Determination of the Depth/Structure/Fracture of Bedrock...(Page 1 of 3)

This section of the web manual discusses the determination of bedrock depths for materials source areas, foundations, mapping bedrock topography and mapping bedrock structure.  The determination of bedrock depths is common to each of these topics, since bedrock topography is simply the lateral changes in bedrock depths.  Bedrock structure is related more to fractures, faulting, and other features, although bedrock depth may also be important.  The section is broadly divided into two parts with the first part describing the methods used to map bedrock depths.  The second part concentrates on the methods that can be used to map fractures.

Unfortunately, no geophysical method, except possibly Ground Penetrating Radar under ideal conditions, can map fine/detailed structure internal to the bedrock.  Generally, only successively deeper layers beneath the bedrock can be mapped geophysically. However, geophysical methods can be used to map fracturing, faulting, locate voids, and for other targets as described in other sections of this web manual.

Many of the methods used to map bedrock topography can also be used to locate fractures; hence, the basic theory of the method is common to both applications.  It is, therefore, convenient to divide this section into two main parts.  The first part discusses the basic theory of each method as well as presents its application to mapping bedrock topography and providing bedrock depths.  The second part of this section will focus on the application of the techniques to locate fractures but will not describe the basic theory of each method, since these are described in the first section.  Exceptions to this will be methods that are used only for detecting fractures but are not used to find bedrock depths and are not described in the first section. Finally, a brief discussion of the use of geophysical methods to map faults is given at the end of this section.

Methods to Map Bedrock Depths and Topography

Determining bedrock depths and topography is commonly done using geophysical methods.  As with all geophysical methods, estimating the physical properties of the geologic section prior to conducting a survey is important in determining the best method to use.  The estimated depth to the bedrock is also an important criterion, since some methods may not be able to penetrate to the required depths.

Determining bedrock depths and topography are somewhat different in that mapping the topography of the bedrock does not necessarily imply that its depth is known.  However, usually, its depth can be found, and the topographic profile can be converted to a depth profile.  There are several geophysical methods that are used to find depths to bedrock and map bedrock topography.  These methods are listed below.

1.      Ground Penetrating Radar (GPR).

2.      Seismic Refraction.

3.      Seismic Reflection.

4.      Resistivity methods.

5.      Time Domain Electromagnetic Soundings (TDEM).

6.      Conductivity measurements using the EM31 and EM34.

7.      Spectral Analysis of Surface Waves (SASW).

8.      Analyzing acoustic noise using the SeisOpt software.

9.      Gravity.

Ground Penetrating Radar (GPR)

Basic Concept:  Ground-Penetrating Radar can be used where the bedrock is expected to be shallow, and the overburden is unsaturated and contains no clay or silt.  If these conditions exist, then penetration depths may be a few meters.  The GPR instrument consists of a recorder and a transmitting and receiving antenna.  Different antennas provide different frequencies.  Lower frequencies provide greater depth penetration but lower resolution.  Figure 1 illustrates the GPR system.  The transmitter provides the high-frequency electromagnetic signals that penetrate the ground and are reflected from objects and boundaries, providing a different dielectric constant exists from that of the overburden.  The reflected waves are detected by the receiver and stored in memory.

Figure 2 shows typical GPR equipment that includes the display and controls for the equipment.

Figure 3 shows a 100 MHz antenna being pulled across the ground as the survey is conducted.  This is a fairly low-frequency antenna and can provide penetration to about 20 m.

Data Acquisition:  GPR surveys are conducted by pulling the antenna across the ground surface at a normal walking pace, as shown in figure 113.  The recorder stores the data, as well as presenting a picture of the recorded data on a screen.

Figure 1.  Ground Penetrating Radar system.

Figure 2.  Ground Penetrating Radar instrument.  (Geophysical Survey Systems, Inc.)

Figure 3.  GPR antenna (100 MHz) used in a survey.  (MALA GeoScience USA, Inc.)

Data Processing:  The data are processed much like the processing done on single channel reflection seismic data.  Processes such as distance normalization, horizontal scaling (stacking), vertical and horizontal filtering, velocity corrections, and migration can all be done.  However, depending on the data quality, it may not be necessary to process the data since the field records may be all that is needed to observe the bedrock.

Data Interpretation:  In order to calculate the depth to the bedrock, the speed of the GPR signal in the soil at the site needs to be obtained.  This can be estimated from ts showing speeds for typical soil types or it can be obtained in the field by conducting a small traverse across a buried feature whose depth is known.

Advantages:  GPR is a relative fast method and presents results as the survey progresses. Different antenna can easily be attached and tested if the resolution or depth penetration is insufficient.

Limitations:  Probably the most limiting factor for GPR surveys is that their success is very site specific, and depends on having a contrast in the dielectric properties of the target compared to the host overburden. Clearly sufficient depth penetration to reach the target is needed.  Penetration depends on the frequency of the antenna, the conductivity of the overburden, and whether clay is present in the overburden.  Also, if a low-frequency antenna is used, then the resolution of targets is less than with a high-frequency antenna.  In addition, with low-frequency antennas, which are usually not shielded, GPR energy radiates in all directions.  Thus, reflections occur from local objects either on the ground surface or above the ground, such as power lines and buildings.  For surveys under bridges, the bridge deck may provide a reflection.  It may be possible to separate the reflection from the bridge deck from the reflection from the bedrock, providing these two reflection times are significantly different.

Seismic Refraction

Basic Concept:  Seismic refraction is one of the most commonly used methods to determine bedrock depths, especially for depths of less than 30 m.  The method requires a seismic energy source, usually a sledgehammer for depths of 15 m or less, and weight drop and black powder ges for depths to 30 m.  The seismic waves produced by the energy source penetrate the overburden and refract along the bedrock surface.  While they are traveling along this surface, they continually radiate seismic waves back to the ground surface.   These are detected by geophones placed on the ground surface.  Both compressional waves (P-waves) and shear waves (S-waves) can be used in the seismic refraction method, although compressional waves are most commonly used.  Figure 4a shows the layout of the instrument, the main seismic waves involved, and the resulting time distance graph.  A typical seismic recorder is shown in figure 4b.  The method can be used for up to about four layers.  However, each layer has to have a higher velocity than the overlaying layer.

Figure 4.  Seismic Refraction: field set up and data recorder.

Data Acquisition:  The design of a seismic refraction survey requires a good understanding of the expected bedrock velocity and depth and overburden velocity.  With this knowledge, velocities can be assigned to these features and a model developed that will show the parameters of the seismic spread best suited for a successful survey.  These parameters include the length of the geophone spread, the spacing between the geophones, the expected first break arrival times at each of the geophones, and the best locations for the off-end shots.  Knowing the expected first break arrival times is also helpful in the field, where field arrival times that correspond fairly well to expected times help to confirm that the spread layout has been appropriately planned, and that the target layer is being imaged.

Data Processing:  The first step in processing/interpreting refraction seismic data is to pick the arrival times of the signal, called first break picking.  A plot is then made showing the arrival times against the distance between the shot and geophone.  This is called a time-distance graph.  An example of such a graph for two-layered ground (overburden and refractor) is shown in figure 4a.

The red portion of the time distance plot shows the waves arriving at the geophones directly from the shot.  These waves arrive before the refracted waves.  The green portion of the graph shows the waves that arrive ahead of the direct arrivals.  These waves have traveled a sufficient distance along the higher speed refractor (bedrock) to overtake the direct wave arrivals.

Data Interpretation:  Several methods of refraction interpretation are used.  One of the most common methods is the Generalized Reciprocal Method (GRM) that is described in detail in Part 2 Geophysical Methods, Theory and Discussion.  A brief, and simplified, description of the GRM method is presented below.  Figure 5 shows the basic rays used for this interpretation.

Figure 5.  Basic Generalized Reciprocal method interpretation.

Figure 6.  Generalized Reciprocal method interpretation.

The objective is to find the depth to the bedrock under the geophone at D.  This is done using the following simple calculations.  The travel times from the shots at A and G to the geophone at D are added together: T1= T(AD) + T(GD).  The travel time from the shot at A to the geophone at G is then subtracted from T1.  Figure 6 shows the remaining waves after the above calculations have been performed.  These are the travel times from C to D added to the travel times from E to D subtracting the travel time from C to E.  The sum of these travel times can be shown to be approximately the travel time from the bedrock at H to the geophone at D.  Since the velocity of the overburden layer can be found from the time distance graph, the distance from H to D can be found giving the bedrock depth.

An example of the results from a seismic refraction survey is presented in figure 7.  The upper figure shows the travel time graph, illustrating the time that the seismic waves take from the shot to each geophone.  The second graph presents the interpreted seismic velocities, and the third graph shows the interpreted bedrock section.

Advantages:  Refraction seismic is generally very effective at determining bedrock depths since bedrock usually has a higher velocity than the overburden.  In addition, the method is able to provide fairly detailed lateral variations in depth since the depth beneath each geophone can also be found.

Limitations:  Probably the most restrictive limitation is that each of the successively deeper refractors must have a higher velocity than the shallower refractor.  However, for determining bedrock depths, this is probably not a significant limitation since, as mentioned above, the bedrock usually has a higher velocity than the overburden.

 

Figure 7.  Example of a SeismicRrefraction interpretation.

If the water table is in the overburden and close to the bedrock, this may obscure the bedrock arrivals since saturated soils have a higher velocity than unsaturated soils.  This may result in a false interpretation of the bedrock depth.

Local noise, for example, traffic, may obscure the refractions from the bedrock.  This can be overcome by using larger impact sources or by repeating the impact at a common shot point several times and stacking the received signals.  If noise is still a problem, a larger energy source may be required.  In addition, since some of the noise travels as airwaves, covering the geophones with sound absorbing material may also help to dampen the received noise.

Seismic Reflection

Basic Concept:  Seismic reflection involves using a surface seismic source to create a seismic wave, which then travels into the subsurface.  At interfaces that have an impedance contrast, (change in velocity and/or density) a portion of these waves is reflected back to the ground surface, and a portion is transmitted through the interface.  These transmitted waves then reflect at the next impedance contrast and return to the ground surface. Geophones on the ground surface record these reflections.  Figure 8 shows the seismic reflection method.

Figure 8.  The SeismicReflection method.

Data Acquisition:  A line of geophones is placed on the ground surface. Shots (hammer or explosive sources) are initiated at regular intervals along the geophone spread and the resulting reflections are recorded by the geophones and stored in seismic recorder. Seismic reflection surveys require that the geophone and shot spacing are appropriate for the particular problem.  The number of channels should be chosen so that the spread length is appropriate for the desired depth of investigation, and the geophone spacing needs to be such that the rugosity of the reflecting surface is imaged properly.

Data Processing:  Many techniques are applied to process reflection seismic data.  These include filtering, correcting for subsurface velocity effects, and stacking the traces that emanate from a common depth point (CDP) on the reflecting surface.  The main objective of these techniques is to provide a gather (group) of seismic traces that can be stacked to image each reflection point as clearly as possible.  The output from processing a line of seismic data is a seismic section showing the reflectors.  This section can be presented as CDP location against record time, or when velocities are assigned to the different layers, as CDP against depth.  More detail is provided in Part II Geophysical Methods, Theory and Discussion. 

Data Interpretation:  For reflection seismic surveys for bedrock depth determinations, a seismic section showing CDP against depth is most appropriate.  Such a section should provide an immediate view of the depths to the bedrock.

Advantages:  Seismic reflection generally requires a less intensive energy source for a given depth than the seismic refraction method.  It is also better able to image greater depths than the refractions method.

Limitations:  The seismic reflection method is usually fairly labor intensive and is often more expensive that other methods.  In addition, when the bedrock depths are shallow, seismic refraction will usually be the more appropriate method.  However, seismic reflection may be a more appropriate method when bedrock depths are more than 30 m.  

Resistivity

Basic Concept:  The resistivity method can be used to find bedrock depths if the overburden and bedrock have different resistivities, which is usually the case.  There are two common resistivity methods: soundings and traverses.  Figure 9 shows the resistivity sounding method.  Resistivity traverses are used to map lateral variations in resistivity and are not usually used to provide bedrock depths.

Data Acquisition:  Figure 9a shows the system used to measure the resistivity of the ground.  Current is passed into the ground using the two electrodes labeled A and B.  The voltage that results from this current is then measured using electrodes M and N.  Using the amount of current passed into the ground along with the voltage and a geometric factor for the electrode layout, the resistivity of the ground is calculated.  The electrode array is then expanded, making the current penetrate deeper into the ground and another reading is taken.  This procedure is repeated for many electrode spacings, providing a set of resistivity values for different electrode spacings.  These values are plotted on a graph of resistivity against electrode spacing, as illustrated in figure 119b.  This graph shows that at small electrode spacings, the resistivity is that of the overburden.  At large electrode spacings, the resistivity approaches that of the bedrock.  The resistivity curve is interpreted using software that provides a resistivity model (depths and resistivities) whose resistivity calculations match the field data.

Advantages:  Unlike the seismic refraction method, which requires that each successive layer have a higher velocity, the resistivity method works whether the deeper layers become more or less resistive.  The field procedures are fairly simple and a sounding to depths of about 50 meters depth can be conducted in less than one hour.

Limitations:  Because electrodes need to be placed in the ground, the method is difficult to use in areas where the surface of the ground is hard, such as concrete or asphalt-covered areas.  In addition, if the ground is dry, water may need to be poured on the electrodes in order to improve the electrical contact between the electrode and the ground.  Generally, the separation between the current electrodes will need to reach a maximum of about three times the investigation depth.  Thus, if the bedrock is 15 m deep, the current electrodes will need to be spaced up to 45 m apart.  Lateral variations in resistivity can affect the accuracy of the depth interpretation, or grounded metal objects near the sounding site may also influence the data.

 

Figure 9.  Resistivity Sounding (a) Data recording geometry, and (b) Sounding curve.

Time Domain Electromagnetic Soundings

Basic Concept:  Time Domain Electromagnetic (TDEM) soundings are another method used to obtain the vertical distribution of the resistivity of the ground.  This method is particularly well suited to mapping conductive layers, but can also be used to map resistive layers, which is likely to be the case for bedrock.  To a significant degree, this method has now superseded the resistivity sounding method since it requires less work for a given investigation depth and generally provides more precise depth estimates.  However, resistivity soundings are still useful for shallow investigations or when resistive targets are sought.  Figure 10 provides a conceptual drawing of the TDEM method. 

Data Acquisition:  TDEM soundings are an electromagnetic method used to provide the vertical distribution of resistivity within the ground.  A square loop of wire is laid on the ground surface.  The side length of this loop is about half of the desired depth of investigation.  A receiver coil is placed in the center of the transmitter loop.  Electrical current is passed through the transmitter loop and then quickly turned off.  This sudden change in the transmitter current causes secondary currents to be generated in the ground.  The amplitude of these currents is related to the conductivity of each of the layers in the ground.  Thus, the conductive layer will generate stronger secondary currents than a less conductive layer.  The voltage measured by the receiver coil does not decay instantly to zero when the current is turned off but continues to decay for some time.  This decaying voltage is caused by the decaying secondary electrical currents in the ground.  The voltage measured by the receiver is then converted to resistivity.

Figure 10.  Time Domain Electromagnetic Soundings.

A plot is made of the measured resistivity against the decay time, as illustrated in figure 11.

The switching and measuring procedure is repeated many times, which allows the resulting voltages to be stacked, thereby improving the signal-to-noise ratio.  This procedure is repeated at different sounding locations until the area of interest has been covered.  A sounding curve is plotted for each location showing the measured resistivity against decay time.

The resistivity sounding curve shown in figure 11 illustrates the curve that would be obtained over three-layered ground.  The near-surface layer is fairly resistive.  This is followed by a layer having a much lower resistivity (higher conductivity) and causes the measured resistivity values to decrease.  The third layer is again resistive.    

Figure 11.  A Time Domain Electromagnetic Sounding curve.

Data Processing:  The field data is transferred from the data recording instrument to a computer.  The only processing required may be the removal of bad data.  The data is then input to software that produces a sounding curve and facilitates interpretation.

Data Interpretation:  The sounding curve is interpreted using computer software to provide a model showing the layer resistivities and thickness.  The interpreter inputs a preliminary model into this software program, which then calculates the sounding curve for this model.  It then adjusts the model and calculates a new sounding curve that better fits the field data.  This process is repeated until a satisfactory fit is obtained between the model and the field data; this process is called inversion.  

Advantages:  Some of the advantages of the method have been mentioned under Basic Concept.  Probably the most important of these is that the method is often more efficient than the resistivity method, especially if the target is at depth greater than about 50 meters.  In addition, compared to the resistivity method, no electrodes have to be planted in the ground.

Limitations:  TDEM soundings are an efficient method for investigating the vertical distribution of ground resistivity.  Generally, the method is better suited to mapping conductive formations than resistive formations.  However, it is also effective at mapping depths to resistive layers.

Probably the most troublesome aspect of data recording is the influence of fences, power lines, and other "cultural" features at the survey site.  For surveys to find the depth to bedrock near bridges and other structures, the metal in these structures may prohibit the use of this method.  However, if the bedrock is fairly horizontal and soundings can be conducted some distance from the site, then TDEM soundings may work.

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