Bedrock is the hard, solid rock most often found beneath the unconsolidated surface materials, such as sand or till, but also as outcrops above the ground surface. The depth to the rock head can vary from zero to several hundred meters and the topography, as well as the quality, of the bedrock can differ significantly.
An understanding of the bedrock is important for many aspects of human activities on Earth. Knowledge of the depth to the bedrock and its properties are essential for:
Today, most bedrock investigation is done by digging or drilling but with the increased use of 3D design tools (on infrastructure and resource mapping projects, for example) the demand for more data coverage is increasing. Attempting to resolve this need with traditional intrusive methods will be extremely expensive and sometimes, with an increasingly ‘crowded’ subsurface, digging and drilling can be most undesirable and risky. Instead, different geophysical tools can be used to assist in the creation of a more comprehensive picture of the bedrock conditions.
Guideline Geo´s wide range of geophysical investigation techniques can solve most of the common questions that instigate some form of bedrock mapping. They will give a non-destructive and cost-efficient way of gaining a better understanding of the ground conditions, providing better data coverage than is normally achieved with traditional, discrete, point-by-point geotechnical investigations, such as drilling or digging.
In Table 1 the most common bedrock applications and a range of possible geophysical techniques for their investigation are listed. Below this, there is a discussion section surrounding the most common solutions provided by Guideline Geo, namely GPR, resistivity, seismics and TEM. Note that all solutions provide output as position (XY) and depth (Z) against some other parameter such as depth to bedrock, which can be transferred easily to GIS or CAD applications.
GPR is an electromagnetic method that can be used for the investigation of more shallow ground conditions, typically down to a maximum of 30-50 meters. The transmitter and receiver antennas are pulled along, or just above, the ground surface and reflections from the subsurface are gathered in a controller. The frequency of the antennas is normally between 25MHz and 2.3GHz, where the lower frequencies give better depth penetration and are suitable for bedrock mapping and investigations.
GPR is suitable for:
Guideline Geo provides a number of different GPR solutions for bedrock applications, most common being the MALÅ GX and MALÅ ProEx ranges; visit the product pages below for more information. The processing and interpretation of the collected data can be done in different software packages, depending on application. More information can be found below.Suitable software is for example Reflexw or GPR-Slice.
Resistivity survey is a galvanic method, where a set of steel electrodes attached to a resistivity meter are inserted into the ground to record the subsurface electrical properties. Different materials have different resistivity associated with them, and the resistivity can vary within a material due to, among other things, changes in mineralogy, water content, and structural integrity. Typically, higher resistivity indicates greater strength in a given geological material.
Depending upon the positions of current and potential electrodes the data will be collected:
The instrumentation can only record a parameter called apparent resistivity, as the measurements are all from the surface, and the measured values are the result of current flowing through all of the ground beneath the electrodes rather than just information from a discrete point or layer. These values, on their own, will not be suitable for making an interpretation and thus a further processing step, called inversion, is required. In this process software is used to find a model of resistivity distribution that would produce the same result as the recorded data.
Resistivity is suitable for:
Seismic surveys rely upon an energy source transmitting elastic waves through the ground, which are then recorded by a seismograph via sensors (geophones or hydrophones for land and water, respectively). The speed and path of the waves is determined by the density, elasticity, and structure of the ground. By analysing the speed and nature of the responses, a model of the sub-surface structure and layer velocities can be produced. The layer velocities can help to predict the type of material present as well as its strength and/or stability. Typically, faster velocities indicate stronger or more stable geology.
Depending upon the sensors configuration, different types of motion can be recorded and the source/receiver layout will determine whether the result is a sounding (point investigation such as vertical seismic profiling, VSP) or something more comprehensive which also reveals velocity variations across an area (profile and/or 3D investigation).
The three main survey approaches are refraction, reflection and surface wave analysis, each with their own advantages and disadvantages but, in general, seismic survey is suitable for:
Guideline Geo’s ABEM Terraloc products are incredibly versatile and can be configured to undertake practically any type of seismic data acquisition; the only choice is the scale of system in terms of how many measurement channels are required. A very wide range of 3rd party processing packages available from Guideline Geo ensures that the perfect solution can be found for your particular application.
Whilst there are many factors involved in deciding upon the correct solution for a given project, these are some of the key considerations:
It is often beneficial to combine different geophysical methods to get the best resulting picture of the bedrock conditions due to the overburden conditions, the physical distribution of the bedrock or level of information required. For instance: