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The frequency selection method (FSM) is the further development of audio frequency telluric electricity method (TEFM), however there are still ongoing debates on the involving mechanisms leading to anomaly genesis. Therefore, the present study intends to explore this using 2D forward modeling of magnetotelluric (MT) sounding, and practical applications of FSM on three Chinese case studies in karst and granitic settings. In the first stage, the profile curves and pseudosection of apparent resistivity (𝜌 𝑠 ) and horizontal electric field component (𝐸 𝑦 ) in Transverse Magnetic field (TM) mode are obtained by forward calculation. As a result, the static shift in 𝜌 𝑠 is observed over the near-surface inhomogeneities, as documented in literature. Additionally, the profile curves of 𝐸 𝑦 showed an obvious static shift in the rectangular coordinate system (i.e., the curve rises with the increase in frequency) which is a well-known phenomenon. The pseudosections of 𝐸 𝑦 also showed static shift characteristics at the horizontal position above the anomaly, referred to as "noodles phenomenon". The FSM results obtained from case studies related to the groundwater and low resistivity clay-filled karst body identification. The ∆𝑉 section curves and pseudo-section showed a significant low potential, and a "noodles phenomenon" respectively, above the low resistive anomalous body. These abnormal characteristics of ∆𝑉 are the basis for delineating the horizontal position of groundwater aquifer applying FSM. It is concluded that the anomaly of FSM is the reflection of the static shift in MT and hence, the FSM can be categorized as a "static shift method". Therefore, this inspired us that the static shift feature of surface 𝐸 𝑦 component can be utilized to explore near-surface geological bodies such as clay-filled or water-filled cavities.Keywords：Profile curve; Pseudo-sections; noodles phenomenon; Geophysics; Frequency selection method of telluric current (FSM); Groundwater; Magnetotelluric sounding (MT)

<p>    The epikarst ecosystem of karst environments plays a key role in biogeochemical cycling and material storage and transport. Many geochemistry and geophysical methods have been used to research the epikarst. Coherence attributes have already showed advantages in many aspects. Although ground penetrating radar (GPR) has been introduced to characterize karst structure long before, GPR coherence attributes are paid less attention in the interpretation of epikarst structure. Coherence attribute can describe the waveform similarity of traces by the classical mutual correlation algorithm and reflect the discontinuities of media. Two typical limestone epikarst profiles which develop shallow and deep fissure soil in Guizhou karst plateau were chosen. We used MALA GPR equipment to acquire data. After the data processed by conventional methods, we then extracted the coherence attribute data from the GPR data by the procedure coded in C Programing Language. The results show that: (i) coherence attribute displays the valid and invalid signals area of the GPR profiles; (ii) the epikarst is corresponding to the valid area and the bedrock below epikarst is corresponding to the invalid area; (iii) the boundary of valid and invalid areas reflects generally the lower boundary of epikarst. Coherence attribute provides additional evidence that the epikarst developed lots of fractures but the bedrock below is complete. The validity of GPR signals is useful to analyze the structure research of epikarst.</p>

Peak-cluster depression (PCD) is a typical karst landform and a crucial aspect in non-invasive geophysical evaluation of the environmental concerns of the karst desertification regions (i.e., canopy growths, animal habitat, among others) related with karstification and its soil stocks and their moisture contents. To that end, a promising non-invasion, less labour-intensive and cost-prohibited geophysical technique (georadar/GPR) being limited in its operations because of the tilt signals and loss of information introduced by the uneven topography of PCD. Therefore, it requires a detailed investigation applying numerical analysis (considering karst topography and soil conductivity) implicating in a case study which is the scope of the present work. The findings of numerical analysis showed that under moderate soil conductivity conditions, the tilt signals could contain the reflections from both surrounding hills and subsurface inclined interfaces. The data processing workflows include the F-K filtering to remove or suppress the tilt signals. Then we applied the attribute analysis as additional information to aid in interpreting soil depth in depression. The soil depth obtained from the auger showed results highly consistent with that of the GPR findings. It is concluded that the application potential of GPR for the karst peak-cluster depression study is moderate to the fact that inferred information can partly be utilised while the rest is mixed or contaminated with other irrelevant information.

Driven by the magnetotelluric (MT), the telluric electrical field frequency selection method (TEFSM) and telluric magnetic field frequency selection method (TMFSM) measure the the horizontal components of the earth's telluric field at several frequencies along the direction of the survey line. The authors have already documented in previous study, the obtained anomalies applying the TEFSM are not free from the static shift effects. This study focuses on investigating the feasibility of the TMFSM in the context of a karst aquifer through two-dimensional forward simulations and field measurements. Utilizing the MT 2D forward modeling theory, we calculated the surface horizontal electric Ex and magnetic Hx field components along the profile in both telluric magnetic (TM) and in telluric electric (TE) polarization modes. Secondly, TEFSM was employed for groundwater exploration in the karst aquifer. Additionally, the field testing of audio-frequency magnetotelluric (AMT) was conducted using V8 electric acquisition system that minimized the interferences from anthropogenic noise sources. The simulation results indicated that the Ex component exhibited significantly high-value anomalies over high resistance bodies, whereas Ey and Hx components did not show clear anomalies. However, all three components displayed apparent anomalies for conductive bodies. Theoretically, TMFSM is feasible for exploring shallow conductive abnormal bodies, although the component Hx component is susceptible to external interference in practical applications. Therefore, designing specialized magnetic sensors with strong anti-interference capability and high accuracy is significant for achieving satisfactory results.

The epikarst together with its soil stocks (subcutaneous structure), resulted from the dissolution and weathering of soft rocks, are crucial to the fact that they may contribute to the canopy growth and can significantly influence the ecological restoration and organic carbon sequestration. For the delineation of these ecological significant karst features, ground penetrating radar (GPR) seemed to be a promising technique because of its noninvasive, cost-prohibited and lesser labor-intensive operations. However, the landscape heterogeneity, connection between surface morphology and underground environments and high vegetative endemism making karst as a complicated environment for any geophysical application. Same is the case with the GPR applicability in SW Chinese catchment as it is affected by numerous features such as epikarst slope, peak-cluster depression, tree trunks and roots, precipitation and moisture contents as well as proximity to high voltage power lines. Considering these factors, the present study analyzes the GPR data acquired at the sites representing each of these aforementioned features. The analysis includes calculation of GPR attributes as average energy, coherence and total energy together with the forward calculations wherever required. Tilt signals from surrounding hills mix with the tilt signals from subsurface inclined interfaces in a GPR image. The information of soil-rock distribution above epikarst in the slope is difficult to obtain completely for GPR. The interpretation of epikarst bottom boundary faces two possibilities considering the affects of moisture. The affects of tree trunks and roots and strong electromagnetic fields of high voltages lines make the GPR data interpretation about subsurface soil-rock structure high difficult. The soil moisture greater than ~ 30% makes GPR inapplicability. These site-specific findings are used for the generalized GPR application potential zonation in the typical SW Chinese catchment (the central Guizhou plateau). The findings of the present study may prove as a reconnaissance and an application paradigm for the future GPR utilities in complex karst characterization especially, in SW China as well as the areas having similar karstic conditions.

The seasonal soil losses and frequent shallow landslides in the Cerrado region of Brazil have high destructive potential with social, economic, and climatic implications. As fluvial systems substantially drive such environmental threats; therefore, it is essential to conduct geological site characterization and continuously monitor the seasonal erosive potentials of the rivers and streams. However, in such unstable and sensitive conditions, traditional intrusive investigation approaches may not be safe; therefore, the geophysical investigation might offer a good alternative. For the present study, a geophysical approach (particularly the seismic method) was adopted to examine the seismic footprints and GPR site characterization of a seasonal stream in the Rua do Matto, Brasilia, Brazil. The monitoring was conducted (at a safe distance) on the intermittent stream over several durations of dry (no rain) and rainy (flood) conditions. After pre-processing the raw data, the power spectral density (PSDs) was computed as a function of several variables (wind speed), time-frequency spectrograms, ambient noise displacement root mean square (RMS), the single station horizontal-to-vertical spectral ratio (HVSR) curves. In addition, change-point analysis was used for comparing the ambient noise with wind speed (both were well correlated). The GPR amplitude and waveform variation features were attributed to the subsurface material and the presence of boulders in the floodplain as well as regions (low coherence value) susceptible to erosion (weak spots). The river flows were evident on the mean probabilistic PSD values, spectrograms, HVSR curves and different patterns of RMS displacements (at selective ranges of frequency). The multi-peaks emerged on the HVSR curve are further analyzed for changes in amplitude, width and troughs possibly related to river activities and soil moisture due to rain. The approach provides the basis for non-destructive monitoring tools enabling the detection of 'seismic signatures' and weak spots of the fluvial channels for improving their environmental management.

Identifying ambient noise-based (ANb) signatures of streams can help in the estimation of their erosive potential (EP) that promotes reverie landslides and soil losses in the fluvial valleys. This is particularly imperative on flooding or rainy days, leading to stronger erosion-prone conditions (colluvium and boulders) of the valley beds inferred from georadar attribute analysis. Developing such research direction can benefit the local communities, as is the case with the Cerrado region of Brazil, where these phenomena have high destructive potential with social, economic, and climatic implications. For the present study, a seasonal stream in the Federal District of Brazil was investigated by ANb monitoring supported by Ground Penetration Radar (GPR) for site characterization. The ANb monitoring was conducted (at a safe distance) with a seismometer over several durations of dry and rainy conditions. The power spectral density (PSDs) was computed as a function of several variables, including weather conditions (rainfall, wind speed, and pressure), time-frequency spectrograms, and ambient noise displacement root mean square (RMS). This analysis also considered the single station horizontal-to-vertical spectral ratio (HVSR), where rain, wind, pressure, river flow and anthropogenic signatures were evident (at selective frequency ranges). Multi-peaks that emerged on the HVSR curve were further analyzed to identify amplitude and frequency changes, and the three peaks shift on average to a lower position during the rainy period. The GPR amplitude and waveform variation features were attributed to the stratigraphy (i.e., the boundary between valid and invalid regions and coherence value) of the floodplain and regions susceptible to erosion (erosion-prone lithological spots). This approach provides the basis for non-destructive monitoring tools enabling the detection of 'seismic signatures' and weak spots of the fluvial channels for improving environmental management.