Analytische Chemie
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The Alpine Fault at the West Coast of the South Island (New Zealand) is a major plate boundary that is expected to rupture in the next 50 years, likely as a magnitude 8 earthquake. The Deep Fault Drilling Project (DFDP) aimed to deliver insight into the geological structure of this fault zone and its evolution by drilling and sampling the Alpine Fault at depth. Here we present results from a seismic survey around the DFDP-2 drill site in the Whataroa Valley where the drillhole almost reached the fault plane. This unique 3D seismic survey includes several 2D lines and a 3D array at the surface as well as borehole recordings. Within the borehole, the unique option to compare two measurement systems is used: conventional three-component borehole geophones and a fibre optic cable (heterodyne Distributed Vibration Sensing system (hDVS)). Both systems show coherent signals but only the hDVS system allowed a recording along the complete length of the borehole.
Despite the challenging conditions for seismic imaging within a glacial valley filled with sediments and steeply dipping valley flanks, several structures related to the valley itself as well as the tectonic fault system are imaged. The pre-processing of the seismic data also includes wavefield separation for the zero-offset borehole data. Seismic images are obtained by prestack depth migration approaches.
Within the glacial valley, particularly steep valley flanks are imaged directly and correlate well with results from the P-wave velocity model obtained by first arrival travel-time tomography. Additionally, a glacially over-deepened trough with nearly horizontally layered sediments is identified about 0.5 km south of the DFDP-2B borehole.
With regard to the expected Alpine fault zone, a set of several reflectors dipping 40-56° to the southeast are identified in a ~600 m wide zone between depths of 0.2 and 1.2 km that is interpreted to be the minimum extent of the damage zone. Different approaches image one distinct reflector dipping at 40°, which is interpreted to be the main Alpine Fault reflector. This reflector is only ~100 m ahead from the lower end of the borehole. At shallower depths (z<0.5 km), additional reflectors are identified as fault segments and generally have steeper dips up to 56°. About 1 km south of the drill site, a major fault is identified at a depth of 0.1-0.5 km that might be caused by the regional tectonics interacting with local valley structures. A good correlation is observed among the separate seismic data sets and with geological results such as the borehole stratigraphy and the expected surface trace of the fault.
In conclusion, several structural details of the fault zone and its environment are seismically imaged and show the complexity of the Alpine Fault at the Whataroa Valley. Thus, a detailed seismic characterization clarifies the subsurface structures, which is crucial to understand the transpressive fault’s tectonic processes.
Sealing and strengthening of the subsoil by injection is a major issue in the field of geotechnical engineering. This involves also jet grouting, which allows creating columns of grouted soil by eroding and mixing the in-situ soil with a thin cement suspension. A general difficulty of this method is to predict the column diameter and its material strength. Here, we present two-dimensional finite-difference numerical simulation results of a promising non-destructive quality assurance testing tool to evaluate the diameter of jet grout columns. This approach incorporates crosshole and downhole seismic measurements. Preliminary tests showed that this tool is applicable under real site conditions.
Innovative seismic and resistivity tools for determining the diameter of jet grouting columns
(2016)
Jet grouting is used for soil improvement, foundation support and groundwater low control all over the world. It is well accepted and subject of standardization in many countries. However, some issues with the method remain. As the grout columns are produced in the subsurface without visual control in an often inhomogeneous soil, the prediction of the column’s diameter is still a challenge. All methods applied so far have their limitations.
The approach presented in this study is twofold. At Colorado School of Mines a resistivity probe has been developed, which is pushed into the fresh grout directly after production. ERT sections are measured and inverted. Given some background information is available, the diameter of the columns can be evaluated.
At BAM the focus has been on post-production investigations using seismic borehole methods. After hardening of the concrete seismic waves are sent through the column downhole (sensors placed in a casing in the column’s axis, source on top) and crosshole (source and sensor in boreholes on opposite sides of the column). We have developed a scheme to evaluate the diameter of the column based on travel time measurements without calibration. As this method is eventually more time and cost intensive we assume its application mainly for test columns. These are casted and dug out for visual inspection to determine appropriate grouting parameters. Our method would replace the often very cost intensive visual inspection.
Both approaches have been tested at three test columns produced at BAM’s test site at Horstwalde, Germany and on an actual constructions site. Both methods have been in good agreement with the diameters predicted by the jet grouting contractors, which were confirmed in one case by mechanical measurements.