Bacterial growth and activities in natural geo-media are rarely considered in the field of geotechnical engineering. However, bacteria have long been known to considerably influence the physical and chemical properties of porous media. In recent years, research into bacterial activities has received increased attention because of their potential in geo-engineering applications and eco-friendly applications. In particular, permeability changes are associated with the growth of bacteria and their by-products in a porous medium. Bacterial growth in a porous medium alters the pore size and surface of soil particles via accumulation of biofilms and biopolymers. Bacterial colonization and the proliferation of biofilms on mineral surfaces are known to decrease permeability by several orders of magnitude and to cause bioclogging, thereby altering the hydraulic flow systems of porous media. Successful microbial bioclogging treatments require geophysical monitoring techniques to provide appropriate spatial and temporal information on bacterial growth and activities in the subsurface; such monitoring datasets can be used to evaluate the status of plugged reservoir sections and optimize re-treatment if the plug degrades. While most biogeophysical investigations have focused on geo-electrical techniques This study investigated the feasibility of using both P- and S-wave responses (velocity and attenuation) of porous media for monitoring in-situ accumulation of a bacterial material in sediments. Column experiments, where Leuconostoc mesenterorides and Shewanella oneidensis MR-1 were stimulated to produce the insoluble polysaccharide biopolymer dextran and the biofilm (a sort of extracellular polymeric substances, EPS) in a sand pack, were performed while monitoring changes in permeability as well as P- and S-wave responses. P-wave responses at ultrasonic and sub-ultrasonic frequency ranges (i.e., hundreds of kHz and tens of kHz) and S-wave responses at several kHz were acquired using ultrasonic transducers and bender elements during accumulation of the biopolymer. The observed changes in permeability, P- and S-wave velocity, and P- and S-wave attenuation were correlated with the amount of biopolymer produced, and a pore-scale analysis of the experiment results provided some insights into the pore-scale accumulation behavior of biomaterial and interactions between biomaterial and grains in porous media.
Column experiments with a fine sand pack were conducted, where the model bacteria Leuconostoc mesenteroides were stimulated to produce an insoluble biopolymer in sucrose-rich media during 38 days. The most important findings are as follows: The permeability of the sand pack was reduced by more than one order of magnitude while the insoluble biopolymer, dextran, produced by Leuconostoc mesenteroides occupied ~10% pore volume. The remarkable reduction of permeability occurring at this low level of biopolymer saturation is presumably attributed to a combination of the pore-throat clogging behavior of the accumulated biopolymer and the large apparent volume of the twined string-like biopolymer having complex internal structures and large specific surface area with an effective thread diameter of $~0.5-2 \mu m$. P-wave velocity at both frequency ranges of tens of kHz and hundreds of kHz was entirely consistent during biopolymer accumulation. In contrast, the S-wave velocity was monitored to increase by more than ~50% for ~10% biopolymer saturation, implying that this particular class of biopolymer possibly stiffens the skeletal frame of an unconsolidated sediment at the present very low effective stress conditions. This distinctive increment in the S-wave velocity suggests that the bacterial biopolymer coats the grain surfaces and grain-to-grain contacts and provides a stiffening effect, while at the same time accumulating at pore throats, as indicated by the permeability reduction. The amplitude of the P-wave signals decreased by ~90% at the frequency range of several hundreds of kHz, and by ~10-40% at the frequency range of several tens of kHz, confirming an increased seismic attenuation process at both frequency ranges. The spectral ratio calculations showed that P-wave attenuation ($1/Q_P$) in both the 600-1000 kHz band and the 60-100 kHz band increased ~1.5-1.8 times for ~10% biopolymer saturation after ~38 days of biopolymer production. The amplitude of the S-wave signals significantly increased during the increase in S-wave velocity, possibly due to the increased shear stiffness of the medium. However, the spectral ratio calculation showed that the S-wave attenuation ($1/Q_S$) in the 4-6 kHz band increased by ~50-60%. A flow-induced loss mechanism based on the permeability reduction and the increased specific surface area between fluids and solids is considered to be the most plausible mechanism accounting for the observed increase in P-wave attenuation in the ultrasonic and sub-ultrasonic frequency ranges. It is hypothesized that the observed increase in S-wave attenuation is attributed to additional motions of solid biopolymers relative to grains or to other biopolymers, which cause additional S-wave energy losses.
For biofilm formation in sand pack, the permeability of sand pack was reduced from $4.1 \times 10^{-12} to 1.2 \times 10^{-12} m^2$ due to the biofilm accumulation in soils during 21 days. P-wave velocity at ultrasonic frequency remained constant. P-wave amplitude was notably reduced by ~60% and P-wave attenuation increased by ~60%. The results suggest possibility that permeability reduction by biofilm formation can be seismically monitored at this laboratory scale. To further extend our results to a field scale, however, seismic responses at lower frequency needs to be examined.
최근 흙 속에서 미생물의 활동에 희한 지반의 역학적 특성변화에 대한 연구가 활발히 진행되고 있다. 특히 박테리아가 생산한 생체 물질인 바이오폴리머와 바이오필름은 흙 속에 축적됨에 따라 흙의 투수성을 감소시키고 흙의 강성을 증가 시킬 수 있다. 본 연구에서는 Leconostoc mesenterorides가 생산하는 바이오폴리머와 Shewanella oneidensis MR-1이 하는 바이오필름이 완전 포화된 모래 속에 축적됨에 따라 변화하는 투수성과 탄성파 전파특성을 관찰하였다.
그 결과로 바이오폴리머가 흙 속에 10%포화될때 투수성을 10-1 이상 감소 시켰으며, 그 동안 탄성파전특성이 변하는 것을 관찰할 수 있었다. S파의 전파속도가 증가는, 이는 바이오폴리머가 흙의 전단 강도를 다소 증가시킨다는 것을 확인할 수 있었다. 또한 P파 와 S파의 감쇠는 바이오폴리머가 축적됨에 따라 점점 증가하는 것을 관찰할 수 있었다. 탄성파의 감쇠는 전파대상의 매질의 물리적 상태를 가장 민감하게 나타내는 지표로 알려져 있다. 본 연구에서는 바이오폴리머의 축적에 의한 흙의 투수성 감소를 감쇠율을 이용하여 모니터링하는 모델을 제시하였고, 그 결과는 탄성파를 이용하여 지반의 투수성을 모니터링하는 기법에 대한 가능성을 제시하였다.
여러 투수성을 예상하는 모델과 비교하여 바이오폴리머의 축적 경향이 두 흙 입자 사이의 간극에서 이루어며 Kozeny pore-filling model과 잘 맞는 것을 확인할 수 있었다. 또 바이오폴리머의 큰 비표면적으로 인해 약 10%의 포화로 인해 흙과 바이오폴리머의 혼합물의 비표면적은 처음기준 4배정도 증가하는 것을 확인할 수 있었다. 이는 바이오폴리머의 큰 비표면적으로 인해 투수성을 효과적으로 감소시킨다는 것을 보여준다.
바이오필름의 축적의 경우 흙의 투수성을 다소 감소시키는 것을 확인할 수 있었으며, P파와 S파 전파속도의 변화는 일정한 것으로 측정되었고, P파의 진폭은 점점 감소하였으며, S파의 진폭은 점점 증가하는 것을 확인할 수 있었다.
