Logic Geophysics now owns a “Summit X1” seismic system from DMT group in Germany. This lightweight, flexible, and robust system provides rapid data acquisition in all that Alaska has to offer! The system also offers enhanced safety during blasting operations (using dynamite as the source) via an improved triggering system. The flexible setup allows geophones at any spacing and distance, decreasing survey time and cost while improving data quality - a true “win-win”.

*Computer and seismograph set up for real-time data quality control in southeast Alaska on a steep seismic line*

Seismic Methods

Near-surface seismic surveys can estimate subsurface properties (for example, overburden stiffness and depth to bedrock) from propagating acoustic energy: reflected, refracted, and/or surface waves. Often we simply use a sledgehammer for the energy source, making acquisition in remote locations possible without the expense associated with explosives or heavy equipment.

Refraction: Seismic refraction uses the refraction of seismic waves on geology layers and rock/soil units to characterize subsurface geologic conditions. The method is governed by Snell’s Law, which describes the relationship between seismic wave angles of refraction when passing through a boundary between two different isotropic media (e.g. soil to bedrock). The method depends on the principal that seismic waves possess varying compressional- and shear-wave velocities within differing types of soil and rock material. Acoustic-energy waves are refracted when they cross over between soil and/or rocks of differing and increasing seismic velocities. Analysis of the refracted wavelet arrival times in conjunction with the geophone and source geometries (usually a straight line) estimates general soil types and approximate depths to strata boundaries. The SR method is also used to estimate rock rippability characteristics for pre-construction planning.

*The Summit X1 telemetry cable, lighter and faster than traditional seismic cables!*

Seismic Tools

Logic Geophysics now uses cutting-edge technology called the X1Summit siesmic system from DMT Group in Germany. This system eliminates the need for traditional seismic cables, greatly reducing weight and therefore cost for deployments to remote locations. The system also enables increased acquisition speed, and it’s easy to train new crews to use the hardware. The trigger is no longer requires a wire attached to the seismograph, which eliminates the need for an entire position/person on the seismic crew and greatly enhances the safety of explosives sources.

Combined with our extensive expertise in data processing and interpretation, these tools allow us to provide cost-effective surveys in minimal time to maximize your results!

Applications - Seismic refraction and shear-wave velocity profiles

These tools are useful for a variety of applications, including estimating depth to bedrock, locating faults, and estimating material properties such as rippability and stiffness. Multi-Spectral Analysis of Surface Waves (MASW) can provide subsurface shear-wave velocity profiles (and inferred corresponding soil properties) needed for earthquake and loading analysis. Since surface waves have much more energy than the body (refracted) wave, the MASW method is more tolerant of noisy sites such as along roads and near moving water. MASW can also image a velocity “inversion,” slow velocity underneath higher velocity, which refraction cannot. 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 its expected velocity contrast with overburden, depth to groundwater, cultural features, and sources of noise or interference in the area.

Downhole surveys

Shear-wave velocity profiles to 100 feet (30 meters) below surface can be provided using a cased, grouted borehole. The measuring device, or geophone, is typically 3-component and tensioned against the wall of the borehole using an actuator motor and metal strip. The source is usually a sledgehammer striking the end of a weighted plank or metal bar, as shown using the UTV in the picture here. The bar or plank has cleats or spikes on the bottom to couple it effectively with the ground at the source location.