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Terra-Fi: Decoupling Sensing and Wireless Depths for Sustainable Deep Underground Monitoring

Kaiqiang Lin, Mohamed-Slim Alouini

arXiv:2610.33202v1eess.SYcs.ETcs.NIcs.SY

TL;DR

Meter-scale underground monitoring is difficult because conventional WUSNs place the whole device at depth, where soil attenuation harms communication and WET. Terra-Fi keeps wireless and energy functions in a shallow hub and connects deeper probes through feeders. In a precision-agriculture case study, it achieves more reliable communication and longer operational lifetime than fully buried WUSNs, particularly at greater sensing depths and higher VWC.

  • Problem

    Conventional WUSNs place the entire device at the target depth, where severe soil attenuation degrades communication reliability and makes WET inefficient for meter-scale monitoring.

    Source: §I · p. 1

  • Method

    Terra-Fi keeps the wireless hub shallow and connects application-specific sensing probes at deeper target layers through power-and-signal feeders.

    Source: A. Simulation Scenarios and… · p. 5

  • Results

    Terra-Fi achieves more reliable wireless communication and longer operational lifetime than fully buried WUSNs, particularly under deep sensing zones and high-VWC conditions.

    Source: §VI · p. 7

  • Takeaways & Limitations

    Terra-Fi’s performance gain can be enhanced through time-allocation and transmission-configuration optimization.

    Source: B. Numerical Results and Discussion · p. 7

    Feeder length is constrained by conductor resistance, probe power demand, voltage drop, power loss, and signal integrity, with long-term deployment also requiring protection against moisture and mechanical stress.

    Source: §V · p. 7

Abstract

from arXiv · show

Wireless underground sensor networks (WUSNs) enable in-situ monitoring of subsurface processes and buried assets to deliver substantial social and economic benefits across diverse verticals. However, meter-scale underground monitoring remains challenging since conventional WUSNs place the entire underground device at the target sensing depth, where severe soil-induced attenuation significantly degrades communication reliability and makes wireless energy transfer (WET) inefficient and even impractical. To address this limitation, we propose Terra-Fi, a novel shallow-hub and deep-sensing architecture that decouples the sensing depth from the wireless communication and energy-replenishment depth. In Terra-Fi, a shallow wireless hub is deployed at a depth favorable for communication, WET, and maintenance, while application-specific sensing probes are extended to meter-scale target layers through power-and-signal feeders. On this basis, we first examine the depth limitations of underground communication and WET and compare representative monitoring architectures. Then, we present its system architecture, operation flow, and potential benefits. Through a precision agriculture case study, our numerical results reveal that Terra-Fi establishes more robust wireless connectivity and a longer operational lifetime than conventional fully buried WUSN architectures, particularly at greater sensing depths and higher volumetric water content. Moreover, its performance gain can be enhanced through time allocation and transmission-configuration optimization. Finally, we pinpoint practical challenges and future research directions for deploying Terra-Fi into WUSN systems.

I. Introduction

WUSNs support subsurface monitoring, but meter-scale sensing places conventional devices where soil attenuation weakens communication and WET. Terra-Fi separates deep sensing from shallow wireless and energy functions, and the paper evaluates it against fully buried WUSNs in a precision-agriculture simulation.

I. Introduction

  • Root-zone measurements support precision agriculture, with active rooting regions extending to 1–1.8 m depending on crop type.Other motivating settings include pipeline monitoring at 0.6–1.8 m [5] and geohazard precursors at approximately 1–3 m [6].Source: §I · p. 1
  • Conventional monolithic WUSNs co-locate sensing, communication, storage, and power replenishment at the target depth, where soil attenuation degrades links and WET efficiency.Higher transmit power, higher-gain antennas, and multi-antenna techniques can partly compensate, but increase energy use, hardware complexity, and deployment cost.Source: §I · p. 1
  • Terra-Fi keeps communication, control, energy storage, and WET in a shallow hub while protected power-and-signal feeders connect it to deeper sensing probes.This decouples sensing depth from wireless communication and energy-replenishment depth.Source: §I · p. 1
  • The paper examines underground communication and WET depth limits, compares monitoring architectures, and presents Terra-Fi’s system, operation flow, and advantages.Its precision-agriculture simulation compares communication reliability and operational lifetime across underground conditions and network scales, then identifies open deployment challenges.Source: §I · pp. 1–2

II. Depth Limits of Underground Wireless Operation

Underground communication becomes increasingly constrained by burial depth and soil conditions. This motivates keeping radios shallower than the target sensing layer.

A. Depth-Limited Wireless Communication

  • WUSNs use underground multi-hop or centralized networking; multi-hop extends coverage with buried relays, while centralized LPWAN connectivity is often preferred for reliable, energy-efficient monitoring.Multi-hop incurs additional underground communication and energy costs.Source: A. Depth-Limited Wireless Communication · p. 2
  • At 433 MHz, LoRaWAN achieved only a 16 m UG2AG range at 0.8 m burial depth and 15% VWC; NB-IoT reach fell from 0.7 m at 5% VWC to 0.3 m at 25% VWC.Soil-induced attenuation also depends on soil texture, operating frequency, and antenna–soil coupling.Source: A. Depth-Limited Wireless Communication · p. 2
  • Greater burial depth and VWC increase underground attenuation, while link-level remedies such as higher transmit power or denser gateways add energy, complexity, or infrastructure cost.These constraints motivate placing probes at target layers while retaining radio components in shallower soil.Source: A. Depth-Limited Wireless Communication · p. 2

B. Depth-Limited Wireless Energy Transfer

Wireless energy transfer and existing monitoring architectures each impose practical tradeoffs at depth. Terra-Fi positions a shallow hub between surface-connected cabled systems and fully buried WUSNs.

B. Depth-Limited Wireless Energy Transfer

  • Ambient energy harvesting is generally weak and intermittent, while dedicated WET offers more controllable replenishment through MI-WET or RF-WET.MI-WET requires close coil separation and accurate alignment; RF-WET offers deployment flexibility but is constrained by soil absorption.Source: B. Depth-Limited Wireless Energy… · p. 2
  • WET is feasible for shallow replenishment but becomes inefficient near meter-scale depths; MI-WET faces coupling and alignment limits, while RF-WET faces harvester-sensitivity and soil-loss constraints.Increasing source power, aperture, charger density, or charging duration can improve delivery but raises energy use, complexity, or cost without removing soil-induced loss.Source: B. Depth-Limited Wireless Energy… · p. 2

C. Existing Underground Monitoring Architectures

  • Fully buried WUSNs preserve a concealed surface but constrain communication, WET, and maintenance at sensing depth; multi-hop variants add radios, energy sources, routing, and distributed maintenance.Cabled systems avoid underground wireless propagation and ease access to the logger and power supply, but exposed electronics face machinery, weather, theft, and vandalism.Source: C. Existing Underground Monitoring… · pp. 2–3
  • Terra-Fi retains concealment and avoids additional buried relays while placing communication, control, energy storage, and WET in a shallow hub connected to deeper probes.Compared with cabled systems and fully buried WUSNs, it introduces feeder integration, sealing, and installation requirements.Source: C. Existing Underground Monitoring… · p. 3

III. Terra-Fi Platform: A Shallow-Hub and Deep-Sensing Architecture

Terra-Fi separates a shallow wireless hub from application-specific deep probes using a power-and-signal feeder. Its architecture also links the hub to aboveground energy, connectivity, and data services.

A. System Architecture

  • Terra-Fi’s functional depth decoupling places probes at target depths while consolidating communication, control, energy storage, and wireless energy replenishment in a shallow hub.The architecture comprises an aboveground service segment, shallow hub, feeder, and deep sensing segment.Source: A. System Architecture · p. 3, Figure 1 · p. 4
  • The aboveground service segment supplies energy, wireless connectivity, and data-processing services, using fixed RF beacons or UAV chargers and terrestrial or satellite links.Collected data are forwarded through backhaul links to a cloud monitoring platform.Source: A. System Architecture · p. 3
  • The shallow hub integrates communication, control, energy storage, power management, WET reception, and feeder interfaces in a sealed enclosure.At a representative depth of 0.3 m, it reduces the wireless propagation path while preserving surface concealment and facilitating maintenance.Source: A. System Architecture · p. 3
  • The feeder delivers regulated power and bidirectional low-rate data, with electrical design governed by resistance, probe demand, voltage drop, power loss, and signal integrity.Sealing, corrosion protection, and mechanical reinforcement address moisture, soil compaction, and installation stress.Source: A. System Architecture · p. 3
  • Deep sensing probes measure application-specific variables such as VWC, temperature, strain, pH, and gas concentration while communication, storage, control, and WET remain in the hub.Multiple probes can share one hub at different depths for vertical subsurface profiling.Source: A. System Architecture · pp. 3–4

B. Operation Flow

Terra-Fi runs an energy-aware cycle in which a shallow hub harvests energy, powers probes for sensing, and forwards their data to aboveground receivers.

B. Operation Flow

  • The shallow hub harvests and stores energy, then supplies activated probes with regulated power and control commands through power-and-signal feeders.Probe activation depends on available energy and application requirements.Source: B. Operation Flow · p. 4
  • Probes measure at target depths and return data to the hub, which aggregates and preprocesses it before transmitting to terrestrial gateways, base stations, or LEO satellites.The choice of aboveground receiver depends on network availability.Source: B. Operation Flow · p. 4
  • Aboveground receivers forward data to a cloud platform for storage and analytics, which can return commands to adjust reporting, transmission, and charging schedules.The passage says these adaptations can prolong UD operational lifetime.Source: B. Operation Flow · p. 4

C. Architectural Benefits and Tradeoffs

Terra-Fi separates deep sensing from shallow wireless and energy functions, combining meter-scale probe access with shallower serviceable electronics.

C. Architectural Benefits and Tradeoffs

  • Terra-Fi extends sensing to meter-scale depths while keeping communication, control, and energy functions in a shallow hub and preserving surface concealment.Only probes and local interfaces operate at the deepest layers.Source: C. Architectural Benefits and Tradeoffs · p. 4
  • Keeping radios in the shallow hub reduces underground propagation length without requiring buried relays, higher transmit power, or denser gateways.The cited benefit is improved wireless connectivity relative to fully buried WUSNs.Source: C. Architectural Benefits and Tradeoffs · p. 4
  • Moving the WET receiver, energy storage, and power-management circuitry to the shallow hub mitigates constraints on meter-scale wireless energy replenishment.Electrical power reaches deeper probes through the feeders.Source: C. Architectural Benefits and Tradeoffs · p. 4
  • Serviceable communication and energy components remain in a shallow buried hub, reducing routine maintenance depth while preserving an unobstructed surface.This is an intermediate arrangement between accessible surface loggers and fully buried WUSNs that may require full-depth excavation.Source: C. Architectural Benefits and Tradeoffs · p. 4
  • Multiple depth-distributed probes can share one hub’s communication, control, and energy resources, enabling vertical profiling with fewer UDs.The passage contrasts this with fully buried WUSNs, which typically require separate UDs at different depths.Source: C. Architectural Benefits and Tradeoffs · pp. 4–5

IV. Proof-of-Concept Case Study: Precision Agriculture

The case study models a Saudi Arabian center-pivot farm and compares fully buried WUSNs with Terra-Fi for meter-scale agricultural sensing.

A. Simulation Scenarios and Configurations

  • The simulated Al Jouf Arable farm covers a 5 km-radius area with randomly and uniformly distributed UDs and target root zones at 1 m and 2 m.The modeled measurements include VWC, soil temperature, conductivity, pH, and nutrient-related indicators.Source: A. Simulation Scenarios and… · p. 5
  • The baseline places each UD at its sensing depth, while Terra-Fi keeps the wireless hub at 0.3 m and connects deeper probes through power-and-signal feeders.The fully buried WUSN is used as the direct reference for evaluating depth decoupling.Source: A. Simulation Scenarios and… · p. 5, Figure 2 · p. 5
  • The simulation evaluates uplink reliability with PDR, packet energy with EPP, and operational lifetime using a complete LoRaWAN Class A energy model.PDR accounts for successful demodulation under SNR and co-channel interference conditions; direct WET to the meter-deep fully buried baseline is not considered feasible.Source: A. Simulation Scenarios and… · p. 5
  • Each reporting period allocates Tw to energy harvesting and Tt to sensing and uploading a 10-byte packet to a gateway.The figure depicts RF-WET from UAVs during Tw; the study uses LoRaWAN in the 433 MHz ISM band with fixed SF and no adaptive data rate.Source: Figure 2 · p. 5, A. Simulation Scenarios and… · p. 5

B. Numerical Results and Discussion

The numerical results favor Terra-Fi over fully buried WUSNs in connectivity and lifetime, with the advantage persisting under denser deployments when SF and WET timing are optimized.

B. Numerical Results and Discussion

  • As UD count increases, collisions reduce PDR and raise EPP, particularly for deeply buried UDs with limited link margin.The study examines network scale, fixed and optimal SF configurations, and RF-WET integration; the figure compares PDR/EPP and lifetime across those strategies.Source: B. Numerical Results and Discussion · pp. 5–6, Figure 4 · p. 7
  • At 2 m and 40% VWC, fully buried 433 MHz received power falls to −209 dBm, while Terra-Fi remains above the SF12 sensitivity threshold across the VWC range.The results also report that 433 MHz outperforms 868 MHz across the considered deployment cases.Source: Figure 3 · p. 6, B. Numerical Results and Discussion · p. 6
  • 4 years at 10k UDs and 3.3 years at 100k UDs: optimized Terra-Fi with RF-WET maintains longer lifetime than the fully buried configurations.At 100k UDs, the optimized 1 m fully buried WUSN has a 0.07-year lifetime, while the 2 m configuration remains non-operational.Source: B. Numerical Results and Discussion · pp. 6–7, Figure 4 · p. 7
  • At 100k UDs, optimal SF gives Terra-Fi PDR above 0.7 and EPP below 0.034 J with SF7, versus 8.24 J EPP for 1 m fully buried WUSNs with SF10.At 2 m, fully buried transmission is infeasible even with SF12; at 1 m, fixed SF12 PDR falls to 0.01 and EPP rises to 4908.3 J at 100k UDs.Source: B. Numerical Results and Discussion · p. 6, Figure 4 · p. 7

V. Open Challenges and Research Directions

Terra-Fi’s practical deployment requires addressing feeder reliability, heterogeneous probe interfaces, shared energy management, and deployment planning. These challenges must be resolved before the architecture can be deployed in real underground monitoring systems.

Deployment Scope

  • Terra-Fi’s quantitative feasibility analysis is for precision agriculture, while other monitoring applications require adapted sensing probes and deployment configurations.The authors identify practical challenges that must be addressed before real underground deployment.Source: §V · p. 7

Feeder Electrical and Mechanical Reliability

  • Feeder length is jointly constrained by conductor resistance, probe power demand, voltage drop, power loss, and signal integrity.Long-term burial also requires waterproof, corrosion-resistant conduits, sealed connectors, and protection from mechanical stresses; these limits need quantification under representative lengths and burial conditions.Source: §V · p. 7

Multi-Probe Interface and Interoperability

  • A single shallow hub may need to serve probes with different supply voltages, sensing interfaces, and communication protocols.Modular interfaces need voltage regulation, signal conditioning, protocol conversion, individual addressing, power gating, and fault isolation to support heterogeneous multi-depth sensing.Source: §V · p. 7

Efficient Energy Management

  • The shallow hub must coordinate energy harvesting, probe activation, sampling frequency, and data reporting across probes or depth layers.This calls for joint optimization of WET scheduling, stored-energy allocation, depth-aware sampling, and transmission configuration under heterogeneous demands, age-of-information requirements, and lifetime constraints.Source: §V · p. 7

Effective Deployment Planning and Installation

  • Deployment planning must jointly balance hub depth, feeder length, probe-depth allocation, gateway density, and charger placement against coverage, reliability, WET feasibility, cost, and maintenance access.Small-borehole or direct-push installation can place deep probes and feeders without full-depth excavation while keeping the hub shallow.Source: §V · p. 7

VI. Conclusion

Terra-Fi decouples deep sensing from shallow wireless communication and energy replenishment through power-and-signal feeders. In a precision-agriculture case study, it outperformed conventional fully buried WUSNs in communication reliability and operational lifetime, with gains enhanced by joint transmission-configuration and WET/data-time optimization.

VI. Conclusion

  • Terra-Fi achieved more reliable wireless communication and longer operational lifetime than conventional fully buried WUSNs, particularly for deep sensing zones and high-VWC conditions.The comparison covered different network scales.Source: §VI · p. 7
  • Joint optimization of LoRaWAN SF configurations and WET/data-transmission time allocation enhanced system performance.The paper also identifies open challenges to support practical deployment.Source: §VI · p. 7
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