An interview with Jerry Harris - Stanford University

INTERVIEW 

12 CSEG Recorder December, 2002 

‘TECHNOLOGY IS IMPORTANT……’ 

– An interview with Jerry Harris 

Jerry Harris is Professor and Head of the Department of 

Geophysics at Stanford University. While in Calgary recently, to 

deliver the SEG/AAPG 2002 Fall Distinguished Lecture on 

Crosswell Seismic Profiling: The Decade Ahead , Jerry was kind 

enough to spare some time for an interview for the RECORDER. His 

impressions and opinions on different aspects of his favourite topic 

are contained in the following excerpts from the interview. 

S: Jerry, tell us about your educational background and experience? 

JH: I did my undergraduate studies at University of Mississippi 

in the early 1970s, and then went to the California Institute of 

Technology and majored in electrical sciences. After my master’s 

degree, I worked for 3 years in atmospheric geosciences, looking at 

microwave attenuation due to rain. You may say I am an electrical 

engineer at heart but I have done wave propagation all my professional 

career, first electromagnetics and now seismic. 

S: Why did you decide to go in for a teaching career? What do like 

best about this profession and what is the most difficult thing about being 

a Professor? 

JH: In a research university like Stanford, we teach in a many 

different ways. Most of us like the variety. It is not just in a classroom 

but all the interaction we have with students. For example, in a Ph.D. 

program we are really involved in research. And through our 

research we are educating and teaching our students. What I really 

like about it is there is always an opportunity to learn yourself and 

an opportunity to work with smart students. So, it is an environment 

that you can never really outgrow because you are always renewing 

continually. I really enjoy the teaching career in a research institution. 

There are no boundaries as to what you can work on; in my case 

electromagnetics, seismic imaging, laboratory, field, etc. So it is 

something always challenging and interesting. The biggest challenge 

is balancing all the demands for your time, such as the 

teaching, your own research, proposal writing, and of course 

project administration. 

Satinder Chopra in conversation with Jerry Harris (Photos courtesy: Al Bradshaw) 

S: At times I hear people say ‘teaching is a thankless profession, 

because teachers do a lot for the students and when the students do well, 

they get the credit’. What do you have to say about it? 

JH: In some ways it is like working with your own children, your 

own family. You teach them and you feel happy when they grow up 

and do good things. You always know that you were a part of the 

beginning, for example when you taught them how to solve a 

problem or code a solution or whatever. In our case, we teach them 

to ask questions and to become critical thinkers. So when our 

students graduate and move on and succeed in their careers in 

industry or academia, we get some personal satisfaction for being a 

part of there training. 

S: How did you get started with crosswell imaging? Who is credited 

with this idea? Was it yours? 

JH: The idea of crosswell imaging is certainly not mine. Crosswell 

seismic imaging dates back to 1940s and 50s. In the early days, small 

explosions were used to locate the boreholes themselves and there 

was always lots of talk about imaging. But there were limitations in 

that there were no practical downhole sources and data acquisition 

was too slow to make it a practical imaging technology. So my 

contribution was really to introduce a modern downhole source, 

rapid data acquisition and modern processing methods. 

S: I think what I am going to ask you now you may have already 

touched on in your talk, but it is just to place on record for the benefit of 

the members not able to attend your talk. What is the power of crosswell 

imaging? 

JH: I think its primary utility is to produce true high-resolution 

images at a scale not available to other methods. By high resolution 

I mean an order of magnitude higher than what is available from 

conventional surface seismic surveys. As I pointed out in my talk, 

resolution is an issue when it comes to reservoir management, 

particularly when you want to understand the detailed stratigraphy, 

Continued on Page 14

INTERVIEW Cont’d 

TECHNOLOGY IS IMPORTANT 

Continued from Page 12 

to develop reservoir models for flow simulation and of course monitoring. 

When you need to estimate small-scale features, crosswell 

may be the only technology capable of producing the kind of reservoir 

models that are needed for flow simulation. Crosswell images 

are actually pushing the limits in terms of resolution of grid blocks 

in the flow simulator itself. Where else do you get this small-scale 

information between wells? So crosswell profiling is the only direct 

measurement technology available today for this problem. 

S: What type of sources are used in crosswell (imaging) data 

acquisition? 

JH: I have used a number of different sources: airguns were probably 

the first modern sources to be used, but piezoelectric sources 

are probably the best and easiest to use and also are relatively 

powerful and easy to operate. The piezoelectric sources run sweep 

waveforms or pulse codes that are easy signals to distinguish from 

background noise so high peak pressures are not required. 

There are other sources of course. One approach was to take low 

frequency surface seismic sources like hydraulic or pneumatic vibrators 

and repackage them for downhole use. Others have tried this 

with limited success. Instead, I took the approach of repackaging 

high frequency logging technology, making it more powerful and 

lower frequency. With logging-based technologies, I could build on 

all the expertise and experience of logging operations, that is high 

temperature, high pressure, and wireline operation. Of course the 

traditional sonic logging tool is not capable of transmitting the long 

distances required in crosswell, say up to a 1000m. So, we had to reengineer 

the piezoelectric devices to produce the right frequencies, 

100 - 2000 Hz, a feature that repackaged surface seismic technology 

was never able to accomplish. 

BM: You spent a lot of time as you were developing this technology. 

You would have many stories to tell. 

JH: Not only about sources, but downhole detectors and operations 

were issues as well. Some of the first downhole receivers we 

used were actually Teledyne marine streamers and hydrophones. 


There were some good stories and bad stories about them. They 

worked very well as detectors but sometimes we would get things 

stuck in the borehole and some of the stuff we put in first actually 

came out first. 

Apart from the bad stories, we had some good times as well, 

for example the first time we detected a 1000 Hz signal over 500 m 

or so. Others could not believe that we could see those high 

frequencies over those distances. It was pleasantly surprising for 

all of us. In fact the piezoelectric source that we use now was 

primarily built as a reference source in many of those early tests. 

We were comparing a number of different sources and needed one 

as a reference. I built the piezoelectric because it was very repeatable 

and reliable. And it turned out to be the best of all the sources. 

So, what started off as a reference tool ended up as a tool of choice. 

And we compared that source to hydraulic vibrators, airguns, 

explosives, all kinds of fancy exotic sources literally from around 

the world. From Norway to Japan and other places, the piezoelectric 

always emerged as the best source. 

S: What type of source spacing are we talking about? 

JH: Typically, 1m; it depends on the imaging objective, that is 

whether only tomography or reflection processing is needed. The 

sampling interval is dictated by the frequency content and the desire 

to avoid spatial aliasing of the low velocity events. Another problem 

is the depth control needed for for small source intervals. Imagine 

now that you are trying to position the source at 1 m intervals at 

5000m depth. How do you do it? If you just start at 5000m and call 

for the source to move 1m at these depths, sometimes it moves, 

sometimes it sticks and doesn’t move the entire 1m. So having a 

source that operates like a logging tool, keeping it moving continuously 

in the borehole, takes care of the problem and keeps the source 

on depth with repeatability of a few inches. As the source moves, the 

computer is telling it to fire on depth. Again we borrowed that operation 

from well logging but we, at Stanford, were the first to use it 

with a modern crosswell source while others were operating by 

stopping, starting and sometimes clamping the source at depth. This 

shooting on the fly became very important to data quality because 

we have to go back and occupy those shot points several times. By 

keeping the downhole source continuously moving, we found we 

could repeat locations and the event moveouts in common-shotgathers 

became much smoother. We introduced that approach, 

shooting on the fly, at Stanford with the piezoelectric source. Now 

it’s used with other downhole sources, such as the airguns. 

S: Is there a limit to the distance between the wells to get good results? 

JH: Certainly there is a limit. But again, it depends on what you 

are trying to image and how you hope to accomplish that. 

Eventually high frequencies are going to be attenuated to a level 

below delectability. So, if you are willing to use the same frequency 

as surface seismic, say 100 hz, then you will be able to see the signal 

over distances comparable to the distances you see in surface 

seismic, say 1000s of meters. But in my opinion, the advantage of 

crosswell comes when you use the higher frequencies. The higher 

frequencies will still have good signal-to-noise ratio over the shorter 





raypaths that are set by well spacing, not the 

target depth. A rule of thumb is that you are 

going to see your signal up to about 200 wavelengths. 

So given the frequency and the velocity 

of sound, figure that you will see adequate 

signal over paths of about 200 wavelengths. If 

that’s 5000m at 100 Hz, it’s probably only 500m 

at 1000Hz. 

S: Since we are using high frequency sources 

and because we are recording close to the borehole 

in the zone of interest, there are smaller associated 

Fresnel zones. All these contribute to the high resolution 

images that we see in crosswell data. Is there 

anything more to that? For processing crosswell 

data, do you use VSP processing techniques or are 

there special techniques? 

JH: I have already said that the distances 

the waves have to propagate are shorter. So 

you can transmit and receive higher frequencies 

over the shorter distances. There isn’t nay 

magic here. The geometry of the survey is also 

different. You are propagating say parallel to 

bedding in most cases rather than perpendicular 

to bedding. We can borrow all the 

processing concepts from seismic and VSP 

technology except the details are different 

because of the frequency content and geometry. 

Conceptually one could process crosswell 

data like the offset VSP, except in practice you 

will have to process 300-500 offset VSPs for 

each pair of wells. Certainly, you cannot do it 

one offset at a time. So, the way you do it is to 

borrow the concepts and algorithms from VSP 

but organize data in a form suited to the crosswell 

geometry. 

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December, 2002 CSEG Recorder 15




The problem with migration of crosswell data is the limited aperture 

of the crosswell survey. Unlike medical imaging or surface 

seismic we are trying to image a complicated structure from data 

collected along two lines. We don’t have the vertical aperture you 

would ideally like to have for migration, so we you use is a limited 

aperture migration. Well, if I keep limiting the migration aperture 

more and more, the migration turns into a CDP reflection mapping 

process. So instead we tend to start with a CDP reflection mapping 

and then open the aperture until we start seeing unacceptable 

migration artifacts. One last comment is that we have incorporated 

Fresnel zones into the tomography algorithms, though not routinely, 

to capture finite bandwidth effects on the inversion. 

S: So it is a sort of trade off? 

JH: It is a trade off between wanting to collapse the Fresnel zone 

effects with a migration technique, but being forced to control the 

aperture to reduce the artifacts with a limited migration aperture. 

Again, the conceptual advantages and disadvantages of migration, 

say regarding Fresnel zone effects, are the same as for surface seismic 

but the practice is different. 

S: You referred to anisotropy determination in your talk. How much 

effort is being put into including anisotropy in processing crosswell data? 

JH: Not nearly enough in my opinion. We can extract 

anisotropy in the plane of the survey but not done routinely. 

Azimuthal anisotropy is a different beast. Tomoseis is recording 

crosswell using multiple wells, so they can in principle detect 

azimuthal anistropy. One challenge though is to separate 

anisotropy from heterogeneity. More sophisticated modeling is 

required. Nevertheless, this is one area where the detailed 

understanding that you get from the crosswell survey could be 

used to enhance the value of surface seismic by unraveling how 

scale affects anisotropy. As you know, heterogeneity below the 

scale of resolution, say due to aligned fractures, may appear as 

seismic anisotropy. In the simple cases of laminated shales that I 

have shown, high frequency crosswell data can resolve some 

scales of heterogeneity while other smaller scales still appear as 

anisotropy. Of course, the scales we resolve are an order of 

magnitude smaller than surface seismic so we may be able to 

resolve heterogeneity that appears as anisotropy in lower 

frequency seismic data. 

S: Are we in a position to do 3D imaging with crosswell technology 

and if yes, does that justify the cost that may be incurred? 

JH: We do have the technology to do 3D imaging, at least the 

velocity and attenuation tomograms in 3D. The issues with migration 

are more complicated. Now that we can survey several wells 

simultaneously, the aperture issues improve but sampling is still a 

problem. The basic technology is there but data are scarce and the 

devil is in the details. 

Now, I’ll move to the other part of your question. The cost is 

acceptable if the result you produce has value or answers the questions 

being asked. For example, someone might ask, “Do I have to 


shut the wells in to do this, because that will cost me money?” The 

answer is, yes you must make the wells available to do this imaging. 

But shutting the wells is not a problem if the results add value. If the 

engineer asks the operator to shut in for a well test, there wouldn’t 

be any question because they know the value. So, if we can establish 

the value crosswell brings to reservoir analysis and monitoring, the 

cost for 2D or even 3D will not be an issue. 

S: With this 3D coverage, is it possible to get an idea about azimuthal 

anisotropy? 

JH: The limitation of crosswell in this respect is constrained by 

where the boreholes are located; you will not be able to get 

uniformly sampled azimuthal data and this lack of uniform 

coverage will be a problem for estimating azimuthal anisotropy. 

From the point of view of tomography, the rate of convergence of the 

different complements of the anisotropy model may be horribly 

different. 

S: It will depend on the location of the borehole. 

JH: Yes, and you’ll want to keep the geometry as uniform as 

possible. You do not want to shoot between wells a few 100 m apart 

and interpret anisotropy with data from other wells that are 1000 m 

away. So what it means is you are forced to work with the geometric 

pattern you have for the wells. 

Remember too, in surface seismic this anisotropy you are seeing 

may be due to heterogeneity. It appears as anisotropy because you 

are not resolving the heterogeneity, say fractures. It may not be 

intrinsic anisotropy at all. If I have the higher resolution imaging that 

crosswell brings you may actually image the isotropic zones 

between the fractured zones or see other wave phenomena associated 

with the fractures like guided waves, etc. So it is no longer an 

anisotropy problem, it is a heterogeneity problem. 

S: Apart from Tomoseis, what other companies offer crosswell imaging 

service? 





JH: There is Schlumberger and Paulsson Geophysical and 

companies that do 3D VSP, though others have not been doing 

surveys for as long a time or as routinely as Tomoseis. The national 

laboratories have active programs. In addition to their work in oil 

and gas, the labs also work on near surface shallow environmental 

applications. 

S: Your present assignment as SEG/AAPG Distinguished Lecturer 

takes you to different places in North America and UK. How does this 

help you professionally? 

JH: It is very interesting. In some ways, it is an opportunity for 

me to see what other people are doing, what their research interests 

are. I am enjoying it, so far, though I am only on my 7th or 8th 

stop. I have 26 in all. 

S: This will take you to North America and UK or beyond? 

JH: This program is limited to North America and Western 

Europe, even though I had requests from South America and the 

Far East. I think the SEG should be responding to these distant 

requests, but there may be a combination of financial and political 

reasons for not doing them this year. 

S: I was wondering if you would like to share with us what 

research you are doing now? 

JH: I am giving this lecture on crosswell imaging now, but in 

fact much of this work was done 5 or more years ago at Stanford 

and elsewhere. Most of the work we do in crosswell at Stanford is 

related to environmental and near surface applications. Moreover, 

my personal interest is mostly on attenuation of seismic waves. 

This interest is driven by the observations of attenuation that we 

see in crosswell data. The problem is that even when we produce 

an attenuation tomogram, we have no good idea about how to 

interpret it. We cannot go to the laboratory and measure attenuation 

on a piece of rock, because the way attenuation is typically 

measured on a core is with ultrasound waves in the megahertz 

range. Attenuation scaling is just not reliable as velocity scaling. I 

have developed this technique called Acoustical Resonance 


Bruce Marion (Tomoseis) and Satinder in conversation with Jerry 

Spectroscopy, for measuring attenuation or Q of a small sample of 

rock in the lab at frequencies of 1000 Hz or so. This will provide 

some ground truthing for later being able to interpret the in situ 

attenuation data. We can also measure Q and velocity on irregularly 

shaped pieces of rock, like cuttings from a borehole. It is difficult 

to measure those in the lab even at high frequencies because of 

their irregular shape. We are looking for a technology that we can 

potentially locate at the well site, so that as the cuttings come out 

from the well, we can determine their acoustic properties. So, we 

spend most of our time working and thinking about numerical and 

physical models for attenuation in porous media, and how to 

process data to produce attenuation images. 

The other area where we have funding for is related to problems 

related to carbon sequestration, that is, using geophysical 

methods to monitor the injection of carbon dioxide in depleted oil 

and gas fields and aquifers. So, you might say that crosswell data, 

because the quality is so good, opens the door for a lot of basic 

research in terms of how waves propagate in porous media. I am 

spending more of my time looking at those basic issues than say 

specific applications. 

S: Jerry, what message do you have for young geophysicists 

entering our industry? 

JH: It is a question that I hear often. My response is that technology 

is important. They need to understand to some extent and 

not fear to use it when a problem calls for leading-edge technology. 

Moreover, they must support its development and think ahead 

longer term than just a few months. The majors need to be competitive 

and leaders in developing and applying technology, and 

should support research and education. Young geophysicists 

should follow their hearts in choosing a career path. They should 

build and basics and strive to be the best at what they do. 

S: Jerry, we thank you for sharing your experience and 

spending this time with us. We wish you good luck with your 

work. 

JH: I enjoyed it. Thank you R

An interview with Jerry Harris - Stanford University

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