SUBSURFACE GEOLOGY

SUBSURFACE GEOLOGY

Subsurface geology is the study of the physical properties and location of rock and soil found below the ground surface

THE IMPORTANCE OF SUBSURFACE GEOLOGY

If a proper subsurface geologic study was conducted in the year 1360, the Leaning Tower of Pisa would just be known as the Tower of Pisa today! These studies can also reveal hazards such as underground coal mine workings and unstable slopes. It is significantly cheaper to design for hazards than to repair the damage later. Thus, engineering companies often obtain geotechnical reports before starting any major project.

The construction industry relies heavily on mining sand and gravel resources. These resources provide the raw materials needed to produce concrete and asphalt for our roads, offices, and homes. Understanding the amount and quality of the sand and gravel in a resource deposit can influence many decisions in the mining process. Total reserve estimates, mine design, and mining method all depend on having quality subsurface data.

Subsurface studies are very important for understanding and protecting groundwater aquifers in order to maintain a steady supply of clean water. The U.S. Geological Survey and Washington State Department of Ecology have published many studies related to aquifers and their use. These studies help us understand how water flows through and is stored in aquifers as well as how contaminants may enter an aquifer. This allows people to plan future development so that aquifers remain viable for years to come.

Subsurface studies can also provide critical inputs for imaging magma chambers, geothermal exploration, locating active faults (paleoseismology), oil and gas exploration, carbon sequestration, and metal mining.

HOW IS SUBSURFACE INFORMATION COLLECTED ?

There are several methods available to collect subsurface information. There are depth and resolution limitations associated with each method. Thus, knowledge of the limitations of each method is critical in undertaking a subsurface survey.

Boreholes

Drilling boreholes is the primary way used to obtain subsurface information. Drilling brings soil and rock to the surface for direct study. These materials are described on site and may also be sent to a laboratory for additional study. Information obtained from drilling includes material properties, changing rock or soil type, fault locations, and depth to water or bedrock. Tests can also be performed within open boreholes, to better understand the materials in place.

Ground penetrating radar (GPR)

Ground penetrating radar (GPR) is a method of imaging the subsurface using nondestructive radar pulses. We commonly think of using radar to locating aircraft or track weather systems. At an airport, for instance, a radio tower will emit a radio wave that bounces off an aircraft and returns to the detector, informing an air-traffic controller of the aircraft’s location. The same concept applies if the radar wave is pointed down toward the earth. The radio wave travels through the subsurface and bounces off of soil and rock layers. Faults, contacts between different rock types, and discrete objects like boulders or voids are then visible. GPR has a variety of effective depths (generally <100 feet) and resolutions, depending on the type of equipment used and soil conditions at each site.

Active seismic surveys

Seismic surveys are similar to GPR in that it relies on analysis of waves bouncing off subsurface layers. Unlike GPR, which relies on radar waves, seismic surveys rely on sound waves. Seismic surveys are commonly categorized as either active or passive. In active seismic surveys, the geophysicist creates a seismic wave by hitting the ground with a hammer or using an air-gun if in water. In rare cases, small explosive detonations may be used to create a seismic signal. In passive seismic surveys, the geophysicist uses naturally occurring seismic waves or the “background noise” as the seismic source. Seismic surveys can be conducted on land or in the water—the effective depth of the survey may vary from tens of feet to several kilometers.

All active seismic surveys, regardless of the size of the study, involve sound waves generated from a sources that travel through the ground, are refracted, and are then received by detectors. The speed at which the waves travel tells us a lot about what layers are made of and how they are arranged.

Passive seismic surveys

Naturally occurring earthquakes in Washington State are located and monitored by the Pacific Northwest Seismic Network (PNSN). This network consists of over three hundred seismometers. Earthquakes are located using a complex method of triangulation using several seismometers and computer modeling. This can help researchers identify faults and other structures, and aids in understanding the seismic hazards of our state.

The locations of earthquake epicenters are determined by finding the intersection of measured distances from multiple seismometers detecting the earthquake.

Gravity and magnetics

Two additional types of surveys, gravity and magnetic, often work hand-in-hand to characterize the subsurface. Gravity surveys identify density differences in rock formations. This is useful for identifying the depth of bedrock under unconsolidated sediment. Gravity surveys can also locate contacts between two distinct densities of rock at depth. Magnetic surveys identify magnetic contrasts in rocks. This is useful for mapping faults, folds, and rocks with more magnetic minerals. Both surveys can be conducted on the ground for high-resolution data. They can also be run from an airplane for lower resolution and greater coverage. These surveys are generally faster and cheaper to acquire than other data types. However, interpreting the data requires more training and is seldom used independently.