Research | W. Steven Holbrook

Geophysics in the Critical Zone

Thu, 16 Jun 2011 20:26:28 -0600

The "Critical Zone" can be defined as the zone where life and rocks interact -- the layer of air, vegetation, soil, water, and rock that bounds the top of the canopy to the base of groundwater. What's fascinating about the CZ is the diversity of scientific disciplines that intersect there -- geology, biology, hydrology, chemistry, geomorphology, ecology, atmospheric science, and soil science.

The National Science Foundation is funding a number of Critical Zone Observatories (CZO's) that span a range of climatic, geological, and ecological variables. My group has been working with Dr. Cliff Riebe, a geomorphologist/geochemist in our department, at the Southern Sierra Nevada CZO.

A key question in surface processes is: what controls the thickness of the regolith (weathering zone) above bedrock? This question sounds deceptively simple, but, perhaps surprisingly, there is no unifying theory that encapsulates the geological, geomorphological, biological and chemical processes that control regolith development and thickness. Part of the difficulty is a paucity of data about weathering zone thickness -- since often the regolith is too thick to trench.

That's where geophysics comes in. Using near-surface geophysical methods like seismic refraction, GPR, and resistivity, we can discern the physical properties of the subsurface. We've been doing this in the SSCZO, and the results show a deeper weathering zone (20-30 m) than previously thought.

In the photo above, we're conducting a "barenaked granite" refraction survey, with geophones plastered onto a granite outcrop.

Geophysics in the CZO's is likely to become an increasing effort in our group. Right now Jorden Hayes is working on this topic as part of her PhD dissertation.

Cascadia Subduction Margin

Wed, 26 Jan 2011 21:58:11 -0700

Together with co-PI's Graham Kent, Katie Keranen, Anne Trehu, and Paul Johnson, I am leading an effort to acquire seismic data (both 2D and 3D) on the Cascadia subduction margin.

You can read about both of those projects by clicking the links below. The 2D survey is scheduled for July 2012 and will be open-access and open-participation. This means that (1) we will release both raw and processed seismic data shortly after the cruise, so that anyone can use the data (or write proposals to work with the data), and (2) we will have about 12 extra berths available for students and early-career scientists.

The 3D survey will be proposed (again) in February 2013, and won't happen before summer 2014.

Deep Structure of Magmatic Arcs

Mon, 02 Jun 2008 14:47:37 -0600

An abiding interest of mine is the deep structure of magmatic arcs -- especially island arcs.

Methane Hydrates

Tue, 27 May 2008 23:37:38 -0600

I've been working on methane hydrates since "before it was cool" -- my first research cruise on hydrates took place in 1992. Our current efforts focus on the relationships between submarine landslides and the methane hydrate reservoir.

Seismic Oceanography

Tue, 27 May 2008 23:37:38 -0600

In 2003, our group made an exciting discovery: seismic reflection profiling, a technique that has been used for decades to image the solid earth beneath the ocean, can also provide detailed images of thermohaline structure within the ocean itself. In a paper published in Science, we presented images of ocean structure across the front between the North Atlantic Current (a branch of the north-flowing Gulf Stream, which is relatively warm and salty) and the Labrador Current (south-flowing, relatively cold and fresh water). We think this opens up a whole new way of looking at ocean structure and dynamics: our data show, at unprecedented lateral detail, structures associated with eddies, double-diffusion, thermohaline intrusions, and internal waves. The pictures below showed the ocean in a way it had never been seen before.

Since 2003, we've been working steadily on exploring some of the many new applications of seismic oceanography (some of which are listed below). In addition, we are delighted to see that a number of other research groups have picked up on this topic, including groups in the US, Japan, Canada, the UK, Germany, France, Spain, Portugal, and Ireland. For a more recent "news" update on the status of seismic oceanography, which highlights the work of the European GO consortium, see this feature published in April 2008 in Science.

You can see a complete list of the media news coverage of our work (including stories in Scientific American, Science, Earth magazine, and AAPG Explorer) on this page.

For research summaries of our work (past and present) in seismic oceanography, click on the links below: