Stefan Jetschny

Rayleigh-to-shear wave conversion at the tunnel face — From 3D-FD modeling to ahead-of-drill exploration

For safe tunnel excavation, it is important to predict lithologic and structural heterogeneities ahead of construction. Conventional tunnel seismic prediction systems utilize body waves (P- and S-waves) that are directly generated at the tunnel walls or near the cutter head of the tunnel boring machine (TBM). We propose a new prediction strategy that has been discovered by 3D elastic finite-difference (FD) modeling: Rayleigh waves arriving at the front face of the tunnel are converted into high-amplitude S-waves propagating further ahead. Reflected or backscattered S-waves are converted back into Rayleigh waves which can be recorded along the sidewalls. We name these waves RSSR waves. In our approach, the front face acts as an S-wave transceiver. One technical advantage is that both the sources and the receivers may be placed behind the cutter head of the TBM. The modeling reveals that the RSSR waves exhibit significantly higher amplitudes than the directly reflected body waves. The excavation damage zone causes dispersion of the RSSR wave leading to multimodal reflection response. For the detection of geologic interfaces ahead, RSSR waves recorded along the sidewalls are corrected for dispersion and stacked. From the arrival times, the distance to the S-S reflection point can be estimated. A recurrent application, while the tunnel approaches the interface, allows one to quantify the orientation of the reflecting interfaces as well. Our approach has been verified successfully in a field experiment at the Piora adit of the Gotthard base tunnel. The distance to the Piora fault zone estimated from stacked RSSR events agrees well with the information obtained by geologic surveying and exploratory drilling.

Seismic prediction ahead of a tunnel face - Modeling, field surveys, geotechnical interpretation -

An important precondition for underground construction is a detailed knowledge of the soil and/or rock conditions in the area of the construction. In order to overcome existing limitations in classical exploration methods, research and development for exploration ahead of a tunnel face focuses on: hardware development for excavation integrated measurements, modelling and processing of data measured under these specific circumstances, and integrative interpretation of seismic results with other data from the excavation, from geological mapping, and from exploratory drilling, where available. Finite difference modelling of seismic wavefields around tunnels has shown the general feasibility of seismic measurements for imaging structures ahead of a tunnel face. The modelling results were confirmed by field measurements in various tunnel sites. The integrated interpretation of seismic data with all available geological and geotechnical information is currently in the state of development and aims, in the middle to long term perspective, at an “a priori” detection of structures ahead of the face.

Towards an automatic seismic localization of human footsteps

Today, many companies or government institutions are required to monitor outdoor areas of office buildings, frontiers or restricted areas. In this paper we present an automated algorithm to monitor and to localize human footsteps with the help of their excited seismic waves. The preliminary goal of the data processing is the calculation of traveltime differences for each receiver pair of an arbitrary array of geophones. The data are filtered with a band-pass filter, which improves the signalto-noise ratio significantly. A short term average / long term average (sta / lta) picker determines the time of the first breaks. After cutting out time windows around the picked arrival times, the traveltime differences are than calculated for each receiver pair using cross-correlation. Finally, a grid search algorithm is used to invert the traveltime differences into locations of the footstep. Therefore, the propagation velocity of the Rayleigh wave has to be approximated. In one field measurement reliable and precise localizations were achieved. Footsteps on a fixed position were localized within an area of 30 m x 30 m with less than one metre deviation and paths on a straight line were reconstructed successfully.

Realistic FD Modeling of the Tunnel Environment for Seismic Tomography

Especially in urban areas, tunneling is the method of choice to built new pathways to improve the infrastructure, for e.g. rail tracks, roads or power cables. In this context, safety threads are not limited to the tunneling construction itself but can occur years later. Cavities or fracture zones that can be weakened by the tunneling are a serious risk, both for the stability and integrity of the tunnel tube and buildings on the surface. A collapse of a cavity can result in sudden load peaks possibly overpowering the stability of the tunnel casing and subsidence damage to buildings and buried gas pipelines (Fig. 1). Seismic tomography is a useful tool to detect such anomalies in the vicinity of the tunnel tube while the tunneling progresses or after completion. In order to do so, seismic receivers can be placed at the tunnel wall or at anchors behind the tunnel wall. The seismic wave field is excited by a hammer blow applied to the tunnel wall. Basis for such a tomography is a profound understanding of the seismic wave propagation in the complex surrounding of a tunnel which can be gained from seismic modeling. We, therefore, investigate the influence of the excavation damaged zone (EDZ) that is usually present as a side effect of the tunneling and the topography of the tunnel wall. Both features will significantly effect the seismic waves excited at the tunnel wall. The modeling is done by the parallel elastic 3-D finite difference (FD) modeling code SOFI3D using a Cartesian coordinate system (Bohlen 2002). Later, we insert an anomaly close to the tunnel wall. This paper will now primary focus on the realistic description of a tunnel example and the accurate modeling of seismic waves with respect to this model.

Detection of Geological Structures Ahead of the Tunnel Construction Using Tunnel Surface-waves

To improve the performance and safety of tunnel constructions, seismic predictions methods can be used to detect relevant geological structures ahead of the tunnel face (e.g. faults, lithological boundaries). We present a simple and robust processing method that can automatically calculate the distance of such a geological inhomogeneity from the seismic response of only a few receivers mounted on the tunnel wall. The method works fully automatic and does need much computational resources which is ideal under tunneling conditions. Our approach has been develop on 3D synthetic finite difference and tested on real tunneling data. In both cases, the distance of a fault zone has been determined accurately and without any a priori information.

Seismic Prediction Ahead of Tunnel Constructions Using Tunnel Surface‐Waves

To increase safety and efficiency of tunnel constructions, online seismic exploration ahead of a tunnel can become a valuable tool. We developed a new forward looking seismic imaging technique to e.g. determine weak and water bearing zones ahead of the constructions. Our approach is based on the excitation and registration of tunnel surface-waves (TS-waves). These waves are excited at the tunnel face behind the cutter head of a tunnel boring machine and travel into drilling direction. Arriving at the front face they generate body-waves (mainly S-waves = ”RS”-waves) propagating further ahead. Reflected S-waves are back-converted into tunnel surface-waves (”RSSR”-waves) and can be recorded by geophones mounted on the tunnel wall. Using 3D Finite Difference modeling, an analytical solution of the wave equation in cylindrical coordinates and field data acquired at the Gotthard massive (Switzerland) we investigated the propagation characteristics of tunnel surface waves in terms of dispersion and polarization. Understanding the excitation and propagation of TS-waves is the key for developing processing and imaging techniques for our seismic look ahead prediction in tunnel constructions.

Seismic Prediction Ahead of Tunnel Construction Using Rayleigh-waves

To increase safety and efficiency of tunnel constructions, online seismic exploration ahead of a tunnel can become a valuable tool. We developed a new forward looking seismic imaging technique e.g. to determine weak and water bearing zones ahead of the constructions. Our approach is based on the excitation and registration of tunnel surface-waves. These waves are excited at the tunnel face behind the cutter head of a tunnel boring machine and travel into drilling direction. Arriving at the front face they generate body-waves propagating further ahead. Reflected S-waves are back-converted into tunnel surface-waves.