Spectrum geophysics

Services: Depth to Bedrock/Rippability

Several methods can be used to determine depth to bedrock at a site. The method chosen depends on a number of factors including dimensions of the area of investigation, depth of investigation, desired resolution of overburden/bedrock interface, type of bedrock (i.e. sedimentary vs. igneous) and expected contrast with overburden, expected induration of bedrock, depth to groundwater, cultural features and sources of noise or interference in the area of investigation. Spectrum commonly utilizes the following methods to identify depth to bedrock:



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  Seismic Refraction
  

  P-Wave
  
  

  Seismic refraction is a surface method that uses the properties of acoustic waves to measure the velocity of the material through which the waves travel. The p-wave seismic refraction method is the most common method for identifying the depth to bedrock because there is generally a sharp contrast in seismic velocity at the overburden/bedrock interface. The seismic refraction method has a long history with the oil industry, and as such the equipment is easy to obtain and deploy. Because this method uses travel time to measure the seismic properties of materials it is highly precise in the measurement of seismic velocity and quite accurate in the measurement of depth to bedrock in many instances. Because seismic velocity is diagnostic for different types of material, and generally increases with degree of induration, the p-wave seismic refraction method not only can measure depth to bedrock but can be used to non-invasively classify the type of bedrock (e.g. soft sedimentary vs. igneous) and rippability of the bedrock encountered. In addition, seismic layers often correlate closely with geologic contacts.

  
  

  Spectrum utilizes the Seistronix RAS-24 twenty-four channel signal enhancement seismograph along with geophones and associated cabling to collect seismic refraction data.During a p-wave seismic refraction survey, a linear array (or spread) of geophones is established along the ground surface and connected to a multi-channel seismograph. A seismic source (often a sledgehammer or a weight drop) is then established at a certain location along the line, and a seismic “shot” is made by making vertical blows to a plate with a sledgehammer. The first arrivals at each geophone location on the seismic record from a given shot are plotted on a time vs. distance graph. The slope of the line segments created by the first arrivals is the inverse of the velocity of the material through which the waves have traveled, and the intercept time or crossover distance can be used to calculate the depth to the target bedrock layer. During a typical seismic refraction survey, shots at several different locations on the spread are made (in line with GRM methodologies) in order to obtain a measure of the two-dimensional variation of seismic velocity with depth along the line. Several different software programs are currently available to process seismic refraction data, and the end product is generally a two-dimensional velocity vs. depth profile providing a detailed image of lateral changes in topography along the bedrock surface.

  S-Wave

  
  Spectrum uses p-wave refraction for most depth to bedrock surveys; however, shear wave refraction can be useful in cases where the water table lies above the bedrock surface and the overburden materials are unconsolidated, or where a higher level of detail is required. Shear wave refraction is preferred over p-wave refraction in the presence of a shallow water table because, while p-waves react strongly to water and essentially sense a “layer” with a relatively high velocity, shear waves don’t propagate in water and therefore are capable of detecting changes in the velocity of sediments as if the water were not there at all. In addition, because the shear wave velocity of a material is always slower than the corresponding p-wave velocity of that material, shear waves are capable of obtaining higher resolution in the data, and thereby can detect smaller changes in velocity along a line than p-waves. Although shear wave velocities are lower than p-wave velocities, the seismic refraction interpretation methodology is the same as for p-waves, and end result of a shear wave refraction survey is a two-dimensional shear-wave velocity vs. dept

  
  
• ReMi Method

  
  The refraction microtremor, or ReMi, method is a seismic method that uses ambient noise and surface waves to generate a detailed image of vertical changes in the seismic shear wave velocity of subsurface materials to depths up to 100 meters. Because the shear wave is highly sensitive to small changes in induration and the degree of competence of materials, and because there is generally a sharp contrast in shear wave velocity at the overburden/bedrock interface the ReMi method is very useful for measuring depth to bedrock. The ReMi method is ideal in many urban environments where seismic refraction is precluded because of large amounts of ambient noise, and is also very useful in complex geologic environments such as a shallow water table over deep bedrock, many unconsolidated layers where velocity gradually increases to the bedrock layer, or where there is a velocity inversion in the layers overlying bedrock. This method can also be used to constrain seismic refraction interpretation in complex cases. Spectrum utilizes the Seistronix RAS-24 twenty-four channel signal enhancement seismograph along with geophones and associated cabling to collect ReMi data.
  
  During a ReMi survey, a linear array (or spread) of geophones is established along the ground surface and connected to a multi-channel seismograph. Once established, ambient and active-source noise records are recorded. In addition, Spectrum collects p-wave refraction data along the ReMi line to provide a constraint to the ReMi interpretation. Noise records are then processed using the SeisOpt® ReMi™ software. First, a wavefield transformation of the noise records identifies the fundamental mode dispersion curve of the Rayleigh wave. This dispersion curve is then picked and a one-dimensional shear wave velocity model is generated by the user that would give rise to the selected dispersion curve. This one-dimensional model is then interpreted for bedrock depth based on site geology and the p-wave refraction results.
  
  

• Electrical Resistivity
  
  
  The DC electrical resistivity method measures both the lateral and vertical variation in electrical resistivity of subsurface materials. In this method a DC circuit is established in the ground via cables and electrodes and the ground acts as the resistor to complete the circuit. The electrical resistivity method is useful for determining depth to bedrock at sites where there is a measureable electrical resistivity contrast between overlying soils and bedrock, such as weathered soils over granitic bedrock, or loose sands over shale bedrock. In addition, because this method is highly sensitive to the degree of saturation of resistive materials such as igneous rocks it can be used to identify the fractured rock/competent bedrock interface in cases where there is a shallow water table overlying competent bedrock. As the electrical resistivity method results in a two-dimensional profile of electrical resistivity vs. depth this method can provide a highly accurate image of the overlying soils/bedrock interface at sites where seismic refraction oversimplifies the geology. Spectrum uses the SuperSting R8/IP system (SuperSting) manufactured by Advanced Geosciences, Inc. to acquire electrical resistivity data.
  
  During an electrical resistivity survey, Spectrum establishes a linear array of equally-spaced electrodes (usually 56, but more can be supplied if necessary) in the ground. Once the electrodes are established, the circuit is connected and a known current is transmitted into the ground via a pair of electrodes. This current travels through the ground and the voltage between another pair of electrodes some distance away is measured. The electrical resistivity of the ground between the pairs of electrodes is then computed using a geometric factor and Ohm’s Law which states V=IR. The distance between pairs of current and potential electrodes determines the depth of investigation of a particular measurement. The SuperSting system steps through suites of readings all the way down the line such that a two-dimensional apparent resistivity vs. depth pseudosection is generated. The data are then processed to generate a two-dimensional color-contoured model resistivity section that represents the subsurface materials along the line. Once the model section has been generated it is used to identify the bedrock layer.
  
  
METHODS IN ACTION


Seismic Refraction – LAKE ELSINORE, CALIFORNIA


A seismic refraction investigation was conducted at a property in Lake Elsinore, California to provide an image of the alluvium/bedrock interface to depths ranging from 60 to 130 feet below ground surface at the Property. Lithology at the Property consisted of discontinuous alluvium along with sedimentary and intrusive igneous bedrock. Spectrum was hired to obtain seismic rippability information where several cuts were planned for a proposed development. A challenge for this project was topographic relief of 200 feet or more and no vehicle access along several transects



Spectrum collected 7 transects of P-wave data, for a total of 6800 line feet, using a Seistronix RAS-24 signal enhancement seismograph, 8-Hz vertical geophones, and a 20-lb. sledgehammer. This seismic refraction profile shown below was approximately 1850 feet long and consisted of 5 overlapping spreads, each consisting of 24 geophones at 20-foot spacing. Seismic data were processed using the GRM method, which allows for an undulating bedrock surface and lateral changes in the velocity of the target refractor.

Bedrock velocities at the Property were found to range from 7,400 to 18,600 feet per second, and depths were found to range from 10 feet to 90 feet below ground surface.