Spectrum geophysics

Services: Fault Location

Several methods can be used to delineate geologic faults. The method chosen depends on a number of factors such as dimensions of the area of investigation, depth of investigation, formation lithology either side of the fault, degree of induration of layers in the formation, depth to groundwater, cultural features and sources of noise in the area of investigation. Spectrum commonly uses the following methods to delineate faults:



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  Seismic Refraction
  

  
  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. As there is often a measureable contrast in velocity across a fault, seismic refraction can be useful for the delineation of fault boundaries. 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, with proper geophone spacing and shot distribution, can be quite accurate in the measurement of the lateral location of faults. The seismic refraction method is ideal for fault delineation in areas where the fault is shallow, where there is a measurable contrast in velocity across the fault and in areas where subsurface structure is relatively simple. Spectrum utilizes the Seistronix RAS-24 twenty-four channel signal enhancement seismograph along with geophones and associated cabling to collect seismic refraction data.
  

  
  

  P-Wave:

  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 layer. During a typical seismic refraction survey to delineate a geologic fault, Spectrum utilizes numerous shot locations on the spread (in line with GRM methodologies) in order to obtain a detailed 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 and velocity of the seismic layers that can be used to delineate faults.
  
  S-Wave:

  Spectrum uses p-wave refraction for most fault location 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. During an s-wave seismic refraction survey, a linear array (or spread) of horizontal geophones is established along the ground surface and connected to a multi-channel seismograph. A seismic source (a shear wave plank or steel brick) is then established at a certain location along the line, and a seismic “shot” is made by making horizontal blows with a sledgehammer to one side of the plank or brick, followed by horizontal blows to the other side of the plank or brick. 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. depth profile which can be interpreted for lateral and/or vertical changes in velocity that may be associated with a fault.
  

  
  

• Electrical Resistivity
  
  
  The electrical resistivity method measures both the lateral and vertical variation in electrical resistivity of subsurface materials, where the electrical resistivity of a material is a measure of the ease with which an electrical current can flow through that material. 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 identifying both the lateral and vertical boundaries of geologic faults where there is a measureable electrical resistivity contrast across the fault. In Spectrum’s experience most faults do exhibit a characteristic resistivity that is measurably different from background (although the actual values vary from site to site), and the electrical resistivity method is extremely useful for imaging both the depth and the lateral location of faults. 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 areas of anomalous resistivity that may be associated with faults.
  
  
• Seismic Reflection
  
  Seismic reflection is a surface method that uses the properties of acoustic waves to obtain highly detailed images of subsurface layers and bedding planes to significant depths. The seismic reflection method has a long history with the oil industry where seismic sections are interpreted for stratigraphic features such as changes in lithology or thickness of layers, and subsurface structural features such as anticlines, faults, unconformities and other sources of oil traps. More recently, seismic reflection methods have been used for environmental and engineering investigations to delineate faults, aquifers, aquitards or other subsurface features of interest. Seismic reflection is an ideal method for the delineation of faults in areas where vertical offset is expected across the fault, where strong reflectors are likely to be present, where the angle of the fault needs to be measured, and where a high level of detail in the data is desired. Spectrum uses a 48-channel Geometrics StrataVisor NZII signal enhancement seismograph (or equivalent) along with high frequency geophones and associated cables to obtain seismic reflection data.
  
  

  P-Wave:

  During a typical p-wave seismic reflection survey, a linear array (or spread) of equally spaced vertical 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 the sledgehammer or weight drop. When a wave from the seismic shot reaches a boundary between two subsurface materials having different acoustic impedances (product of velocity and density) a portion of that seismic wave will be reflected back to the surface and received at the geophones established in the array. Reflections from subsurface boundaries occur on the seismic record at a time that depends on the shot-to-receiver distance, the average velocity of the materials above the reflected layer and the depth of the reflected layer. Because the seismic reflection method is based on arrivals in time and the geometry of the reflected raypath the resultant sections are highly detailed and often highly accurate images of subsurface geologic layering and structural features. Spectrum uses the common midpoint (CMP) configuration of shooting where seismic shots are made between each and every pair of geophones (typically at 10-foot to 20-foot intervals for p-wave reflection-depending on objectives) along the line, so that the same point in the subsurface is sampled multiple times to cancel noise and enhance reflections. After reflection data are collected they are carefully processed and the final product is a 2D seismic section in time or depth that represents subsurface layers of interest. These sections are then interpreted for anomalous features that may be associated with faults.

  
  

  S-wave

  Spectrum uses shear wave reflection for delineation of faults in areas where the water table is shallow or where sediments are unconsolidated and not expected to give rise to strong p-wave reflections. The shear wave reflection methodology works similar to p-wave reflection; except that shear waves are generated - not p-waves, and the geophones used are sensitive to horizontal motion – not vertical. The seismic shear wave source is generally a shear wave plank or a steel brick, and a seismic “shot” is made by making horizontal blows with a sledgehammer to one side of the plank or brick, followed by horizontal blows to the other side of the plank or brick (this procedure is necessary both because shear waves are polarized and to ensure cancellation of p-wave energy). Shear wave reflections from subsurface boundaries occur on the seismic record at a time that depends on the shot-to-receiver distance, the average shear wave velocity of the materials above the reflected layer and the depth of the reflected layer.

  Shear wave reflection is preferred over p-wave reflection in the presence of a shallow water table because the water table gives rise to very strong primary reflections and multiples in p-wave data (which can dominate the section), whereas shear waves don’t propagate in water and therefore respond to the 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 much higher resolution in the data, and thereby can detect smaller changes in velocity and density along a line than p-waves. As a result, shear wave reflection sections are often far superior to p-wave reflection sections in the presence of a shallow water table or unconsolidated sediments. Spectrum uses the common midpoint (CMP) configuration of shooting for shear wave reflection where seismic shots are made between each and every pair of geophones (typically at 2.5-foot to 5-foot intervals, depending on objectives) along the line, so that the same point in the subsurface is sampled multiple times to cancel noise and enhance reflections. After shear wave reflection data are collected they are carefully processed and the final product is a 2D seismic section in time or depth that represents subsurface layers of interest. These sections are then interpreted for faults and other features of interest.
  

METHODS IN ACTION


Fault Location using Electrical Resistivity – LONG BEACH, CALIFORNIA


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  An electrical resistivity investigation was conducted for proposed development at a site in Long Beach, California to delineate the surface trace of the Newport-Inglewood Fault and possible splays associated with it (collectively referred to as the NIFZ). The site was adjacent to a marine channel and consisted of marine sediments that were saturated 6 feet below ground surface. A concern for this project was the presence of several active oil wells and a few shallow utilities in the area of investigation, as these could cause interference in the electrical resistivity data. Because of this concern, Spectrum used utility locating methods to locate and mark shallow utilities at the Site and generated a detailed site map of cultural features at the Site.
• Spectrum collected Schlumberger DC electrical resistivity data along two parallel 220-meter (722-foot) transects using AGI’s Sting/Swift resistivity system, four-meter cable and an array of 56 electrodes. As anticipated, the presence of the oil wells and utilities did not preclude the use of the electrical resistivity method. The electrical resistivity data along Line 1 revealed three anomalies extending from depth to the near surface, which were interpreted as the Main Fault (Station 92) and two splays of the fault (Station 64 and Station 168). Using the interpreted resistivity sections, an interpretation map was generated that identified the trace of the Main Fault and fault splays in the NIFZ, in accordance with the Alquist-Priolo Act. A subsequent marine seismic investigation in the adjacent channel conducted by Legg Geophysical confirmed the presence and location of the identified faults.