Peering into the earth: Geophysical techniques can find rock for forest roads

Fresh off the stump and replete with water, fresh-cut timber is heavy stuff. A fully loaded logging truck can tip the scales at 68,000 pounds, which is the weight equivalent of approximately 17 average-size cars.

To support this kind of weight, forest roads must be built strong with good, hard rock. How hard? Generally, the best rock has few fractures and requires blasting for extraction.

DNR obtains the rock it needs from existing rock pits located across state trust lands, but new sources often are needed. The farther rock must be hauled from the pit, the more it costs to build the road. Haul rock far enough, and a promising timber sale becomes infeasible.

Finding good rock can be a challenge, especially in steep, heavily forested terrain. Current methods include field reconnaissance and use of geolog­ic maps to locate deposits and predict which direction they run beneath the surface. Promising areas may be explored with test pits and drilling.

Unfortunately, rock deposits are seldom uniform, thick in some places and thin in others, soft and fractured here but harder there. It can be difficult to know exactly where to dig or what type of equipment to use. All of this begs the question: surely, there must be a better way?

As it turns out, there is.

Looking for Answers

In early 2013, managers in DNR’s South Puget Sound and Pacific Cascade region offices approached DNR’s Geology and Earth Resources Division for a new way to approach this problem. The challenge intrigued Recep (Ray) Cakir, a hazards geophysicist in the division’s Geo­logic Hazards group. Ray organized a team of specialists to answer the following question: was there a cost-effective, practical geophysical technique that could be used to locate rock for forest roads, character­ize rock quality, determine the thickness of the overbur­den soils, and identify any concerns with groundwater, all without breaking the ground surface? To answer this question, the team tested a combination of geologic re­connaissance and the following geophysical techniques: active and passive seismic, electromagnetic induction, electrical resistivity, and ground penetrating radar (GPR). These techniques will be explained in this article.

Test Sites and Equipment

The testing was conducted at three sites on state trust lands: the BB Pit timber sale on Tiger Mountain, Perry Creek rock pit in Capitol State Forest, and the Eastern Panhandle timber sale in the Elochoman block near Cathlamet, Washington. The Perry Creek rock pit was the prima­ry test site at which all techniques were tested; selected techniques were tested at the other sites. Since DNR had only seismic equipment in-house, some equipment was rented and some was provided by the geotechnical sup­pliers on the team (see Sites and Methods Tested sidebar).

At each of these sites, the primary rock types were basalt, basalt breccia, and andesite. Basalt is a dark, fine-grained rock that forms when molten rock cools on the ground. Andesite is similar to basalt, but has less iron and magnesium and more silica. Basalt breccia forms when the outer layer of a mass of molten rock cracks as it cools because the inner layer is still flowing.

Geophysical Techniques Explained

Active and Passive Seismic Techniques. When something shakes the earth, such as an earthquake, an explosion, or a heavy weight falling to the ground, waves of seismic energy travel out from the point of disturbance. By measuring the velocity (speed) of these waves, inferences can be made about the composition of the ground. Generally speaking, the higher the velocity, the harder the rock. The types of waves discussed in this section are shown in Figure 1.

Figure 1: Types of seismic waves

Seismic techniques can be active or passive. The active technique involves hitting the ground with a hammer, dropping a heavy weight, or setting off explosives to generate seismic waves. By contrast, the passive technique involves measuring the small seismic waves or micro at-surface tremors caused by day-to-day life: waves on the beach, water running in a stream, wind blowing across a field, traffic rolling past. With both techniques, one or more ground sensors (“geophones”) are placed directly on the ground. The geophone(s) feed data through a cable to a seismograph that records the arrival of the seismic waves, and the data collected are analyzed using computers.

The team tested two types of active seismic techniques: P wave refraction and multichannel analysis of surface waves (MASW). P wave refraction examines P waves, or primary waves. P waves are the fastest of the seismic waves (Figure 1) and are sometimes called compression waves because they push and pull the earth as they pass. When a P wave hits an area of density contrast, for example the boundary between a layer of rock and a layer of soil, some of its energy travels a distance along that boundary and then bends or refracts back to the surface, where its arrival is sensed by the geophones (Figure 2). The remainder of that energy continues to travel down until it hits another area of density contrast and refracts upward. The velocity of each layer is calculated using the time it takes the wave to arrive and the distance the wave traveled to reach the geophone. An example of a cross-section built with data from this technique is shown in Figure 3.

Figure 2: P wave refraction technique

Fig3: P wave refraction technique cross-section
Fig3: P wave refraction technique cross-section

 

Once collected, P wave values must be translated into information a manager can use to make decisions. For example, if the P wave velocity is 2.4 meters per second, how hard is the rock? Does it require blasting? To answer these questions, the team created a classification system that correlates P wave velocity values to hardness characteristics of western Washington rock.

The starting point for this work was the basalt portion of the D8R/D8T Caterpillar Chart (Figure 4). Developed in a study sponsored by Caterpillar, Inc., the Caterpillar Charts define a range of P wave velocity values for rock that is rippable (can be excavated manually, usually with a ripping head mounted on a tractor, dozer, or other heavy equipment [Figure 5]), non-rippable (requires blasting), and marginal (hard, but not hard enough to require blasting) using different types and sizes of equipment. The “D8R/D8T” chart (Figure 4) is geared toward a D8R/D8T Caterpillar dozer, which is one of the larger Caterpillar dozers. Using this type of dozer, for example, basalt with P wave velocities between 0.8 and approximately 2.4 is rippable; basalt with P wave velocities above 2.4 is not.

Fig4: Excerpt from D8R/D8T Caterpillar Chart & Team's Classification systemThe team’s classification system (Figure 4) assigns ranges of P wave values from the D8R/D8T Caterpillar Chart to one of three categories: hard, which is the same as “non-rippable” on the Caterpillar chart; “intermediate,” which combines the Caterpillar chart’s “rippable” and “marginal” categories; and “soft,” for rock that is soft enough to dig with a shovel and therefore useless for road construction. Each category includes a description of the physical characteristics of the rock.

Fig5This classification system likely is the first to correlate P wave velocity values from a Caterpillar Chart to local rock hardness characteristics. It may be expanded in future studies, as will be explained at the end of this article.

The MASW technique (Figure 6) involves examining Rayleigh waves, which are a type of seismic surface wave. Rayleigh waves cause the ground to ripple and heave like the ocean, and this motion causes particles at the surface to move in counter-clockwise circles as the wave moves past (Figure 1).

Fig6: Multichannel analysis of surface waters (MASW) technique.Because Rayleigh waves are influenced by the shear strength or stiffness of the rock, their velocities can be used to derive S wave (shear wave) velocities. Those derived S wave velocities are then used to create a cross section of the subsurface. S waves are a type of body wave that moves through the ground like a waving flag (Figure 1) and are also called secondary waves, because they are the second waves to arrive at a seismograph or geophone (P waves arrive first).

The team also tested two passive seismic techniques, one performed with an array of geophones and another performed with a single, three-component seismograph (three sensors built in one) called a Tromino seismograph. Passive techniques usually measure surface waves.

Electromagnetic Induction and Electrical Resistivity Techniques. Both of these techniques use the electrical properties of the ground to make inferences about its composition. One technique measures conductivity and the other measures resistivity.

The electromagnetic induction technique measures the ability of the ground to conduct electricity. In this technique, a transmitter coil sends an alternating current (AC) into the ground (Figure 7). That current generates an electromagnetic field (electromagnetic fields are generated when electricity moves from one place to another, for example along the cord of a lamp when the lamp is turned on). This field, called the primary electromagnetic field, causes the receiver coil to react by generating a secondary electromagnetic field. By measuring the size of the secondary field—in other words, determining how much the receiver coil responds to the primary field—it is possible to infer the conductivity of the ground.

The transmitter and receiver coils often are mounted at opposite ends of a long pole, which is carried across the ground. The drawback to this technique is depth: the shorter the pole, the shallower the depth. For example, if the distance between the centers of the coils is three meters, then the depth of the survey is approximately 3 meters.

Fig7" Electromagnetic induction technique; Fig8: Electric resistivity technique

The electrical resistivity technique measures the ground’s resistance to transmitting an electrical current. In this technique, transmitter dipoles (a pair of equally and oppositely charged poles) induce an electrical current directly into the ground, and receiver dipoles are placed nearby. The difference in electrical potential between the transmitter and receiver dipoles is then measured and used to calculate resistivity.

At the Perry Creek site, the team used an instrument called an OHM Mapper, which consists of transmitter and receiver dipoles mounted on a cable or rope that is dragged along the ground (Figure 8). Similar to electromagnetic induction, the depth of penetration is determined by how far apart the transmitter and receiver diodes are placed.

GPR Technique. The GPR technique makes use of radio waves, which are a type of electromagnetic wave. Electromagnetic waves can be arranged in a spectrum depending on their frequency (Figure 9). Frequency is the number of waves that pass a fixed point in a given amount of time, and is often measured in hertz, kilo­hertz, megahertz (MHz), or gigahertz.

Fig9: Electromagnetic spectrum & Fig10: GPR techniqueWith the GPR technique, high-frequency (for example, 12.4 to 1,500 MHz) radio waves are pulsed into the ground by a radar antenna on the GPR unit. When radio waves encounter an interface between areas with different dielectric (insulating) properties, such as soil horizons (layers of soil that differ from layers above and below), soil/rock interfaces, or man-made objects, a portion of the wave is reflected up and detected by the GPR unit (Figure 10). From hun­dreds of these measurements it is possible to map the properties of the subsurface.

The depth of the survey is affected by wave frequen­cy. Pulses of lower-frequency radio waves penetrate more deeply but provide less detail than pulses of higher-frequency waves.

Commonly, the GPR unit is pushed or pulled along the ground. Distance is measured by a wheel on the back of the unit, or by an additional GPS unit with a built-in antenna for positioning. Other types of GPR units can be mounted on a car or even a helicopter for aerial surveys. The team used a ground-based unit for this study.

Results

Of all the techniques tested, GPR provided the most detailed information for the depths at which most rock would be extracted, approximately the first 10 meters beneath the surface. In addition, GPR was the easiest technique to perform, although the instrument requires contact with the ground and may be difficult to deploy in some forested areas. The GPR technique had the added benefit of being fully described in the American Society of Testing Materials (ASTM) standards, which makes its results more legally defensible.

Active seismic techniques (P wave refraction and MASW) provide good detail at the right depths; how­ever, results were less detailed than GPR (Figure 10). Of the two active seismic techniques, P wave refraction was better, because P wave velocities can easily be tied to rippability of various rock types and overburden material such as topsoil. The P wave refraction technique also is fully described in the ASTM standards.

Passive seismic techniques provided information for depths of 100-200 meters, but did not provide enough detail for the first 10 meters.

Electromagnetic induction and electrical resistivity provided detailed conductivity and resistivity informa­tion (respectively) for the shallow soil layers, but did not penetrate deeply enough to provide adequate informa­tion for locating rock sources. However, both of these techniques provided more detailed groundwater infor­mation than either GPR or P wave refraction.

For the most complete and accurate information, it is best to combine techniques. The team recommended that DNR use both GPR and P wave refraction tech­niques to image the subsurface, determine the thickness of overburden soil, and locate good rock. At sites where groundwater conditions are a concern, these methods could be combined with electromagnetic induction or electrical resistivity techniques.

Validation

Up to this point, results were promising but theoretical. To validate the results, the team compared the GPR and active seismic data with results obtained through drilling at the Perry Creek site. The drilling results demonstrated the accuracy of these techniques.

Fig10

 

For further validation, the team will conduct drilling at the other two sites (BB Pit timber sale on Tiger Moun­tain and Eastern Panhandle timber sale in the Elocho­man block). The team also will measure the groundwater depth at Perry Creek.

Next Steps

The next step is to begin using these techniques (P wave refraction and GPR) in the field to locate rock for for­est roads. DNR recently purchased a GPR unit, which will be useful not only for rock source surveys, but for other applications such as locating underground pipes or culverts, surveying for cultural resources, or develop­ing more detailed geologic maps.

As these techniques are implemented, the team will continue to refine its classification system to make it more specific to western Washington geology. One goal is to expand the classification system to include more geophysical parameters, including S wave velocities and electric properties. This information will be highly useful in defining boundaries between rocks of different quali­ties.

Over time, geophysical surveys and the new classifica­tion system should not only provide greater certainty for developing new rock sources, but significantly reduce the amount of drilling needed. Geophysical surveys are far less expensive than drilling, which could increase revenue to our trust beneficiaries.

This effort also demonstrated that working across divi­sion lines can be an effective way to solve one of DNR’s daily challenges. And that, indeed, is a better way to find rock for forest roads.