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The present geophysical study was motivated by the need to determine suitable coring locations for paleolimnological studies in two karst lakes (Metzabok and Tzibaná) of the Lacandon Forest in Chiapas, southern Mexico. We used seismic and transient electromagnetic methods to map the sediment thickness below the lake floor. When lakes were filled in March 2018, we collected seismic data with a sub-bottom profiler (SBP) and transient electromagnetic (TEM) data with a floating single-loop configuration. The latter aimed at assessing the TEM method as an alternative to seismic methods for the investigation of lake sediments and geology. After the first campaign, water levels of both studied lakes dropped dramatically by July 2019, leaving Lake Metzabok (maximum depth ~ 25 m) dry and Lake Tzibaná (~ 70 m) with a water level decreased by approx. 30 m. After the sudden drainage of the lakes, we complemented water-borne measurements by a survey carried out on the exposed lake floor in October 2019, when lake levels were still low. During this second campaign, we collected time-domain induced polarization (TDIP), and seismic refraction tomography (SRT) data on the desiccated bed of Lake Metzabok and some dry parts of Lake Tzibaná. By comparing the various data sets, we find that (i) SBP and TDIP phase images consistently resolve the thickness of the fine-grained lacustrine sediments covering the lake floor, (ii) TEM and TDIP resistivity images consistently detect the upper limit of the limestone bedrock and the geometry of fluvial deposits of a river delta, and (iii) TDIP and SRT images suggest the existence of a layer that separates the lacustrine sediments from the limestone bedrock and consists of collapse debris mixed with lacustrine sediments. While our results do not imply that resistivity-based methods could generally replace seismic reflection surveys for lake-bottom reconnaissance, they clearly show that TEM and TDIP surveys can provide important complementary information and resolve additional geological units or bedrock heterogeneities.

The karst lakes of the sparsely-populated Lacandon Forest in Chiapas, southern Mexico, and their associated sediment infill are attracting increasing attention as high-resolution and continuous environmental and climate archives. To evaluate the information stored in the sediments, paleolimnologists retrieve sediment cores and analyze multiple biological and non-biological indicators. Our geophysical measurements presented here were motivated by the need to determine coring locations providing continuous sediments records from a total of four lakes of the Lacandon Forest. Therefore, we mapped the sediment thickness on the lake floor by applying seismic, electrical, and electromagnetic methods. The measurements were carried out with floating devices – and, after the sudden drainage of two of the studied lakes, complemented by measurements on the exposed lake floor.During a first campaign in March 2018 when lakes were filled, we collected seismic data with a sub-bottom profiler (SBP). Furthermore, we collected transient electromagnetic (TEM) data with a floating measuring device to investigate the potential of the method for the determination of sediment thicknesses as an alternative to seismic methods. After the lake-level maximum that coincided with the first campaign, the water levels of two of the studied lakes dropped dramatically by July 2019, leaving lake Metzabok (maximum depth ~15 m) dry and lake Tzibaná (~70 m) with a water level decreased by approx. 30 m. In October 2019, when lake levels were still low, we conducted a second survey covering the dry lake floor of lake Metzabok and some dry parts of lake Tzibaná. During this second campaign, we collected electrical resistivity tomography (ERT), induced polarization (IP), and seismic refraction tomography (SRT) data along selected lines of the 2018 survey.Our 2018 results from the water-borne survey show that sediment thickness estimates from seismic (SBP) and electrical (TEM) data agree well for water depths up to 20 m and sediment thicknesses ranging from 2 m to 10 m. The 2019 data collected on the dry lake floor confirms the findings of the first campaign and – due to the smaller distance between measuring devices and target – results in a more detailed picture of sediments and the underlying limestone bedrock.