BEDROCK MAPPING

Description

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:

  • Infrastructure projects, allowing safe and stable construction of buildings, bridges, roads, railroads, airports etc. Aside from the safety aspect, failing to determine the depth to, or quality of, bedrock can also cause both severe cost increases and large delays.
  • Building tunnels or different types of repositories (storage, wells etc.) within the bedrock.
  • Extraction of different types of mineral resources (iron, copper, gold etc.) as well as aggregate and fossil fuel (coal, oil and natural gas).
  • Characterization of groundwater aquifers in bedrock formations, such as porous sandstone or fractured crystalline rock.
  • Mapping areas that can be used for storm water management, due to climate changes and increasing precipitation.
  • Mapping pathways for analysis of groundwater and contaminant flow.

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.

Legend

BEDROCK MAPPING WITH GPR

BEDROCK MAPPING WITH RESISTIVITY

BEDROCK MAPPING WITH SEISMICS

BEDROCK MAPPING WITH TEM

COMBINING METHODS

CASE STORIES

Solutions

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

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:

  • Bedrock level / detection
  • Topography of bedrock surface
  • Cavities within the bedrock
  • Fractures in bedrock (from the surface, from within boreholes, or inside tunnels e.g. walls and roof)

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

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:

  • from different depths beneath a fixed point at the surface (vertical electrical sounding, VES);
  • at a constant depth but under different surface positions (usually referred to as profiling);
  • at both different depths and lateral positions to produce a 2D or 3D dataset (most commonly referred to as electrical resistivity tomography, ERT, or imaging).

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:

  • Bedrock depth / detection
  • Bedrock topography
  • Fracture and fault zone identification
  • Cavity detection and mapping
  • Ores / mineral deposits
  • Lithology or bedrock type / change

 

Seismics

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:

  • Bedrock depth / detection
  • Bedrock topography
  • Fracture and fault zone identification
  • Cavity detection and mapping
  • Lithology or bedrock type / change
  • Mechanical (engineering) parameters

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.

TEM

TEM measurement out in the fieldTEM (transient electromagnetics) is an inductive method, which means that it does not require a direct galvanic / physical contact with the ground. A simple wire transmitter loop on the ground is used to generate a varying magnetic field, and the receiver antenna(s) then measures the rate of the magnetic field’s diffusion in the subsurface. The received signal can be recalculated to give an indication of resistivity. Data are collected as a point investigation (sounding), and multiple soundings can be combined to produce profiles or spatial images and models. Similar to the resistivity method, the raw data represents only apparent resistivity values and a process of inversion is required to get a model of true (hopefully) resistivity versus depth.

TEM is suitable for:

  • Bedrock depth / detection
  • Coarse bedrock topography
  • Ore / mineral deposits
  • Bedrock type / change

The scalable TEM solutions offered by Guideline Geo allow a tailored package of hardware and software to be configured to meet each customer’s specific survey needs. The different hardware options primarily define the depth capability of the system, based on the power and loop sizes, but also in-field convenience including features such as on-board automated processing and inversion. The available software can be limited to an intuitive and fast package for processing, analysis and interpretation in 1D, or configured for advanced data handling and visualisation of TEM and other datasets 1D, 2D and 3D space.

TEM Transient electro magnetics method

What method and technique to use?

Whilst there are many factors involved in deciding upon the correct solution for a given project, these are some of the key considerations:

  • GPR is suitable in non-conductive conditions (e.g. without clayey soils) down to approximately 30-50 meters
  • TEM is unsuitable in urban areas due to sources of electromagnetic disturbance
  • TEM is suitable for deep investigations (>500 m) where resistivity and seismic methods might be limited by access, equipment power or general survey practicalities
  • Resistivity requires galvanic contact via electrodes of some description which can be both hard and time consuming in environments with bare rock or asphalt etc.
  • If an active (as opposed to passive) seismic method is to be employed the source needs some consideration; it needs to be big enough to get the energy to and from the desired depth and there may be legislative or environmental constraints upon how this is achieved
  • TEM or VES may be sufficient for broad prospection but GPR, ERT and seismic methods are best for more detailed results
  • Seismic methods can provide quantitative appraisal of geological strength and stability
  • TEM is not well suited to differentiating between highly resistive geological units; the method responds better to conductive stratigraphic elements
  • 1D methods (VES, TEM, VSP) are not well suited to identifying confined features like fracture zones and intrusions compared to 2D methods which offer lateral detail

 

Combining methods

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:

  • if the project covers a wide geographical area, GPR can be used more efficiently in highland areas with till and sands, whilst resistivity investigations may be more successful in lowland areas with high proportions of clay and silt
  • it may be beneficial to supplement ERT or seismic methods with TEM soundings through those areas where the bedrock was too deep to be detected by the primary methodology
  • GPR can give excellent detail on the stratigraphy, where the interfaces between geological units lie, but a method which gives a more quantitative result (such as resistivity or seismic refraction) can aid with the interpretation of what material makes up each of those units

 

  • This field is for validation purposes and should be left unchanged.