A comprehensive literature search was conducted using Web of Science (WoS) database covering all the publications utilizing ERT, EM and GPR for groundwater recharge study. The keywords and combinations used for search were “groundwater recharge,” “electrical resistivity tomography” or “ERT,” “electromagnetic survey” or “EM methods,” “ground penetration radar” or “GPR,” “geophysical methods” and “recharge estimation” to get the relevant literature. PRISMA search strategy (Figure 1) guidelines were adopted for the literature search and selection process which provided a standardized approach for systematic and scoping review analyses.

After the literature search, all the papers were screened and included based on criteria. The criteria used was that the papers which focused on geophysical methods in estimating or identifying recharge, published in peer reviewed journals, conference or academic reports which provide the case study data, methodology description and result synthesis related to recharge estimation. The PRISMA diagram lays out the steps utilized in narrowing down the articles selected for review in a non-biased manner. Non-articles were removed from the records due to the lack of review associated with other forms of scientific media. Finally, words such as “airborne, cavities, caves,” and “slopes” were also used to screen the geophysical literature sources. Proceedings. Furthermore, articles were excluded which contained “cavities, caves, and slopes.” Once the screening phase reduced the number of articles down to a sample size of 93 total surveys, manual cataloging and screening of each article was conducted. Each type of survey, following the same step-by-step filtering and screening, yielded a different number of articles. While this is consistent with other PRISMA analysis in other fields, this poses a fundamental problem when examining the frequency of trends within the data. There were no even distributions between the number of studies selected for analysis, and this certainly affected the trend graphics displayed in other sections of this study.

The selected literature after screening were extracted for detailed analysis using geophysical method, geographical location of the study, publication year, climate of the area, hydrogeological setting, configuration type and statistical metrics. An overview of the three geophysical methods and comparisons was also employed based on equipment, survey type, electric configuration, penetration depth, measurement method, range, resolution, data acquisition with pros and cons were summarized. Furthermore, yearly trends, geographical distribution, climatic analysis and the factors influencing groundwater recharge estimation through geophysical survey methods are summarized in detail.

Using the publication database “Web of Science,” the study featured the examination of publications per geophysical method. This includes 38 ERT survey publications, 34 EM publications, and 21 GPR publications. Using the PRISMA method for screening and selection, 93 published journal articles featuring geophysical surveys were recorded. The search words queried included “groundwater” and then the name of the survey. The top relevant articles that appeared were examined for each of the methods. The year of publication, the country where the study site resided, the climate of the study area, and key terms were cataloged for analysis. The analysis involved the creation of word clouds to visualize the frequency of items within the papers for bulk comparison. Graphics and trends were examined and thoroughly investigated to provide possible insight into why the observed trends were seen. A detailed overview and comparison of the geophysical method used in this systematic and scoping review is also discussed.

While many geophysical surveying methods exist today, it would not be a comprehensive scoping review of geophysical methods without mentioning the merits of the seismic surveying of the 1900s that allowed for future geophysical work to be conducted. German scientist Ludger Mintrop (1880–1956) is often credited with the invention of seismic testing that involved striking the ground to measure seismic refraction patterns for salt dome and oil exploration (Bednar, 2005). This work involved measuring the relationships of different waves as they attenuate through the subsurface. Mintrop’s work is the predecessor of the seismic refraction surveys that are conducted today with dynamite or by dropping weight on a steel plate to measure P and S waves as they refract through various layers of lithologies in the subsurface. This scoping review is primarily concerned with groundwater investigation. Furthermore, the identification of saturated or semi-saturated layers using seismic surveying is difficult due to the wide range of possible reflection velocities that register when conducting this type of survey (Grelle and Guadagno, 2009). For these reasons, seismic geophysical surveying was omitted from the search query list. Other geophysical methods show much more promise at imaging the subsurface for highly saturated areas. The three geophysical methods that will be examined in this scoping review are the following: ERT, EM, GPR surveys. These three methods will be compared in this review and Table 1 shows a summary of each method.

Upon review of the three geophysical methods for investigating groundwater and recharge, trends exist within the publication keywords. These are words that the authors deemed key to adequately describe their paper. After examination of all the keywords of all the 93 articles reviewed, several trends emerged (Figure 2). The term “inversion” appeared 11 unique times within key terms across the sample, suggesting that the studies conducted required data manipulation and additional computer model inversions. Many models (2D or 3D) require an inversion of the 1D raw data to create final figures. An example is depth to replace frequency on the 1D models of EM surveys to create a conductance vs. depth graphic that is more understandable to the community (McLachlan et al., 2021). The term “coastal” appeared 12 unique times to suggest that coastal aquifers and groundwater are of particular importance to the public as seawater intrusions pose a threat to coastal aquifers (Klassen and Allen, 2017). It should be noted that the word “intrusion” was observed 9 times within the keywords analysis. In addition, coastal areas typically experience a higher population density compared to their water resources; therefore, one can reasonably infer that groundwater surveying in these areas would be important (Boretti and Rosa, 2019). The term “hydrogeology” appears 8 unique times and demonstrates the importance of understanding the aquifer content and recharge rates and how important a comprehensive understanding of the local geology is to these studies.

Other trends observed involve the years published and the location of said studies. Groundwater exploration surveys suggest an apparent stress on the water supply or a lack of understanding of subsurface aquifers. Thus, it can be inferred that areas that experience heavy groundwater usage or lack of plentiful, easily accessible water resources are more likely to conduct a study. This can be directly observed when examining the climate of the study areas. The climates of each of the study areas of each survey publication were noted and archived to examine trends. Unsurprisingly, a frequent climate survey was “arid” in 16 unique publications. This points to arid regions receiving little rainfall and often having water-stressed resources. The issue of water scarcity in arid regions is exacerbated by the effects of global climate change (Morante-Carballo et al., 2022). The other popular terms were tropical (7) and subtropical (19). Two explanations exist for this trend. The first is that the majority of the tropical and subtropical areas surveyed lie within Africa or the Indian subcontinent. African regions are more susceptible to water resource issues, and thus, more exploration and understanding of where water is located within Africa is of public importance and increases the likelihood of geophysical research. Studies that fall under tropical and subtropical within India can be attributed to the topography of the Himalayas and the remoteness of the study areas. Isolated villages and towns within India often deal with unique geographical and mountainous terrain that makes water infrastructure difficult (Bathla, 1999). This would explain the need for groundwater surveys to sustain the largest populated country in the world.

Over the past few decades geophysical methods are gaining attention due to their potential to non-invasive and indirect characterization of subsurface hydrological processes, especially for groundwater recharge estimation. Understanding the historical development and progression of geophysical methods is essential for groundwater resources research. Highlighting the development of those methods, a literature search from Web of Science revealed that earliest application of EM appeared in 1981, followed by GPR in 1990 and ERT in 1992. Figure 3 illustrates the growing interest in geophysical mapping for groundwater. The trend is increasing yearly, with 2022 being an exception due to the COVID-19 pandemic. There is enough evidence to support the notion that surveys will continue to be useful in mapping and identifying aquifers around the globe. An examination of the publication number trends was also considered. The yearly publication trends can be explained for EM and GPR, which are often cheaper methods of geophysical mapping. While the equipment prices of all three methods are a substantial investment, ERT, until recently, has been extremely expensive. This also deals with the fact that the depth of investigation of ERT is directly proportional to the length of cable spacing. Moreover, the more cables are needed, the larger the cost of the system. This creates an economic barrier to entry in terms of ERT for groundwater investigation. ERT begins to accelerate to become the dominant groundwater survey tool in 2021 for maximum publications across all years. The number of surveys conducted also trends upward in all types, indicating that all three methods provide viable means for groundwater exploration given the study area. The increase in groundwater surveys can be linked to global climate change affecting water resources. The change in temperatures, precipitation, and growing global populations continue to put the water supply of the world under strain (Al Atawneh et al., 2021). Most geohydrological models indicate that groundwater recharge is depleting across many regions. This could provide an explanation for the increase in groundwater recharge mapping and aquifer monitoring using geophysical surveying.

The map compiled in Figure 4 highlights the number of geophysical groundwater papers published by each country. It should be immediately noticeable that the three countries with the three largest populations on earth are the top three leading countries publishing groundwater geophysical surveys. The U.S. (14), India (7), and China (4) are the three largest countries in the world in terms of population. Population and demand for groundwater are directly related, as water is the most important source for living things (Boretti and Rosa, 2019). The cost of the survey equipment should be noted as well. The high cost of these mapping tools reflects on the countries that use them with U.S., China, Germany, England, Canada, India, Italy and Brazil representing 8 of the top ten highest GDP’s in the world. In correlation, each of the countries listed above has published at least 2 or more geophysical surveys on groundwater. The data collected from this scoping review supports the correlation that countries with higher GDP publish more research (National Science Board, 2021).

It should be noted that most U.S. surveys were conducted in arid or water-stressed areas. The effect of global climate change is altering traditional weather patterns and affecting groundwater resources. So, while the whole country might be highlighted in the map above, the areas within the countries where the surveys are taking place are likely an area where water resources in a region where water is stressed or will become stressed in the future. The map is a visual representation of the amount of geophysical groundwater surveys being conducted around the globe. The analysis also shows the wide extent of groundwater geophysical surveying. While no analysis was done on the preferential types used in various areas, the map above serves as a testament to the fact that groundwater geophysical surveying for resource mapping can be applicable to practically all corners of the globe.

A review of the climates in which geophysical surveys on groundwater were conducted yielded the following bar graph shown in Figure 5. Figure 5 perhaps demonstrates the most poignant point of all; groundwater estimation and mapping are important in all climates. The effects of global climate change are directly impacting the rates of recharge and precipitation on a global scale (Davamani et al., 2023). The presence of groundwater surveys across all the biomes on the planet indicates that no region is more stable than others due to sub-surface processes not being fully understood. Granted, arid and mountainous climates appear in bulk due to their natural disposition to water resources, but temperate and subtropical climates are also equal targets of these groundwater surveys. In fact, temperate climates have seen the most geophysical groundwater surveys while tropical climates see the least.

Upon review, there are differences between the climate areas studied for the survey sites. The tropical biome experiences the lowest amount of geophysical groundwater surveys. The likely explanation for this trend is the fact that tropical climates receive the most rainfall of any of the other biomes. This increase in rainwater leads to more surface water likely used to supply population centers and less water stress on humans living in these areas (Konapala et al., 2020). Moving forward, the impacts of global climate change are likely to cause an increase in groundwater surveys in the subtropical zones. The slow conversion of subtropical lands into arid environments is well documented, with the Sahara Desert increasing in size by 10 percent in the last 100 years and growing with the shifting of rainfall patterns. The lands that once were subtropical surrounding the desert are becoming more arid due to the effects of climate change (Thomas and Nigam, 2018).

The same trends are observed in Mediterranean climate data. Mediterranean climate encompasses a small geographical area, yet there is a high population density due to the desirable climate and economic opportunities. This small, unique climate has seen and will continue significant desertification due to climate change and, therefore, lends itself to explaining the significant increase in groundwater surveying (Lionello et al., 2014). As the Mediterranean climate continues to convert to arid, the large population centers will continue to seek out groundwater resources. The effect of this is also worsened by all Mediterranean climates being located next to saltwater bodies. Coastal aquifers are known to face issues with saltwater intrusions, rendering them ineffective for drinking water once contaminated with salt (Klassen and Allen, 2017). This would be an added incentive for researchers or academics to conduct geophysical groundwater surveys in the Mediterranean climate zone atop the aforementioned.

The temperate climate zone is described by four distinct seasons with varied precipitation. Much of the world lives within a temperate climate zone. The temperate climate zone features the bulk of all agricultural operations in the world. This makes temperate groundwater for human consumption and irrigation for crops important to a sustainable economy (Dornik et al., 2024). This climate zone also saw the most geophysical surveys. This was unexpected in the findings as we assumed that these areas maintained a large amount of water resources from surface water or their precipitation. The reason for large amounts of groundwater geophysical surveying perhaps lies in the motivation trends behind the papers than with climate alone but also population growth and demands are the promising factors.

The 16 arid geophysical surveys concern themselves with the words of “exploration” and “resource identification.” This mainly owes to the notion that semi-arid and arid environments receive less rainfall and maintain lower levels of surface water resources. These features are certainly exacerbated by the onset of global climate change, and therefore, these surveys are more concerned with identifying areas of water saturation in the subsurface. Other studies in the temperate climate category appear more concerned with the “quantification of groundwater resources” or “identifying aquifer size and health.” This difference likely occurs from the shift in priorities with arid regions concerned with finding more water and temperate regions concerned with the management of known water resources (Wiederhold et al., 2021). These surveys are an attempt to visualize the hydrogeological features under the surface and study how they are changing with time in respect to climate change. The reasoning and factors behind groundwater geophysical surveying are numerous, however, observing the trends has led to the above theories as to why the trends presented were observed.

Detailed analysis of the data extracted from the selected papers illustrates different configurations, array types, investigation depths, and the promising factors which influence geophysical methods performance. The statistical metrics and configurations used for survey methods are shown in Table 2, which indicates the overall low Root Mean Square (RMS; <5%) and low L2 norm (Euclidean norm) for diverse configurations reflect accuracy with best fitness. The geological setting affects the performance and choice of the geophysical method to use. Table 3 shows the distribution of the geophysical survey method based on geological settings and soil types. ERT ranked first based on the geological setting used in a diverse range of soil, followed by EM and then GPR (comparatively less literature for ground water recharge estimation). Depth of investigation (DOI) is also an important factor for considering the geophysical method selection for investigation. Figure 6 presents the investigation depth (m) across geophysical methods with array types. Figure 6 showed that maximum investigation depth was noted for EM followed by ERT and GPR. The configuration or array type used in the selected research is plotted against the number of papers (Figure 7). Overall, the 2D ERT configuration was found to be the maximum followed by pole-dipole and dipole–dipole, VES, TDEM and Wenner array. Various environmental and subsurface factors complicate the data and interpretation of the geophysical methods. Details of the factors and their characteristics affecting the geophysical survey methods are presented in Table 4. Among the factors, subsurface heterogeneity and high material conductivity often reduce resolution and distort signals, influencing all methods in conductive soils. ERT and EM measurements are affected by salinity and saturation. The GPR signals are attenuated in saturated and saline conditions. Data quality is affected by poor electrode-ground contact topography and cultural noise. Seasonal variability and frequency-depth trade-offs affect the resolution and accuracy of the survey. The effectiveness and repeatability of the geophysical survey methods are influenced by weather conditions and water table depth. Considering these factors, an adaptable and well-planned geophysical survey method selection in diverse geological settings can enhance the precision of groundwater recharge estimation.

Besides the critical insights into geophysical models for groundwater recharge estimations, the limitations reported in the included literature are summarized. Among the limitations of the geophysical methods, studies reported that depth and resolution affect the survey performance. ERT is limited to shallow depths (up to 50 m), which restricts deep subsurface exploration, and additional techniques like jump in magnetic potential and magnetic data are needed to assess the deeper investigations (Gao et al., 2018). GPR penetration is limited (1–3 m) assuming uniform resistivity thereby affecting precision (Albuquerque et al., 2022; Mesbah et al., 2017). In TEM and ERT, the limited vertical resolution affects the small-scale variations (Christensen et al., 2020; Ronczka et al., 2015). The potential inaccuracies due to variations in vertical resolution and characteristics are reported while merging SSR and GPR datasets (Cardimona et al., 1998; Mohammadi Vizheh et al., 2020). Data quality and environmental influence are also reported to influence recharge estimation through geophysical models. Environmental factors such as salinity, soil heterogeneity, and other instrumental errors (noise and system failure) lead to inaccuracy (Busato et al., 2019; Heggy et al., 2023). Surface conditions and electrode spacing also affect the geological results (Andrade, 2011; Singh and Tripura, 2023). Jiang et al. (2020) reported geometric issues in 2D and MRT assumptions in the vadose zone leading to artifacts. Methods integration and modeling are reported to be used for more accurate results (Ma et al., 2024). Zhang et al. (2021) stated that non-uniqueness and interpolation techniques limitations of models leads to uncertainty in resistivity. Terrian and seasonal variability are also reported by De Carlo et al. (2024), Robinson et al. (2005), and Singh and Tripura (2023). Certain studies also reported application-specific limitations due to shallow penetration and anthropogenic influence (Thapa et al., 2019; Zarroca et al., 2011) and limited clarity in conductive layers need for improved petrophysical models (Gómez et al., 2019). ERT faces certain challenges regarding data reliability due to electrodes mislocution distorting tomographic images, noise sensitivity, and environmental factors interfering in data interpretation. Some spatial constraints for electrode array like vegetation, compaction, pipes, fences and time consumed during large survey due to resolution issue in detecting shallow water without small electrode spacing (Oldenborger et al., 2005). GPR contains some limitations related to conductive soils compared to ERT and depth of penetration in saturated soils due to high attenuation of radar signals (Toushmalani et al., 2010). The TEM resolution decreases in depth, inefficient resistive materials and challenges in laterally or near-vertically confined structures (Binley et al., 2015).