the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Feb 2024
Combining crosshole and reflection borehole-GPR for imaging controlled freezing in shallow aquifers
Abstract. During test operation of a geological latent heat storage system as a potential option in the context of heat supply for heating and cooling demands a part of a shallow quaternary glacial aquifer at the “TestUM” test site is frozen. To evaluate the current thermal state in the subsurface the dimension of the frozen volume has to be known. With the target being too deep for high resolution imaging from the surface, the use of borehole Ground-Penetrating-Radar (GPR) is assessed. For imaging and monitoring of a vertical freeze-thaw boundary, crosshole zero-offset and reflection measurements are applied. The freezing can be imaged in ZOP, but determination of ice body size is ambiguous, because of lacking velocity information in the frozen sediment. Reflection measurements are able to image the position of the freezing boundary with an accuracy determined through repeated measurements of ±0.1 m, relying on the velocity information from ZOP. We found, that the complementary use of ZOP and reflection measurements make for a fast and simple method, to image freezing in geological latent heat storage systems. Problematic is the presence of superimposed reflections from other observation wells and low signal-to-noise ratio. The use in multiple observation wells allows for an estimation of ice body size. A velocity model derived from zero-offset profiles (ZOP) enabled to extrapolate geological information from direct-push based logging and sediment cores to a 3D-subsurface model.
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CC1: 'Comment on egusphere-2024-257', Giacomo Medici, 29 Feb 2024
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General comments
Good geophysical research that can be improved following the comments below that aim to bring the impact out of your research.
Specific comments
Line 17. “Geological latent”. Unclear. Please, revise the statement.
Lines 22-60. Any link between your research and Equivalent Porous Medium models to enhance geothermal energy usage in quaternary deposits? See relevant literature below:
- Case studies of geothermal system response to perturbations in groundwater flow and thermal regimes. Groundwater, 61, 255-273.
Lines 43-44. “Electrical Resistivity Tomography (ERT) has been regularly used for monitoring of geological storages”. Please, insert recent review paper that discusses electrical geophysical methods in the field of geothermal energy:
- Review of Discrete Fracture Network Characterization for Geothermal Energy Extraction. Frontiers in Earth Science, 11, 1328397.
Line 60. You should have only one aim and 3-4 specific objectives looking at the content of this paper. Please, revise the final part of your introduction.
Lines 63-64. Insert detail on the thickness of the quaternary glacial sediments and nature of the bedrock below.
Line 67. Provide detailed description on the sedimentary heterogeneities and lithologies in glacial deposits. Several experts in geophysics, hydrogeology and civil engineering might be not familiar.
Lines 181-190. Have you got information on the stratigraphy of the boreholes to describe the nature of low and high hydraulically conductive layers?
Lines 196. You should have cores to address the comment above.
Line 340. Remind to the reader that the sediments are of glacial origin in the conclusions.
Line 366. Please, integrate the relevant literature suggested above.
Figures and tables
Figure 1a. The spatial scale of the figure is unclear.
Figure 6b. The black line is unclear in the graph.
Citation: https://doi.org/10.5194/egusphere-2024-257-CC1 -
AC1: 'Reply on CC1', Peter Jung, 23 Apr 2024
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Thank you for the comments and suggestions. We appreciate your effort and tried to make the
remarked sections more clear and added relevant information. Please find our details answers
below:
Line 17: “Geological latent”. Unclear. Please, revise the statement.
-> Latent heat storage is a fixed term. The use of geological formations as storage volume is tested in
this research project, therefore geological latent heat storage. This is explained in line 25-35.
Lines 22-60: Any link between your research and Equivalent Porous Medium models to enhance
geothermal energy usage in quaternary deposits? See relevant literature below: - Case studies of
geothermal system response to perturbations in groundwater flow and thermal regimes.
Groundwater, 61, 255-273.
->Of course the plant operation will affect groundwater flow and the thermal regime in the subsurface,
however in this manuscript we do not address this topic. The focus of our work is on the feasibility of
imaging the development of a frozen volume in the subsurface with geophysical methods.
Lines 43-44: “Electrical Resistivity Tomography (ERT) has been regularly used for monitoring of
geological storages”. Please, insert recent review paper that discusses electrical geophysical methods
in the field of geothermal energy; Review of Discrete Fracture Network Characterization for
Geothermal Energy Extraction. Frontiers in Earth Science, 11, 1328397:
-> Many thanks for the reference to the inspiring paper. We checked it carefully and do not see
relevance of the proposed article, since it does not address the use of ERT and focuses on fracture
characterization, while this project is set in unconsolidated sediments, where no fractures are
present. The remark, that the whole project setting is in unconsolidated sediments will be added in
the site description.
Line 60: You should have only one aim and 3-4 specific objectives looking at the content of this
paper. Please, revise the final part of your introduction.:
-> The paragraph will be changed to specify the goals more clearly.
Line 63/64: Insert detail on the thickness of the quaternary glacial sediments and nature of the
bedrock below.
-> We have no information on the total thickness of quaternary glacial sediments and only operate in
the first <20m. Prior site investigations give no indications on being close to bottom of the
quaternary deposit, so bedrock does not have an influence on our measurements.
Line 67: Provide detailed description on the sedimentary heterogeneities and lithologies in glacial
deposits. Several experts in geophysics, hydrogeology and civil engineering might be not familiar.:
-> Many thanks for pointing out these issues. We will extend information concerning glacial deposits.
Line 181-190: Have you got information on the stratigraphy of the boreholes to describe the nature
of low and high hydraulically conductive layers?:
-> We do have a core from drilling point MP055. See lines 186-188: “Sediment coring at MP055
matches the high hydraulic and low electric conductivity with a sand layer and the low hydraulic and
high electric conductivity with higher clay content.” Information will be provided in Figure 4
additionally.
Line 196: You should have cores to address the comment above.
-> See comment above, information on the stratigraphy are provided.
Line 340: Remind to the reader that the sediments are of glacial origin in the conclusions.:
- > We will add in the conclusion that our investigations are performed in glacial sediments.
Figure 1: The spatial scale of the figure is unclear.:
-> Thank you for pointing that out. Depths information will be added to get an idea about spatial
scale.
Figure 6: The black line is unclear in the graph.:
-> The legend will be updated to clarify the meaning of the black lines.
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AC1: 'Reply on CC1', Peter Jung, 23 Apr 2024
reply
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RC1: 'Comment on egusphere-2024-257', Anonymous Referee #1, 11 Mar 2024
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Section 2.1. Geological description of the area is poor. It does not say much if you state that there are "quaternary glacial sediments". More detailed description must be added. If this is glacial till.. what is the clay contet for example? As sediment properties will influence effectiveness of GPR as well as changes in volume of the soil if freezing will accour.
Line 106. ERT resolution is affected by many factors (electrode spacing, applied electrode configuration, 2D or 3D survey, overall electrical properties of the soil and numerous settings that you can change during data inversion process) and it is not good idea to state that 10m level is some magical depth where resolution drops. Also it is much more better to have high conductivity layers then low conductivity layers for ERT. My suggestion is to avoid discussion about ERT at all. No necessity to include it in this paper.
Line 110. Also for GPR it is not streightforfard to get the distance. It is also product of your interpretation.
Line 112. If you would previously described soil type in moire detail it would be clear why 10m is already far for GPR. In sandy sediments you can get in depth of 20+m with ~200 MHz.
Line 128. What GPR signal propagation speed was used and how did you determined it? This is crytically important question that must be explained in detail. Yes later you explain method for ZOP, but whas obtained values used also for reflection data? Also I suggest to use not only velocity values but values of dielectric permittivity (neglecting all the complications and assuming that this equation is valid v=c/sqrt(e)). Most GPR specialists use values of permittivty instead of velocity.
Line 175. Are there no spatial variations in GPR propagation speed without frozen zone? This must be explaned.
Line 175. I did not found explanation what value for Vfrozen was used?
Line 182. Obtained information regarding sediments indicate multiple layer situation. As a result explanation of GPR signal propagation speed determination in the section of methods must be improved in order to explain that different propagation speeds were used for different layers.
Line 287. Theoretically also relfection GPR resolution depends of FIrst Fresnel zone as we approximate area of the reflection via First fresnel zone.
Citation: https://doi.org/10.5194/egusphere-2024-257-RC1 -
AC2: 'Reply on RC1', Peter Jung, 23 Apr 2024
reply
Thank you for the comments and suggestions. We appreciate your effort and tried to make the remarked sections more clear and added relevant information. Please find our detailed answers below:
Section 2.1: Geological description of the area is poor. It does not say much if you state that there are "quaternary glacial sediments". More detailed description must be added. If this is glacial till.. what is the clay contet for example? As sediment properties will influence effectiveness of GPR as well as changes in volume of the soil if freezing will accour.
-> Many thanks for pointing out these issues. We will extend information concerning glacial deposits and soil properties.
Line 106: ERT resolution is affected by many factors (electrode spacing, applied electrode configuration, 2D or 3D survey, overall electrical properties of the soil and numerous settings that you can change during data inversion process) and it is not good idea to state that 10m level is some magical depth where resolution drops. Also it is much more better to have high conductivity layers then low conductivity layers for ERT. My suggestion is to avoid discussion about ERT at all. No necessity to include it in this paper. & Line 110: Also for GPR it is not streightforfard to get the distance. It is also product of your interpretation.
-> It was not the intention to sound like ERT resolution would drop significantly below a specific depth. We are sorry if this may have been formulated inadequately. Of course ERT resolution depends on multiple factors, yet resolving the exact position of a vertical layer boundary in the target depth of >10m is not very accurate, and further decreased by the high conductive layer above the target, because then less current runs through the aquifer. We will revise this paragraph.
RC Line 112: If you would previously described soil type in moire detail it would be clear why 10m is already far for GPR. In sandy sediments you can get in depth of 20+m with ~200 MHz.
-> The statement will be adapted to make clear, that determined position is indeed just an interpretation of GPR reflections converted to distance using measured velocity profiles. In combination with the improved site description we will make more apparent why GPR penetration is limited in our setting.
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AC2: 'Reply on RC1', Peter Jung, 23 Apr 2024
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