Detecting Blind Ground Fissures With Seismic Surface Waves Using the Square Array Void Mapping SAVM Technique
Authors: Fliedner Moritz , Butler Heather , Pfeiffer James , Hild James , Sowers Janet , de Bock Stephan 
Published: November 2020

Abstract
The Square Array Void Mapping (or SAVM) seismic method is used to identify ground fissures and open voids that may affect structures built over, or adjacent to such features. It is relatively simple to deploy and can be applied to investigation beneath existing structures. The method is designed to identify and map discontinuities with greater sensitivity to these features than linear surface wave methods designed to derive the shear wave velocity structure for geotechnical design. The SAVM method uses a square array of standard single-component geophones and engineering-scale impact or vibratory sources to map variations in seismic surface wave parameters underlying the square in a frequency band corresponding to the depth range of interest.
SAVM data were collected over a series of adjacent squares along the edge of the footprint of a planned greenhouse expansion in Guanajuato, Mexico. Extensive linear ground cracks had formed beneath the existing greenhouse as the result of subsurface erosion of soil beneath a discontinuous carbonate soil layer that created voids, which subsequently collapsed. The dimensions of the greenhouse expansion required 14 square arrays of 48-meters by 48-meters to cover the perimeter of the footprint. In addition, SAVM data were collected outside the footprint in a 36-meter by 36-meter square array as a test over the projection of a known ground fissure that developed beneath the existing greenhouse. Spacing between geophones was 3 meters and a cross of geophones was located within the center of the square. A slide hammer source was used at offset 2 meters outside the nearest geophone around the edge of the square. The data were analyzed for surface wave attributes and seismic refraction velocity variations that are indicative of disruptions in the subsurface caused by ground fissures. These anomalies include a decrease in seismic wave velocities, amplitude changes or scattering. Since the fissures are linear and have a significant extent, the absence of identified fissures within the perimeter squares would indicate the absence of fissures within the full footprint of the new greenhouse.
The SAVM results identified surface wave anomalies that may represent blind ground fissures. The SAVM dataset was also processed for 3-D P-wave refraction tomography. Compressional wave velocity anomalies that coincide with surface wave anomalies increase the confidence in the interpretation because they measure different wave trains sensitive to different soil moduli. In the test square the SAVM technique showed a strong lineation of anomalous Rayleigh wave amplitude and velocity coincident with the projected trace of the know ground fissure. In one of the SAVM squares covering the proposed building footprint, we mapped linear SAVM anomalies coinciding with low P-wave refraction velocities and trending in line with the test square anomaly. Based on these results, the client was advised that subsurface exploration and possible remediation of this linear anomaly should be conducted.