Lithology is a description of rocks, especially sedimentary rocks, mostly from hand specimens and outcrops.  This description includes the color, structure, mineralogical composition, and grain size.  Geophysical methods measure the physical properties of rocks, including seismic wave velocity, resistivity, induced polarization, magnetization, and, at shallow depths, dielectric properties.  To use geophysical methods to map lithology, there has to be some relation between the lithological parameters and the physical properties of the rock. 

The two lithological parameters that can be most readily correlated from geophysical measurements are structure and grain size.  Color cannot be easily translated from geophysical methods to a physical property.  Mineralogical composition is loosely related to seismic velocity, although this relationship is probably quite complex. The only minerals that can be detected directly using geophysical methods are magnetite and metallic sulfides, such as pyrite.

The distribution of grain size within a rock is related to porosity.  Porosity is the ratio between the volume of pore space and the total volume of rock. A uniform wide grain size distribution usually results in a lower porosity than a narrow distribution, since the smaller grains can fill in the spaces between the larger grains.  The critical grain size cannot generally be readily determined from porosity. Porosity can provide an indicator of the relative grain size distribution (wide or narrow) since materials with wide grain size distributions will typically have less pore space or lower porosity than materials with narrow distributions.  Structure is probably related to seismic velocity and less to resistivity.  Seismic velocity is strongly related to density, with denser rocks having a higher seismic velocity.  However, since there is a considerable overlap of densities for different rock types, it is difficult to predict rock type, and hence lithology, from density.  Likewise, the resistivity of any particular rock type varies over a wide range, and there is a large overlap of rock types having similar resistivities.  Probably the most definitive relationship is between seismic velocity and porosity, although, as with density, there is a large overlap of different rock types with similar velocities.  Figures 1 and 2 show the resistivities and seismic velocities of numerous rock types and illustrates the overlap in rock type for both resistivity and velocity.

Figure 1.  Resistivities of different rock types.  (From TN5, Electrical Conductivity of Soils and Rocks, Geonics Ltd)

Figure 1.  Resistivities of different rock types.  (From TN5, Electrical Conductivity of Soils and Rocks, Geonics Ltd)

Figure 2.  Seismic velocities of different rock types. (Modified from Exploration Seismology, R.E. Sheriff and L.P. Geldart)

The minerals comprising a rock are almost always electrical insulators.  Thus, electrical conduction occurs because of the moisture contained within the pores of the rock or soil. The resistivity of soil or rocks depends on several parameters. These include the clay content, moisture salinity, degree of saturation of the pores, and the number, size, and shape of the interconnecting pores.  For soils, the degree of compaction (influencing porosity) is also an important factor. Archie (1942) developed an empirical formulae relating resistivity to porosity, degree of saturation and resistivity of the saturating moisture, shown below.

                                                                                 (1)

where f  is the fractional pore volume (porosity), s is the fraction of the pores containing water, ris the resistivity of the water, n is approximately 2, a and m are constants with a varying between 0.5 and 2.5 and m varying between 1.3 and 2.5.

Porosity determines the amount of water that a rock can contain, and this influences resistivity.  However, because resistivity is also influenced by several other factors as described above, the absolute value of porosity can probably not be obtained from resistivity.  However, changes in resistivity may possibly be used to indicate changes in porosity, assuming that the other factors influencing resistivity remain constant over the area of interest. In this case, resistivity changes may possibly be used to indicate changes in lithology.

It appears, therefore, that it is not easy to predict lithology from either seismic velocity or resistivity, although rock type is often interpreted from resistivity and seismic velocity data.  As discussed above, these methods may be used to map changes in lithology and will be discussed within this context.

Magnetic methods will also be briefly described, since these can be used to estimate the magnetic susceptibility of a rock, which is related to the amount of magnetite it contains.  Induced polarization is also described, since this technique is used to locate pyrite and other metallic sulfides.

Some of the methods discussed can be used for investigations to depths of hundreds of meters and beyond.  These include the seismic reflection and magnetic methods.  Seismic reflection methods are used extensively for hydrocarbon exploration surveys.  Although the general techniques employed for hydrocarbon exploration are similar to those used for shallower surveys, this web manual concentrates only on techniques applicable to these shallower surveys.  It is assumed that the primary depths of concern for this web manual are generally less than 100 m. 

Methods

Six different geophysical methods will be discussed.  These are listed below along with the observed or derived properties that are measured.

       Method                                                          Observed/Derived Property

Seismic Reflection                                                  Layer stratigraphy and depth

Seismic Refraction                                                  Rock velocity, layer depth

Time Domain Electromagnetic Methods            Resistivity, depth

Resistivity Methods                                                 Resistivity, depth

Magnetic Methods                                                   Magnetic susceptibility, magnetite content

Induced Polarization                                               Metallic sulfides, clay

Seismic Methods

Two seismic methods will be discussed.  These are seismic refraction and seismic reflection.  Seismic refraction can be used to obtain the velocity of up to four layers, along with the thickness of these layers.  Seismic reflection can also be used to estimate the velocity and measure the thickness of rock layers, but is more commonly used to map the continuity of reflecting horizons to much greater depths than the seismic refraction method and is not restricted to four layers.

Seismic Refraction

Basic Concept:  The seismic refraction method can be used to measure rock velocities and the depths to refractors.  The method requires a seismic energy source, usually a hammer for depths less than 15 m and black powder ges for depths to 30 m.  The seismic waves then penetrate the overburden and refract along the bedrock surface.  While they are traveling along this surface, they continually refract seismic waves back to the ground surface.  These are then detected by geophones placed on the ground surface.  Both compressional waves and shear waves can be used in the seismic refraction method, although compressional waves are most commonly used.

Figure 3 shows the StrataVisor NZ instrument manufactured by Geometrics.  This system can record 64 channels of data.  In addition, the number of channels can be increased using attached secondary instrumentation.  This instrument is used for both seismic refraction and seismic reflection surveys.

Figure 3. Seismic Refraction data recorder.

Figure 4.  Seismic time-distance graph over four-layer ground.

The main seismic waves involved and the resulting time distance graph are shown in figure 5a.  This figure shows the shot locations commonly used in seismic refraction surveys along with the geophone layout.  In addition to the shot locations shown, more shots internal to the geophone spread are sometimes used to better map the near surface velocities and stratigraphy.

Figure 5. Seismic Refraction: field set up.

The ray paths and time distance graph over four-layered ground are shown in figure 4.  The inverse of the slopes of the line segments on the time-distance graph provide the velocities of the refractors.

Data Acquisition:  The design of a seismic refraction survey requires a good understanding of the expected bedrock and overburden.  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, and the expected first break arrival times at each of the geophones.  Knowing the expected first break arrival times is 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 layers are being imaged.  If lateral changes in velocity are required over a wide area, contiguous spreads will be used along traverses crossing the area of interest.

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 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 5 and that over four-layered ground is shown in figure 4.  In figure 5, the time-distance plot shows the waves arriving at the geophones directly from the shot (Velocity 1).  These waves arrive before the refracted waves.  The refracted waves arrive ahead of the direct arrivals (Velocity 2).  These waves have traveled a sufficient distance along the higher speed refractor (bedrock) to overtake the direct wave arrivals.  A similar argument is made for all of the waves shown in figure 4, where the first arrival waves from each successively faster refractor eventually arrive earlier than the first arrival waves from the slower refractors

Data Interpretation:  There are several methods of refraction interpretation, the most common of which is the Generalized Reciprocal Method (GRM) that is described in detail in the Geophysical Methods section. A brief, and simplified, description of the GRM method is presented below. Figure 6 shows the basic rays used for this interpretation.

Figure 6. Basic 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). The travel time from the shot at A to the geophone at G is then subtracted from T1.  Figure 7 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.  This process can be extended to apply to several layers.

Figure 7. Generalized Reciprocal method interpretation.

Once the velocities of the layers are assigned, these should be interpreted to give appropriate geologic layers.  For example, a layer with a velocity of 2,000 m/s suggests a soft rock such as shale, whereas a velocity of 4,000 m/s indicates a hard rock such as limestone. Lateral changes in the velocity of the layers may indicate changes in lithology.

Advantages:  Seismic refraction is usually a good method to obtain the velocity of  rocks, which one of the criteria for establishing lithology.

Limitations:  Probably the most restrictive limitation is that each of the successively deeper refractors must have a higher velocity than the preceding shallower refractor.  If this is not the case, errors in depth estimates of the deeper layers can occur, since a lower velocity layer beneath a higher velocity layer will not be observed in the data.

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 also lower the velocity contrast between the saturated overburden and the bedrock.

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.  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:  Geological layers can be mapped using the seismic reflection method. This can be done using either a shear wave source or a conventional compressional wave seismic source.  One shear wave source is called the Microvib and is illustrated in figure 8.  Compressional wave sources range from a simple hammer and base plate, to black powder and conventional explosive sources, to vibrators.  For relatively shallow investigations, a hammer and base plate, weight drop, and black powder sources can be used.  The actual depth of investigation depends on the geology and site conditions, but can be 50 m and deeper.  For deeper investigations, a small vibrator can be used.  Examples are the Microvib (figure 7) shear wave source and the Minivib source (figure 8), applicable for depths over 1,000 m.

Figure 8. The Microvib shear wave generator. (Bay Geophysical)

Seismic sources, producing either vibrations or impulses can be used for seismic reflection surveys.  Special processing techniques are applied to the vibrational sources to make the resulting data compatible with impulse sources.

Figure 9.  The Minivib seismic source.  (Industrial Vehicles, Inc.)

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 (related to velocity and density), a portion of these waves is reflected back to the ground surface, and a portion is transmitted through the interface. Geophones on the ground surface record these reflections.  Figure 10 shows the seismic ray paths for reflections from two layers.  The signals at two geophones are illustrated to the right of the ray path diagram.  The reflection from the interface between layers 1 and 2 arrives first at geophone 1.  A short time after this arrival, the same interface provides a reflection that arrives at geophone 2.  Some time later, the reflection from the boundary between layers 2 and 3 arrives at geophone 2.

Figure 10. The Seismic Reflection method.

Data Acquisition:  In a seismic reflection survey, shots are positioned at regular intervals along the line to be surveyed, and the seismic reflections recorded by a series of geophones placed on the ground surface, called a geophone spread.  The number of geophones varies from 12 to 60 or more for non-hydrocarbon seismic surveys.  The data are stored in the seismic recorder and transferred to a computer for processing.  The spacing between the geophones, the number of geophones used, and the shot spacing have to be determined before the survey begins, and depend on the exploration depth and the target resolution required. If long lines are to be recorded, a special switch box is needed, called a roll box, which allows the recording geophones to be advanced along the line as shooting progresses.

Data Processing:  If a vibration source is used, such as the Microvib, then special processing techniques have to be applied to make the data compatible with an impulse source.

Many techniques are used to process reflection seismic data including filtering, correcting for velocity effects, and stacking the traces that emanate from a common depth point (CDP, sometimes called common mid point CMP) on the reflecting surface.  The main objective of these techniques is to provide a gather of seismic traces that can be stacked to image each reflection point (CDP) 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.

Data Interpretation:  Reflection seismic surveys are usually used for stratigraphic exploration, and a seismic section showing CDP against depth is most appropriate.  Such a section should provide an immediate view of the seismic stratigraphy, both vertically and laterally.  The relation between the reflector stratigraphy and lithology will need to be constrained using other data such as a well log.  This may enable the seismic reflectors to be associated to a particular rock type.  Lateral changes in the acter of the seismic reflector may indicate changes in the lithology.

Figure 11 presents a seismic reflection section showing several reflectors and interpreted faults.  As can be seen, the acter of some of the reflectors changes across the section.  In particular, some of the reflectors are more pronounced and some are less pronounced between the faults.  Presumably, at least some of these changes are related to lithological changes.

Figure 11. Seismic Reflection section showing faults.

Advantages:  The seismic reflection method provides a pictorial section that resembles the subsurface layers.  The method is not restricted, as is the seismic refraction method, to a section in which the layer velocities successively increase.

Limitations:  The main attribute of the seismic reflection method is that it provides a visual image of the continuity of the reflectors along the surveyed line.  Layer velocities are also interpreted but are probably not as reliable as those found using the seismic refraction method.  In addition, the method is best suited for investigation depths greater than 10 to 20 m, depending on the geology.

The seismic reflection method is one of the more expensive methods geophysical methods, and requires a significant amount of knowledge to process the data. In addition, any local vibrational noise will reduce the signal-to-noise ratio and make the resulting seismic section less definitive.

Time Domain Electromagnetic Soundings

Basic Concept:  Time Domain Electromagnetic soundings (TDEM) are used to obtain the vertical distribution of resistivity.  This method is particularly well suited to mapping conductive layers.  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. 

TDEM soundings are an electromagnetic method used to provide the vertical distribution of ground resistivity.  The method uses pulses of electromagnetic energy rather than continuously oscillating electromagnetic sources.  Figure 12 illustrates the system layout and the current and voltage waveforms.

Figure 12. Time Domain Electromagnetic Sounding.

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 currents in conductive layers (shale) decrease more slowly than those in resistive layers.  The relation between the time after the current turns off (called delay time in this web manual) and layer depth and conductivity is complex.  However, in general, longer decay times correspond to greater depths. 

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 coil is then converted to resistivity.

A plot is made of the measured resistivity against the delay time, as illustrated in figure 13.  In this graph, the shallow layers are imaged at early decay times and the deeper layers at later decay times.

Figure 13. A Time Domain Electromagnetic Sounding curve.

The resistivity sounding curve shown in figure 13 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.  This method can be used for up to four or five layers.

Data Acquisition:  The transmitter loop and receiver coil are set up as described earlier.  Switched current is passed through the transmitter loop, and the receiver coil measures the resulting voltage. The switching and measuring procedure is repeated many times allowing the resulting voltages to be stacked and improving the signal-to-noise ratio.  Soundings are recorded at different sites until the area of interest has been covered. A sounding curve is plotted for each location showing the measured resistivity against decay time.

Data Processing:  Usually the only processing required is the removal of bad data points.

Data Interpretation:  The sounding curves are interpreted using computer software to provide a model showing the layer resistivities and thickness.  The interpreter inputs a preliminary model into this software program that then calculates the sounding curve for the 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. The process is called inversion. 

If the TDEM soundings are conducted along a traverse, a section can be drawn showing the variation of resistivity with depth along the section, thus also providing the lateral variations of resistivity.  The geologic interpretation then requires that the resistivities be assigned rock types. For example, a shale or clay layer will have a low resistivity, probably less than 30 ohm-m, whereas an impervious limestone may have a resistivity of over 1,000 ohm-m.  If a well log is available, rock types can be assigned to the resistivity values with much greater confidence.  Lateral changes in resistivity along a particular horizon will possibly indicate changes in lithology.

Advantages:  For a given depth of investigation, the method is much more efficient in the field than the resistivity method. The TDEM method is particularly good at defining conductive layers, but less effective at defining resistive layers.

Limitations:  Generally, the method is limited to four or five layers.  Resistive layers usually have to be fairly thick in order to be resolved, with the thickness increasing with depth.  Thin conductive layers can be detected, and, in fact, the method responds to the product of conductivity multiplied by the layer thickness.  Thus, thin conductive layers may respond as well as thicker, but less conductive, layers. In these cases, it may not be possible to accurately determine either the thickness or conductivity of the layer.

Metal fences and other above or below ground metal features may prohibit recording interpretable data.

Resistivity

Resistivity methods can be divided into three groups.  The first two are resistivity soundings and traverses.  The last group combines both resistivity sounding and traverse data to form a resistivity section.  Data for this latter group are recorded using relatively recently designed resistivity instruments. Resistivity soundings are useful if the depths and resistivities are needed, although resistivity traverses are now rarely used.  This is partially because the ground conductivity can be more easily measured using electromagnetic methods, where no ground contact is required. In addition, resistivity measuring instruments are now available that automatically record data from several different electrode spacings very efficiently, combining traverse and sounding data.

These newer instruments, called automated resistivity systems, use electrodes that are addressable by a central control unit.  This means that a large number of electrodes can be placed in the ground prior to starting the survey and connected to the central control unit.  The data recording parameters and the electrode array to use are input to the central control unit.  Once the electrodes are all connected, the measurements are automatically recorded by the central control unit. Because of the large amount of data obtained with these systems, a more detailed and reliable resistivity interpretation is obtained, making them the preferred instrument for lithology determination.

Resistivity is usually measured using one of the electrode arrays shown in figure 14.  Electrical current is put into the ground using two electrodes, and the resulting voltage is measured using two other electrodes.  Since the measured resistivity is a composite of the resistivities of several layers, the correct term is apparent resistivity.  In this web manual, the term "measured resistivity" is understood to mean apparent resistivity.

Figure 14. Electrode array used to measure resistivity.

These arrays are used for different types of resistivity surveys.  The Schlumberger array is often used for resistivity soundings, as is the Wenner array. The Pole-pole array provides the best signal, but is cumbersome because of the long wires required for the remote electrodes, and it is rarely used. The Dipole-dipole array was originally used mostly by the mining industry for induced polarization surveys.  Readings were taken using several different separations of the voltage and current dipoles providing measurements of the variation of resistivity with depth.  Long lines of data were recorded requiring many readings. This array has now become common for resistivity surveys using the automated resistivity systems. If more signal (voltage) is needed than can be provided with the Dipole-dipole array, the Pole-dipole array can be used.

Automated Resistivity Systems

Basic Concept:  As mentioned above, newer resistivity systems are now available that make taking resistivity measurements much more efficient.  Figure 14 shows one such system called the Sting/Swift automated resistivity measuring system.

Figure 15. SuperSting R1 IP resistivity meter. (Advanced Geosciences, Inc.)

Data Acquisition:  When using these automated resistivity systems, the electrodes for each spread are first placed in the ground and connected to the control unit.  This is then programmed with the electrode array desired and other data recording parameters and then instructed to record the data.

Data Processing:  Usually the only processing required is the removal of bad data points.

Data Interpretation:  Data from the automated resistivity systems are interpreted by inversion software, although much information can be gained visually.  An example of the results from such a survey is presented in figure 15, which shows two high-resistivity zones, each of which contains sand and gravel.  The data have been inverted showing the resistivity variation, both laterally and vertically, against depth. 

Figure 16 . Resistivity data showing stratigraphic changes.  (Advanced Geoscience, Inc.)

The low-resistivity areas (less than 20 ohm-m) correspond to clay layers and high-resistivity zones (greater than 100 ohm-m) correspond to sand and gravel lenses.  Inasmuch as the change from clay to sand and gravel is clearly observed, and this likely corresponds to a change in grain size, the method has provided some information about changes in lithology.

Advantages:  Automated resistivity systems obtain much more data than the simple resistivity systems and therefore are able to give a more detailed interpretation.

Limitations:  Since electrodes have to be inserted into 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. If the ground is dry, water may need to be poured onto the electrodes to improve the electrical contact between the metal electrode and the soil.

If a survey is conducted along a single line, resistivity variations normal to this line are not accounted for in the interpretation. Recording lines parallel to each other, of course, can rectify this problem.  In addition, three-dimensional resistivity surveys can now be recorded and interpreted.