Urban underground space detection solutions

Seismic Solutions：Urban underground space detection solutions

Ⅰ. Overview

With the rapid development of cities, urban safety issues are becoming more and more prominent and the demand is becoming more and more urgent, including 0-100m high-precision detection of road collapse, pipeline safety, subway safety, etc., which is in a technical gap. At present, urban construction, emergency response, disaster prediction, and environmental protection urgently need high-precision underground detection.

Problem 1: Road collapse

Problem 2: Leakage in pipelines and air-raid shelters

Problem 3: Subway safety collapse

II. Technical Status

Urban underground space detection mainly includes the detection of underground stratum structure, bedrock surface depth and ups and downs, active faults, ground fissures, boulders, cavities, water-richness, bearing capacity, rock and soil quality and other geological information. It is the information basis for conducting geological surveys of multiple elements (space, resources, environment, disasters) of urban underground space and promoting transparent urban construction.

Difficulties:

• There are many interference factors, such as noise interference, human flow interference, industrial interference, electromagnetic interference, current interference, traffic track interference, building interference, etc.;
• Urban geophysical exploration requires high accuracy, and the accuracy generally needs to reach the meter level, even the centimeter level and millimeter level;
• Sometimes it is limited by the site, construction time, etc.
• Currently, shallow layer detection is mainly based on drilling – destructive, time-effective, costly, difficult to construct, and poor spatial control ability.

Application requirements:

• Urban geophysical exploration methods must be practical, flexible, fast, targeted, and have strong anti-interference capabilities.
• The industry urgently needs “two highs and one low” (high efficiency, high precision, and low cost) technical methods to improve service capabilities.

III. Integrated solution

Scale-optimized deployment of vibration sensors and other sensing devices, construction of data transmission networks and data centers, and use of big data and artificial intelligence algorithms to obtain a variety of application results.

• Sensing: High-precision, wide-band sensors, low-noise digital node instruments are the main ones, supplemented by more than 20 high-precision sensors developed including fiber optic sensors, inclinometers, gyroscopes, etc., which are suitable for land, water, and underground scenes, effectively replacing imports, greatly reducing costs, and laying the foundation for large-scale deployment.
• Transmission: Low-power 4G/5G transmission technology, self-organizing network transmission technology, drone data recovery technology, compressed sensing data compression technology, etc. to achieve large-scale sensor access and high-speed transmission of massive data.
• Knowledge: Spatiotemporal big data promotes the development of cloud computing with technological innovation and model innovation.

IV. Technical Principles

The imaging method based on scattered waves in the seismic wave field has the biggest advantage of not being restricted by material for pipeline detection (especially for PE pipes and cement pipelines), and the imaging method based on wave theory has high accuracy (refer to sonic logging), which can complete high-precision pipeline positioning and imaging within a few meters underground.

IV. Application scenarios

1. Application scenarios for underground space resource planning and design

Using technologies such as three-component frequency imaging (internationally leading) to obtain a three-dimensional urban underground model with multiple elements (stratum structure, bedrock surface, caves, faults, weak interlayers, geotechnical indicators), it provides a data basis for urban underground resource assessment, planning, utilization and development.

The physical parameter data of 105 boreholes in the preliminary and detailed survey stages of this area (including: water content, mass density, gravity density, porosity, saturation, plasticity index, liquid limit index, compression modulus, permeability coefficient, bearing capacity) were collected, and the data were sorted and loaded.

2. Application scenarios of geological disaster detection

Combined with satellite remote sensing, ground equipment uses resonance frequency, amplitude attenuation, dispersion, settlement, displacement, deformation and other characteristic calculations to achieve landslide and other geological disaster monitoring.

Landslide detection application in a certain area: a total of 13 two-dimensional survey lines were collected, processed, interpreted as layers and sliding surfaces.

The landslide has the characteristics of multi-stage sliding. This study believes that there are mainly three sliding surfaces. Among them, the sliding surface F1 developed in the weak layer near the top of the bedrock is the main sliding surface. The sliding surfaces F2 and F3 are located in the weak layer above F1, with a gradually decreasing range, and are secondary sliding surfaces.

3. Application scenarios of road collapse detection

The time-varying characteristics of underground imaging amplitude, attenuation and dispersion characteristics are used to realize road collapse detection and monitoring.

3.1 Application of road collapse caused by water pipe leakage

3.2 Application of loess subsidence survey

Loess is eroded by rainwater, forming multiple underground cavities, and the surface of the larger cavity area collapses.

4.Road Damage Subgrade Detection

This loose layer is about 8 meters deep and may not have much impact in the short term, but may be potentially dangerous in the long term.

5. Subway application scenarios

Subway survey, construction safety, and monitoring. Taking a certain city subway as an example, a collapse occurred in front of a certain line shield.

(1) Key engineering geological issues: During the shield tunneling process of the subway tunnel, an underground cavity was encountered and a collapse occurred at the face. Due to the existence of important buildings on the surface, it is urgent to detect the scope of the underground cavity encountered by the subway tunnel, and then evaluate its impact on the surface buildings.

(2) Survey line deployment: One two-dimensional survey line is deployed in the direction parallel to and perpendicular to the subway tunnel. Each survey line has 24 points and the point spacing is 1m.

In the frequency imaging section parallel to the tunnel direction, the tunnel and the shield machine are clearly imaged. It is inferred that the blue area in front of the shield machine is the underground cavity development area.

On the frequency imaging section perpendicular to the tunnel direction, the tunnel cross section is clearly imaged and is consistent with the tunnel burial depth on the section parallel to the tunnel direction.

6. Underground pipeline application scenario

Sewer pipeline detection in a park

The left picture shows the on-site photos of underground pipeline detection and the photos of the exposed cement pipeline (pipeline buried 1.6 meters deep, pipe diameter 50 centimeters).

The right picture shows the geological radar imaging of the cement pipeline. The red dotted circle is the image of the underground pipeline. The buried depth of the pipeline can be accurately read in the red frame, which is consistent with the exposed measurement depth value.