## 5 Opportunities for In Situ Monitoring at Field Sites  
Field monitoring of biofilm development in situ is still in its infancy, and more mechanisms and experimental studies are needed. Research on geophysical signal response of biofilm development provides opportunities for monitoring subsurface bioclogging in situ in applications of biofilm in biological barriers [61; 107], underground carbon dioxide sequestration [108] and bioremediation [109; 110].  
In a biofilm growth experiment, the polarization response was proven to be directly derived from the biofilm formation process. This study suggested that the imaginary conductivity could be used as a proxy for microbial growth and biofilm formation in porous media [111]. [PERSON] and [PERSON] [112] analyzed the results of hydraulic conductivity measurement, cell count, and environmental scanning electron microscopy (SEM). They concluded that the changes in complex conductivity correspond to different stages of microbial plugging. The growth of biofilms not only changed the electrical properties but also changed the acoustic properties of the porous media [19; 50]. For example, a study by [PERSON] [19] indicated that the measured heterogeneity of the elastic experimental sand column was originated from the morphology and structure differences of biofilms in the porous media. Studies on the acoustic and electrical properties of biofilms in porous media so far remain within a laboratory setting. Further research on the mechanism of these signal changes caused by the development of biofouling is needed.  
In addition, NMR technology as a biosensor has been used to detect the presence and development of biofilms in porous media due to its excellent performance in oil exploration and water resource search at subsurface [113; 114; 115; 116; 117]. [PERSON] et al. [60] investigated the applicability of 2D NMR relaxation techniques (T\({}_{2}\), T\({}_{1}\)-T\({}_{2}\), and T\({}_{2}\)-T\({}_{2}\) measurements) by measuring pore and surface changes caused by biofilm growth in different porous media (different bead packs and natural Berea sandstone). However, under high field intensity, the measurement result cannot be used to accurately describe the pore size distribution of the porous media. The narrow range of the uniform field leads to the narrow measurement range, which is a disadvantage in researching the growth of biofilms in natural porous media. Therefore, [PERSON] et al. [118] subsequently studied the sensitivity of low-field NMR to the growth of biofilms in a porous media. [PERSON] [61] further studied the applicability of in situ NMR logging techniques to the growth of microorganisms at the subsurface. The two studies implied that low-field NMR is sensitive to microbial growth in the porous media, that is, microbial growth corresponds to the abnormalities of T\({}_{2}\) relaxation signal and can even be detected in the field site. However, the test results cannot be completely explained by the existing low-field NMR technology, so a more detailed and targeted analysis of the NMR relaxation signals of microorganisms in porous media is still needed.  
These studies focused on the relationship between biofilm growth in unconsolidated porous media (silica sands) and the geophysical signals. Further study should cultivate biofilm in porous media from the subsurface and design experiments to evaluate how the biofilm development process affects the geophysical signals. In addition, since the NMR T\({}_{2}\) relaxation measurement is a direct response of biofilms and water in the porous media, efforts could be made on to determine the permeability reduction induced by biofilm formation in the subsurface by NMR logging.