisition were 40 cm near offset, 20 cm receiver spacing, and 25 points of surface vibration measurements by the accelerometer, respectively. The 1D MASW were repeated 25 times by moving the shaker and receiver array 20 cm forward along a straight line after each MASW run, a procedure similar to the Common-Mid-Point (CMP) - style data acquisition. The HF- MASW tests scanned a straight line with an overall length of 10 meters. In processing the MASW signals, the overtone images of three overlapping frequency bands were combined to form one overtone image. Of the 25 total overtone images taken, 24 of them are displayed in Fig. 2. For all overtone images, two energy patterns can be recognized, representing the fundamental mode and one higher mode of Rayleigh waves. The dispersion curves were extracted from the maximum of these energy patterns, displayed in Fig. 2 as dots for the fundamental mode and solid line for the higher mode of Rayleigh waves. The variations of these dispersion curves reflect the lateral inhomogeneity of the test site.

5 Figure 2: The 24 overtone images, where dots and solid lines represent the dispersion curves of the fundamental and higher modes of Rayleigh waves respectively

The inversions were performed and the first attempt was to use only the fundamental mode to construct an S-wave velocity image. The result is displayed in Fig.

6 The inversions were performed and the first attempt was to use only the fundamental mode to construct an S-wave velocity image. The result is displayed in Fig. 3(a), where the x-axis is the horizontal surface location, representing the mid-point number of the scanned line, the y-axis is the depth, and the color-scale bar represents the range of the S-wave velocity. Figure 3: The S-wave velocity image obtained by using the dispersion curves of (a) the fundamental mode and (b) the fundamental and higher modes of Rayleigh waves respectively The soil profile image of Fig. 3(a) shows a general trend that the S-wave velocity increases with depth. Soil layers are generally visible but the spatial properties are smoothed in Fig. 3(a), as compared with the results illustrated in Table 1 and Fig. 1. Therefore, one may conclude that the inversion using only the dispersion curves of the fundamental mode of Rayleigh waves may yield soil profiles that do not sufficiently describe the test site s layering and spatial heterogeneity. Fig. 3(b) shows the result of the inversion by taking both the fundamental and higher modes of Rayleigh waves into account, where a dashed line at the depth of 80 cm delineates the bottom line of the penetration test. It is clear that the addition of the higher modes of Rayleigh waves improves the accuracy and quality of the S-wave velocity image. The top hard layer is now noticeably featured as high S-wave velocity at the depth around 20 cm, manifesting a surface crust or dry layer. However, as compared with Fig. 1, the top layer of the S-wave velocity image is not as sharp as that of Fig. 1. This is because of the inherent characteristic of the MASW method which spatially averaged soil properties that smears the image. The insufficiency of higher frequency energy of the current MASW also gives rise to vertical resolutions that are not high enough to ascertain a very thin layer. The overall agreements between Fig. 3(b) and Fig. 1 are quite satisfactory. As in the case of penetration test, the S-wave velocity image shows three distinct layers as characterized by their S-wave velocity: a top dry rigid layer, a middle moist soft layer, and a layer featured as monotonically increasing S-wave velocity with depth, at the depths of cm, cm, and below 50 cm, respectively. The formation of these S-wave velocity layers can be explained by using the concept of the effective stress (Terzaghi et al, 1996; Bishop and Blight, 1962) and its power law relation with acoustic velocity (Santamarina et al., 2001; Lu et al., 2002; 2004; Lu and Sabatier, 2009; Lu and Wilson, 2012; Lu, 2014a). For soils in the vadose zone, the effective stress is controlled by total stress and water potential. For shallow surface soils, water potential dominates the effective stress, which causes S-wave velocity to increase as soils become dryer and decrease as soils get wetter. For deep soils, total pressure in terms of overburden pressure prevails, leading to an increase in S-wave velocity with depth. In addition to the three layers, a high velocity layer (HVL) is apparent in Fig. 3(b) at the depth around 80 cm. The HVL stretches across the entire horizon with varying depths from 60 to 140 cm. The spatial distributions, in terms of depth, size, and shape, of the HVL are in good agreement with the

7 characteristic of a fragipan. Several observations can be made to justify the identification of the presence of fragipan found in Fig. 3(b). The HVL manifests distinctive contrasts among their adjacent soils, in accordance with the general fact that fragipans are usually characterized as high mechanical strength relative to overlying and underlying horizons, even when the upper layer is an argillic horizon (Norfleet and Karathanasis, 1996), resulting in higher values in penetration resistance and S-wave velocity than those of its surrounding materials. The depth of presumed fragipan horizon in the S-wave velocity image matches the ones observed from the visual inspection of the pit face and the ground truth listed in Table 1. The irregular boundary, varying thickness, and horizontal orientated formation of the HVL are in accord with the morphological characteristic of naturally formed subsurface structure of a fragipan. Discussions Two issues in the vadose zone exploration using the MASW method need to be addressed. First, to investigate the very thin top-layer, a fourth frequency band ranging from 450 to 1000 Hz can be added. A self-adaptive MASW method with linearly increased receiver spacing was recently developed and can be employed to determine the dispersion curves of Rayleigh waves at such high frequencies (Lu, 2014b). Secondly, the situation in which a HVL overlies a low velocity layer, such as a fragipan, creates a velocity inverse situation and leads to a challenging problem in the process of inversion. Current available algorithms for inversion often fail to deal with a velocity inverse model and will introduce errors in determining the S-wave velocity below the HVL (Pan et al., 2013). A new forward modeling method should be incorporated into the inversion algorithm (Pan et al., 2013). Conclusions In conclusion, the study demonstrates the capability of the HF-MASW technique for noninvasively imaging dense subsurface layers such as fragipans. From the contrast of the S-wave velocity image, the presence, depth, and extent of a fragipan are determined. The result of the HF- MASW is in good agreement with those of site characterization by the invasive methods, as illustrated in Table 1 and Fig. 1. It is found that the addition of the higher mode of Rayleigh waves into inversion significantly improves the accuracy and resolution of the S-wave velocity image, especially in identifying structural layering information and spatial heterogeneity. Acknowledgements This work was supported by the U. S. Department of Agriculture under Specific Cooperative Agreement The authors wish to express their appreciation to Alan Hudspeth, Allen Gregory, and Xiaobin Wu for their outstanding technical support and to the Mississippi Agricultural and Forestry Experiment Station for the use of their facilities in Holly Springs. The first author also would like to thank Julian Ivanov and Jianghai Xia for inspiring discussions. References Bishop, A.W., and Blight, G.E., 1962, Some aspects of effective stress in saturated and partly saturated soils: Géotechnique, 13,

8 Glocker, C.L., and L.A. Quandt What's up in fragipans. 2nd ed. Soil Surve. Qual. Assurance. USDA-SCS National Soil survey Center, Lincoln NE. Howard, W., and Hickey, C.J., 2009, Investigation of the near subsurface using acoustic to seismic coupling, Ecohydrology, 2, Leary, D., DiCarlo, D.A., and Hickey, C.J., 2009, Acoustic techniques for studying soil-surface seals and crusts: Ecohyrology, 2, Lindbo, D.L., Rhoton, FE., Bigham, J.M., Hudnall, WH., Jones, FS., Smeck, NE., and Tyler, D.D Loess toposequences in the Lower Mississippi River Valley: I. Fragipan morphology and identification. Soil Sci. Soc. Am. J. 59, Lu, Z. and Wilson, G.V., 2012, Acoustic measurements of soil pipeflow and internal erosion: Soil Science Society of America Journal, 76, , doi: /sssaj Lu, Z., 2014a, Study of seasonal and weather effects on shallow surface soils using a seismic surface wave method: Journal of Environmental & Engineering Geophysics, (in print). Lu, Z., 2014b, Self-adaptive method for high-frequency dispersion curve determination: Proceedings of 2014 SAGEEP, Boston, Mar , Lu, Z., and Sabatier, J.M., 2009, Effects of soil water potential and moisture content on sound speed: Soil Science Society of America Journal, 73, Lu, Z., Hickey, C.J., and Sabatier, J.M., 2004, Effects of compaction on the acoustic velocity of soils: Soil Science Society of America Journal, 68, Norfleet, M.L., and A.D. Karathanasis Some physical and chemical factors contributing to fragipan strength in Kentucky soils. Geoderma, 71, Pan, Y., Xia, J., Gao, L., Shen, C., and Zeng, C., 2013, Calculation of Rayleigh-wave phase velocities due to models with a high-velocity surface layer: Journal of Applied Geophysics, 96, 1-6. Park, C.B., Miller, R.D., and Xia, J, 1999, Multichannel analysis of surface waves: Geophysics, 64, Park, C.B., Miller, R.D., Xia, J., and Ivanov, J., 2007, Multichannel analysis of surface waves (MASW) active and passive methods: The Leading Edge, January 2007, Santamarina, J.C., Klein, K.A., and Fam, M.A., 2001, Soils and waves- particulate materials behavior, characterization and process monitoring: John Wiley & Sons, LTD, Chichester. Smeck, N.E., and Ciolkosz, E.J., (eds) Fragipans: Their occurrence, classification, and genesis. Soil Sci. Soc. Am. Spec. Publ. 24. SSSA Glossary of Soil Science Terms. Soil Sci. Soc. Am., Madison WI. Terzaghi, K., Peck, R.B., and Mesri, G, 1996, Soil mechanics in engineering practice: 3rd Ed. John Wiley&Sons, Inc. New York. Xia, J., Miller, R.D., and Park, C.B., 1999, Estimation of near-surface shear-wave velocity by inversion of Rayleigh waves: Geophysics, 64, Xia, J., Miller, R.D., Park, C.B., and Tian, G., 2003, Inversion of high frequency surface waves with fundamental and higher modes: Journal of Applied Geophysics, 52,

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