Focus of activities

How stable is the mine structure?

The following project was initiated to guarantee the long term security of a number of redundant potassium salt workers in the South/Harz, Saale and Unstrut area. The project became necessary ...

Geophysical Exploration of a Hardrock Deposit

Within the framework of the Wülfrath Rhenisch limestone works, Kali-Umwelttechnik was entrusted with the geophysical aspects of exploration to assess the viability of mining the limestone deposits. The exploration program involved the following tasks detailed below :

• Proof of the complex of limestone and differentiation of this from the surrounding rock.
• Establishment of the depth and extent of the Limestone deposits.
• Identification of abnormalities in the rock structure, i.e. tectonic related disturbances ( faults, fissures ), subterranean weathering etc...
• Establishment of the inclination of the limestone strata.
• Establishing the extent of the overburden covering the Limestone deposits.
• Recommendations for borehole sites for the subsequent drilling programme.

To clarify these questions, K-UTEC has put various geophysical exploration methods into use in order to attain meaningful and reliable results required for this project:

A seismic profile of the exploration project is shown in Picture 1. From the results of seismic surveys, significant conclusions can be derived about fluctuation and variation within the limestone complex and where they occur. At the same time, the depth and extent of the overburden is ascertained. In addition, the existence of intrusions are easy to recognize in the seismograms.

Seismic profile of the exploration project

From the direct current geoelectricity survey, differences in resistance are recorded in the underground rock formation. The various types of rock demonstrate typical resistance behaviour. As a result, different rock classifications can be assigned to the recorded values. Picture 2 is an example of results of a resistance survey that correspond with core samples recovered at the Angertal drill site. The results of the geoelectric survey support the conclusions attained from a seismic survey.

Geo-electric resistance profile of the exploration project with geological interpretation.

Using a borehole radar, the situation and extent of karsification zones can be proved as well as the slope of the layers. With the help of deep penetrating radar waves, an accurate estimate of the quality of the limestone body can be achieved as illustrated in Picture 3.

Qualitative limestone evaluation based on the results of borehole radar

By bringing together all geophysical results as a complex interpretation, reliable statements about the investigated rock body can be obtained:

• Condition of limestone deposit
• Inclination and angle of the layers at various depths
• Direction of slope
• Degree of weathering
• Existence, position and extent of crevasse systems
• Boundaries of the limestone deposits (Picture 4)

Representation of overburden (top), topography (middle) and limestone surface (bottom) after the removal of the overburden.

Detailed investigation of the Sondershausen Castle foundations

The historically significant Sondershausen Castle is structurally unsound. The present condition of the building requires urgent safety and renovation measures to ensure it can remain standing.
However, a successful safety and renovation programme reqired clarification of the causes of damage and their development in perspective. The existing pictures of the damage in the form of cracks etc. indicate that, for the main part, the damage is caused by unstable foundations and underground phenomena.

In the framework of a complex foundation investigation, Kali-Umwelttechnik was entrusted to establish the causes for the damage with inter-disciplinary work between geology, geophysics, geomechanics, ground mechanics, building statistics and architecture and to find solutions and constructive renovation suggestions.

On the plan of the castle complex in Picture 5, the profiles of some geophysical measurements have been entered. For the investigation project, the following geophysical procedures were put to use:

• Seismic: Seismic refraction, seismic reflection, seismic ultra-sound (sonar)
• Seismic tomography
• Geoelectric tomography: so called psuedo sections
• Radiometric surveys
• Radar tomography

Area map of the geophysical profile

In conjunction with surface and borehole procedures, stonework investigation by radar was also carried out, i.e. a geophysical-geotechnical assessment of the exploration site. By combining the results of all the disciplines involved comprehensive and detailed conclusions can be made regarding the state and depth of the overburden and the nature of the bed rock (multi-coloured sandstone).

Differences in sonar readings prove underground anomalies.

For example from seismic measurements, differences in frequency are noted in different regions of the castle as shown in Picture 6. These differences are a definite indication of irregularities in the underground structure. Areas with low frequency feedback are interpreted as badly weathered or unstable, while high frequency feedback indicates solid, compact rock forms.

From the results of the seismic survey of the surface of the sandstone bedrock along with the visual topography, one can calculate the depth and nature of the overburden covering the multi-coloured sandstone at the site. A presentation of the different thicknesses of overburden can be seen in Picture 7.

Profile of overburden in proximity of castle complex.

An electromagnetic, tomographic survey provides more results. By use of borehole radar equipment the speed and absorption rates of x-rays were measured between two boreholes.The spread and absorbtion of electromagnetic waves between two boreholes is established by borehole radar. In this example the boreholes do not run parallel but lie at an angle to each other. The results of the survey are illustrated in Picture 8.

Radiation matrix of the tomo- graphic radar survey

Pictures 9 and 10 depict the specific electrical resistance resultant from the tomogram. The zone between the two boreholes approximately central (blue) represents the detection of an area where a cavity exists. The results however show that the cavity is not of significant size.

Radar tomogram depicting speed of x-rays		Radar tomogram of the specific electrical resistance

With geophysical investigations of Sondershausen Castle the quality of the building and its foundations could be plausibly portrayed. Along with the proof of cavities and fractured zones of various expanse, other factors also contributed to the overall state of the buildings and its foundations such as a very loose overburden, in the form of embankments as well as highly weathered sandstone.